Course Manual - University of Manitobahome.cc.umanitoba.ca/~mlast/rpublications/assets/coursemanual.pdf · beyond just the bare facts to the environmental implications and interpretations

Environmental Earth Science 007.136

Course Manual

2005 Printing

Copyright © 2000 All rights reserved. No part of the material protected by this copyright may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or otherwise without the prior written permission from the copyright owner.

University of Manitoba, Distance Education Program

Acknowledgments

Content specialist: William M. Last, Ph.D. Department of Geological Sciences Faculty of Science University of Manitoba

Bill Last was born in Illinois and emigrated to Canada shortly after receiving his B.Sc. degree in Geology from the University of Wisconsin in 1971. After working four years as a petroleum exploration geologist with Shell Canada Ltd., he moved to Winnipeg where he completed his Ph.D. at The University of Manitoba. He worked as a research officer in the Tar Sands/Heavy Oil Division of the Alberta Geological Survey until 1980 when he joined the faculty at The University of Manitoba in the Department of Geological Sciences.

Professor Last’s main research interests lie in the fields of sedimentology, environmental geology, and global change. With over 250 publications to his credit, he has maintained a long research involvement in lake sedimentology in western Canada. His research efforts are currently directed mainly at geolimnology and paleolimnology in western and northern Canada, northern United States, South America, Australia, and central Asia. He is editor-in-chief of the Journal of Paleolimnology, associate editor of the International Journal of Salt Lake Research, Sedimentary Geology, and Prairie Forum, and past associate editor of the Bulletin of Canadian Petroleum Geology. He teaches undergraduate courses in petroleum geology, environmental geology, global change, sedimentology, energy resources, and basin analysis. His graduate course offerings include advanced clastic sedimentology, petroleum geochemistry, and evaporite sedimentology and geochemistry.

Instructional designer: Cheryl Martin, M.Ed. Distance Education Program University of Manitoba

Desktop publishers: Lorna Allard Distance Education Program University of Manitoba

Cheryl Martin, M.Ed. Distance Education Program University of Manitoba

Special acknowledgment We gratefully acknowledge the Geological Survey of Canada for providing the Geological survey of Canada map and the brochure entitled The science of change.

Table of Contents

Introduction to the Course ....................................................................1

Course description..............................................................................1

Course goals .......................................................................................1

Learning objectives ............................................................................2

Course materials.................................................................................3

Course content....................................................................................4

Learning strategies for students .........................................................6

Evaluation and grading ......................................................................7

Unit 1 What is Earth Science: Basic Concepts and Historical Development...........................................................................13

Study notes .......................................................................................14 Earth system science and environmental Earth science..............14 Definition of environmental geoscience .....................................16 Historical development of environmental Earth science ............17 Modern environmental Earth science .........................................18 Sources of environmental problems............................................20

Unit 2 Introduction to Earth System Science .................................25

Study notes .......................................................................................27 Introduction to Earth system science ..........................................27 Systems .......................................................................................28 Dynamic interactions ..................................................................31 Cycles..........................................................................................33 Summary .....................................................................................37

Unit 3 Techniques, Data, and Investigative Procedures................39

Study notes .......................................................................................40 Topographic maps.......................................................................41 Geological maps..........................................................................41 Soils maps ...................................................................................42 Hydrological and hydrogeologic data .........................................42 Remote sensing ...........................................................................43

Unit 4 The Earth’s Cycles I: The Lithosphere ...............................53

Study notes .......................................................................................54 Introduction to the lithosphere ....................................................54 Geological framework of time ....................................................55 The crustal system.......................................................................56

Chemistry and mineralogy of the lithosphere .............................56 Major rock types .........................................................................57 Major components of the lithosphere system .............................59 Transfer of matter and energy.....................................................60 Summary .....................................................................................62

Unit 5 Endogenic Geologic Hazards: Earthquakes .......................65

Study notes .......................................................................................67 Introduction to geologic hazards.................................................67 Overview of geologic hazard mitigation.....................................69 Earthquakes .................................................................................70 Tsunamis .....................................................................................78 Summary of mitigation of hazards associated with earthquakes .........................................................................79 Earthquake prediction .................................................................80

Unit 6 Endogenic Geologic Hazards: Volcanoes ............................87

Study notes .......................................................................................88 Introduction.................................................................................88 Volcanoes and volcanic activity .................................................89 Hazards from volcanic eruptions ................................................91 Volcanic hazard prediction, mitigation, and evaluation .............94

Unit 7 The Earth’s Cycles II: The Hydrosphere and the Atmosphere......................................................................97

Study notes .......................................................................................98 The hydrosphere..........................................................................98 The atmosphere .........................................................................104 Concluding remarks ..................................................................107

Unit 8 Floods....................................................................................109

Study notes .....................................................................................110 Historical aspects of flood hazard.............................................110 Definition of flood hazard.........................................................111 The fluvial setting .....................................................................112 Causes of floods ........................................................................113 Controls of flooding ..................................................................115 Geologic factors in flood analysis and control..........................116 Flood analysis ...........................................................................117 Flood frequency ........................................................................119 Flood prevention and mitigation of losses ................................121

Unit 9 Exogenic Geologic Hazards: Landslides and Mass Movements .................................................................127

Study notes .....................................................................................128 Definitions and classifications of landslides/mass movement ..129

Basic mechanics of landslides...................................................132 Causes of landslides ..................................................................134 Landslide assessment, prevention, and control.........................136

Unit 10 Exogenic Geologic Hazards: Subsidence and Problem Soils .......................................................................141

Study notes .....................................................................................142 Subsidence ................................................................................143 Mechanisms of human-induced subsidence..............................145 Subsidence management ...........................................................148 Expansive soils..........................................................................148 Permafrost .................................................................................150

Unit 11 Water Resources and the Environmental Geoscientist ..........................................................................155

Study notes .....................................................................................157 Groundwater..............................................................................159 Water use...................................................................................162 Wetlands....................................................................................164 Dams .........................................................................................164 Drought .....................................................................................173

Unit 12 Coastal Zone Processes and Environmental Geoscience ............................................................................177

Study notes .....................................................................................178 Special coastal area problems ...................................................179 Types of coastal zones ..............................................................180 Waves and mechanisms of sediment movement.......................181 Beaches and shoreline erosion ..................................................188

Answers Appendix..............................................................................201

Environmental Earth Sciences 007.136 1

Introduction to the Course

Course description The Undergraduate Calendar of The University of Manitoba describes 007.136 as follows:

An integrated approach to Environmental Earth Science. The effect of Earth’s internal processes on the external processes in the atmosphere and hydrosphere. Topics include: the water cycle, weather, climate and climate development, pollution. Not to be held with 007.124 or the former 007.132. Prerequisite: one of 007.123, 007.134, 007.144, or 007.225 (or the former 007.126, 007.127, or 007.133).

Modern environmental Earth science is a broad subject encompassing virtually every aspect of the traditional topics of geology, geophysics, and geochemistry and including many associated scientific and engineering subdisciplines. During this term our investigations and discussions of environmental Earth science will revolve around two major themes: • Earth system science; and • the geoscience of natural hazards.

A third major component of environmental Earth science, namely that of pollution geoscience or how humans affect the Earth’s environment, is covered in the course 007.239 Environmental Geology.

Environmental Earth Science will examine selected aspects of both of these two perspectives of environmental Earth science. This course is intended to provide an overview of the salient aspects of environmental Earth science, some of which are unique to the field, others being shared with allied disciplines such as soil science, engineering geoscience, hydrology, and geochemistry. The ultimate objective of the course is to foster your analytical and critical thinking skills. We will have plenty of facts to learn, but we will always want to go beyond just the bare facts to the environmental implications and interpretations.

Course goals Why do we study environmental Earth science? It is clear that the environment and environmental problems have become matters of intense concern on local, national, and international levels. With this increasing awareness, the need for rational, informed decision making by the public and by policy makers is imperative. Environmental Earth Science 007.136 has three main goals:

• to examine the global interconnectedness of the Earth’s air, water, rock, and life systems and the changes within and among these components;

• to present and discuss the role that natural geologic processes play in creating conditions that are detrimental to human activities; and

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• to assess how society can best mitigate the adverse affects of geologic hazards on local, regional, and global scales.

Over the past 100-200 years reductionism, or the technique of attempting to understand the whole by examining its parts, has served science and society well. The emergence of a more holistic approach, such as Earth system science, is leading to the gradual breakdown of disciplinary boundaries within science. Holistic approaches to the Earth emphasize the linkages and feedbacks between its different components, and stress the connecting movements of material and energy. It is important to realize that Earth system science is not replacing the past reductionist approaches but rather simply drawing the various relatively narrow subdisciplines and parts together to build a multidisciplinary picture of how complex systems work as a whole. The approach we are taking with this course is that environmental Earth science (including hazard geoscience and related processes) is a subset of the broader holistic approach of Earth system science.

Learning objectives The skills necessary for environmental Earth scientists to excel in the field of environmental assessment and mitigation today are quite broad and certainly different from what is contained in a standard university undergraduate curriculum. Above all, the environmental geoscientist must be well acquainted with the Quaternary record, should understand quantitatively the development of sedimentary sequences and methods for their description, and should understand the dynamic of fluids both on the surface and in the subsurface.

As you systematically progress through the course material during the next thirteen weeks, you will: • define the relationship between environmental Earth science and other

branches of physical, chemical, biological, and social science;

• demonstrate how nearly all of our major environmental concerns and hazards are rooted in basic geologic processes;

• explore the key dynamic interactions among the various Earth systems;

• discuss how environmental Earth science is a collage of many different geological subdisciplines, from hydrology to geochemistry, from oceanography to geomorphology;

• outline how our perception of geologic hazards and human interaction with geologic processes has evolved over time and is different in various other cultures;

• describe the interrelationships between feedback, thresholds, and flows in cycles within and through the Earth’s systems;

• identify the differences between mission oriented geoscience and problem-solving pursuits;


• locate, on a regional and global basis, areas most prone to naturally occurring geologic hazards, and outline what types of actions can be used to reduce risk and mitigate losses from these hazards;

• describe humans’ role in aggravating normally nonhazardous geologic processes to the point that a threshold is exceeded, resulting in rapid and often catastrophic changes; and

• show how the organization, control, and coordination of new industrial and urban development can be integrated with a basic knowledge of geologic processes to protect environmental, cultural, and aesthetic characteristics of the land.

As you work through the course, you will have the opportunity to apply the theoretical knowledge you are accumulating to solve site-specific problems and environmental dilemmas. The objective of these “real world” exercises is to integrate the concepts of environmental Earth science with practical, often quantitative, information in order to resolve, or at least lessen, the impact of the hazard. It is, therefore, important that you study and adequately understand the assigned problem sections and diligently work through the exercises and review questions in your textbook.

Finally, several words about the textbook assignments: do them! In a distance education course such as this, the textbook is every bit as important as the notes you are reading now. Not only will the textbook readings help you to understand concepts and ideas summarized in these notes, but the textbook provides an abundance of graphical material, tables, charts, and photographs which cannot be included in these course notes. Take advantage of your textbook!

Course materials Required text The following required materials are available for purchase from the University of Manitoba Book Store. Please order your materials immediately, if you have not already done so. See your Distance Education Student Handbook for instructions on how to order your materials.

Merritts, D. J., A. de Wet, and K. Menking. 1998. Environmental geology: An Earth system science approach. New York: W. H. Freeman and Company. (452 pages)

Included with your course manual The following map and brochure should be included in your course materials package:

_____. Geological survey of Canada map. Ottawa: Geological Survey of Canada, Minister of Supply and Services.

_____. 1991. The science of change. Ottawa: Geological Survey of Canada, Minister of Supply and Services.

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Online _____1994. Geological highway map of Manitoba. 2d ed. Winnipeg, MB: Manitoba Minerals Division.

The map is required for assignment 1 and is located online in WebCT. Please follow directions provided in your copy of the Student Handbook to learn how to access WebCT. If you need a printed copy of the map, please see your instructor’s letter of introduction for direction.

Distance Education Student Handbook Along with your course manual you will receive a Distance Education Student Handbook. Keep this Student Handbook and your Distance Education Program Guide handy throughout the year, as they will provide you with detailed information regarding the management/administrative aspects of this distance education course. The Handbook tells you how to access the following:

• Your instructor; • Distance Education Student Services; • Using technology; • The University of Manitoba Libraries; • Information on ordering your course materials through the University of

Manitoba Book Store; and • Information on accessing your grades and submitting assignments on-line

using WebCT.

Course content This course is about the planet Earth or more specifically, the human-planet relationship: How Earth processes influence human habitation and activities on a daily basis and how human actions, in turn, affect the functioning of Earth systems. It is equally important to understand at the outset what this course is not about! This is not a traditional course in either physical or historical geology, although environmental Earth science certainly does encompass aspects of both of these topics. Nor is this course designed to cover all aspects of the human-planet interaction. Components such as environmental implications of resource development, waste disposal, and human health and geology are covered in the subsequent course 007.239 Environmental Geology.

Over the past few years scientists have come to the realization that an understanding of the interconnectedness of air (atmosphere), water and ice (hydrosphere and cryosphere), rocks and soils (geosphere), and life (biosphere) on a global basis is often preferable to the traditional approach of studying separate individual units. This approach to investigating the Earth as a whole with many interacting parts is the basis of Earth system science one of the two major themes.

Coupled with this enhanced understanding of the complex dynamics between the various Earth ‘reservoirs’ is a renewed appreciation of how the geologic


environment and geologic processes interact (usually adversely!) with human activities and communities the second major theme. This is often termed hazard geology or the geoscience of natural hazards. Hazard geoscience most commonly involves the traditional pursuits of hydrology; volcanology; earthquake geoscience; and land subsidence, landslide and mass movement studies, combined with climate change investigations.

In practice, environmental Earth science is a very broad branch of applied science that focuses on the entire spectrum of possible interactions between people and the physical, chemical, and biological environment. Obviously no single course can fully cover the wide range of topics germane to environmental Earth science as it is viewed by modern professional geoscientists. The topics you will cover during the next thirteen weeks represent an overview of selected concepts, processes, problems, and solutions of critical importance to a practising environmental scientist today. The selection and coverage of these topics are based not only on the traditional view of environmental geoscience as a “corrective” science (the treating of environmental problems after they occur) but also on its role as a “preventative” science (anticipating the problems induced by interaction with the geologic environment).

The course is broadly organized in such a way as to familiarize you first with the developmental history and techniques of environmental Earth science as a modern approach to understanding our planet, then with the deep-seated Earth processes that influence human settlements, and finally with the near-surface and surficial processes that must be understood by planners and policy makers to undertake proper environmental management. Within this broad framework, we will delve into selected aspects of the particular system pertinent to each specific group of geologic hazards. Attention will be given to the following topics (in order of coverage):

What is environmental Earth science and what tools do we use? The birth of a new paradigm

The general concept, evolution, and perception of environmental Earth science in science and society

Techniques, tools, and analytical skills commonly used by practising environmental Earth scientists

How do we “know” something? The scientific method and approaches to scientific reasoning The limitations of science

The system concept Closed, open, and isolated systems and conceptual models

Dynamic interactions among systems, reservoirs, fluxes, and cycles that are important on a global and regional scale

Energy: A key cycle in Earth system science

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The major Earth systems The Earth’s lithosphere: Critical aspects and concepts involving plate tectonics, the Earth’s interior, the crust, and the rock cycle

The Earth’s atmosphere and hydrosphere: Atmospheric water and energy, the oceans and ocean circulation, the land-based hydrosphere

Endogenic geologic hazards Earthquake hazards: Mechanisms, potential dangers and sources of damage, prediction, and mitigation

Volcanic hazards: Mechanisms, types of hazards, prediction, and protective measures

Exogenic geologic hazards Flood hazards: Fluvial hydrology, flood magnitude and frequency analysis, identification, prediction, and mitigation

Landslides and mass movements: Landslide processes, slope stability analysis, types of mass movements, hillslope development and management, landslide hazard mitigation techniques

Subsidence and problem soils: Types and causes of subsidence, mechanisms of natural subsidence, hazard recognition, clay mineralogy and expansive soils, permafrost, and hazard mitigation

Coastal zone geoscience and hazards Coastal zone processes and environmental geoscience: Special problems of the coastal zone; types of coasts; mechanisms of sediment transport and wave dynamics; seiches, surges, and tides; and coastal geoengineering and management

Global change and Earth science issues Climate and causes of climate change, influence of plate tectonics and ocean circulation, Earth’s orbital parameters, and climate

Indicators of global environmental change

Geological records of climatic and environmental change, and short-term and long-term climatic fluctuations

Global warming: The geological perspective, modelling, and prediction

Learning strategies for students At the end of each unit, you should be able to summarize each reading in your own words. Ask yourself, “What are the authors’ main points?” “What are the key concepts?” “What do I think about these ideas?” You may wish to discuss your answers to these questions on-line with others in your class.


Interacting Learning by interacting with the new concepts presented in your course materials is very important. For example, you will be actively thinking about the course content as you complete your readings and assignments, and participate in on-line discussions.

On-line with other students Take advantage of communication tools in the course website to learn by interacting with others in your class! The tools include e-mail, discussion, and chat. If you think that you are about to ask a question that others in your course may have, ask other students first using the “discussion tool.” This provides a great opportunity for comments, questions and debate. Through exchanging information with other students you may find that you quickly get the answers to your questions. Consider creating on-line study groups. Exchanging information, questions, and ideas with other students can also enrich your learning experience. Your link to the Distance Education Student Handbook on-line in the course website contains instructions on how to access your course in WebCT.

With your instructor Questions? Concerns? Discussion? Address any concerns regarding assignments directly with your instructor. Caution: Not all instructors interact on-line so you should check your instructor’s contact information to determine the best way to communicate.

Using the library Additional readings will enrich your learning experience and your understanding of your course topics. Textbooks and course materials often contain suggested reading lists and you can search any library using on-line library search tools to find these and other related materials. Check your Distance Education Student Handbook for information about accessing the University of Manitoba library.

Evaluation and grading You should acquaint yourself with the University’s policy on plagiarism, cheating, and examination impersonation as detailed in the General Academic Regulations and Policy section of the University of Manitoba Undergraduate Calendar. Note: These policies are also located in your Distance Education Student Handbook or you may refer to Student Affairs at http://www.umanitoba.ca/student.

Assignments You will be asked to use the knowledge you have assimilated in the course to examine, evaluate, and solve a variety of practical environmental geoscience problems during the term. There will be four assignments that will help you to bridge the gap between the theoretical aspects of the science and the practical application of these concepts. It is very important that you think about and work

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through these problems as completely as possible. Environmental Earth science can be a practical, applied, and pragmatic science whose goal is to generate viable and reasonable solutions to perceived or anticipated problems. Your ability to apply what you have learned is one of most critical factors in successful completion of this course. Although the problems you will be solving are based on real world data and situations, in order to complete the tasks asked for in a reasonable amount of time, simplifying conditions and constraints are often built into the exercises. In total, the problem sets you are assigned during the term are worth 40% of your final mark.

Assignment due dates Assignment Sept.-Dec. Jan.-Apr. May-Aug.

1 September 30 January 21 May 21 2 October 15 February 7 June 7 3 October 30 February 28 June 30 4 November 15 March 15 July 15

Note: If the assignment due date falls on a Saturday, Sunday, or statutory holiday, it will be due on the next working day. If the assignment due date falls during the Mid-term Break in February, it will be due on the Monday following the Mid-term Break. If you are unable to submit an assignment on time, contact your instructor well in advance of the due date, for we cannot guarantee that the instructor will accept late assignments.

Review the guidelines on assignment due dates in the Student Handbook.

Distribution of marks Item Percentage

1. Environmental Earth science data and tools 10 2. Flood hazards 10 3. Landslides 10 4. Subsidence 10 5. Final examination _60

Total 100

Final examination At the end of the course a final examination will be written which will be worth 60% of your final mark. This examination will be designed to test not only your grasp of the theoretical concepts of environmental geoscience, but also the more practical critical evaluation and problem-solving abilities you have acquired. Normally, the final examination is weighted approximately equally between material covered in the course notes and that covered in the assigned textbook readings. The format of the examination is normally a combination of short answers, long answers (essay), and multiple choice questions.


General guidelines for assignment and exam preparation A word of caution about the assignments and the final examination Some students find that they do very well on the assignments, but they do not do nearly as well on the final examination. While your grades on the assignments will give you some idea of how well you are mastering the material, they may not indicate how well you will do on the examination, because the examination is written under very different circumstances. Because the assignments are open book, they do not require the amount of memorization that a closed-book examination requires nor are they limited to a specific time period. Some students have told us that, based on the high marks they received on the assignments, they were overconfident and underestimated the time and effort needed to prepare for the final examination.

Please keep all this in mind as you prepare for the examination. If your course has a sample exam or practice questions, use them to practice for the examination by setting a time limit and not having any books available. Pay careful attention to the description of the type of questions that will be on your final examination. Preparing for multiple choice questions involves a different type of studying than preparing for essay questions. Do not underestimate the stress involved in writing a time-limited examination.

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Notes

Environm

ental Earth S

ciences 007.136 11

007.136 Environmental Earth Sciences Your Course at a Glance

Week 1 Introduction to the course

Unit 1: What is environmental Earth science: basic concepts and historical development

Send in request for examination form if you live outside Winnipeg.

Week 2 Unit 2: Introduction to Earth system science

Week 3 Unit 3: Techniques, data, and investigative procedures

Assignment 1 due

Week 4 Unit 4: The Earth’s cycles I: The lithosphere

Week 5 Unit 5: Endogenic geologic hazards: Earthquakes

Week 6 Unit 5: Endogenic hazards: Earthquakes (continued from week 5)

Unit 6: Endogenic hazards: Volcanoes

Assignment 2 due

Week 7 Unit 7: The Earth’s cycles II: The hydrosphere and the atmosphere

Week 8 Unit 8: Exogenic geologic hazards: Floods

Assignment 3 due

Week 9 Unit 9: Exogenic geologic hazards: Landslides and mass movements

Week 10 Unit 10: Exogenic geologic hazards: Subsidence and problem soils

Assignment 4 due

Week 11 Unit 11: Water resources and environmental Earth sciences

Week 12 Unit 12: Coastal zone processes and environmental geosciences

Week 13 Unit 12: Coastal zone processes and environmental Geosciences

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Notes


Unit 1 What is Earth Science: Basic Concepts and Historical Development

Topics Earth system science and environmental Earth science

Definition of environmental geoscience

Historical development of environmental Earth science

Modern environmental Earth science

Sources of environmental problems

Introduction We begin by describing the basic concepts and fundamentals of environmental Earth science and looking at key historical developments of this new scientific paradigm. We will trace the evolution of environmental Earth science and examine how this evolution has been affected by cultural preferences and scientific discovery.

Learning objectives In order to profit from your exposure to environmental Earth science in this course, you must first be aware of how this branch of science is viewed by geoscientists today and how it is related to other branches of geological science as well as to other sciences. You should also be aware, if you are not already, of the parallel historical developments and evolution of the conservation/environmental movement in western society and the role this movement played in the formulation of environmental Earth science as we now practise it.

By the end of this unit you should be able to: • identify why environmental Earth science is more pertinent now than it was

10, 50, or 100 years ago;

• discuss the factors that distinguish scientific thought from other types of thought;

• summarize why the scientific method is the most effective strategy yet devised for learning about the Earth’s physical events;

• discuss the limitations of scientific thinking;

• provide examples on how science can be used in the interest of conserving Earth’s resources and avoiding environmental risks;

• define environmental geoscience and Earth system science;

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• summarize the advantages of Earth system science for understanding the global environment;

• relate environmental Earth science to ecological and environmental ethics;

• name the factors of “western civilization” that are most responsible for environmental degradation;

• discuss the problem with the term environmental geoscience and explain why it is essential to maintain the use of this terminology;

• differentiate between corrective geoscience and preventative geoscience;

• summarize the more noteworthy accomplishments of environmental geoscientists of the late 19th and early 20th century;

• describe the role(s) of environmental Earth science in military activities;

• examine the ways in which environmental geoscience and Earth system science has changed since World War II;

• critically evaluate why society has been slow to accept geological conservation;

• list the basic underlying, unchanging, unifying concepts that control nearly all aspects of environmental Earth science;

• relate the concepts of feedback and threshold to environmental Earth science;

• assess how rate and changes in rates of geologic processes are viewed in environmental Earth science; and

• identify the two most pressing problems or sources of problems in environmental management and discuss these problems in light of environmental Earth science.

Learning activities 1. Read the study notes and answer the review questions found at the end of

this unit in the course manual.

2. Read pages xv - xxvi, and chapter 1 in your textbook; review the key terms and answer the review questions on page 26 of your textbook.

Study notes Earth system science and environmental Earth science During much of the twentieth century, the natural sciences have been concerned with examining individual physical, chemical, and biological processes in the four major Earth spheres (air, water, land, life). This approach is referred to as a reductionism method, and it has served science and society well over the past 100 years. Relatively recently, however, international concerns with environmental issues on a global scale (e.g., acid rain, global warming, resource


depletion, nuclear contamination) have forced scientists to adopt a more planetary approach to the investigation of Earth. This new approach makes use of a variety of physical, chemical, and biological sciences and integrates them into a broad, global view of Earth and how the Earth functions as an integrated unit. This major change in the way we conduct our scientific investigations (referred to in the popular scientific press as a ‘paradigm shift’) has resulted in a new discipline – Earth system science. In Earth system science, the Earth is viewed as a complex, evolving planet that is characterized by continuously interacting physical, chemical, and biological change over a wide range of temporal and spatial scales.

Much of environmental Earth science revolves around a diverse suite of geoscientific subjects, including geochemistry, geophysics, process geology, engineering geology, economic geology, sedimentology, historical geology, and structural geology. Indeed, few persons ever become experts in all aspects of environmental Earth science. The breadth of knowledge required by a practising environmental geoscientist is a function of the wide range of geologic factors that can interact with human activities and the great many ways in which people can adversely affect the geologic environment. It is ironic that as humans increasingly modify (control?) the surface of the Earth, they become increasingly vulnerable to hazardous or destructive Earth processes. The demand for environmental geoscientific data and interpretations, and for qualified environmental geoscientists has never been greater in history.

A basic law of ecology (stated very simply) is: everything is related to everything else. This is equally true in the realm of environmental Earth sciences. Geology—the study of the Earth and Earth materials and processes—has significant and direct applications to the atmosphere (air), hydrosphere (water), and biosphere (life), as well as to the lithosphere (land). The lithosphere is the ultimate source of nearly all of our minerals and fuels. The weathered lithosphere, or soil, is essential for life. The atmosphere controls, to a major degree, what raw materials are required by people, what can be derived from the weathered lithosphere, and ultimately what type of weathering the lithosphere is undergoing. The hydrosphere is likewise fundamental to all life, plus it serves very important roles in waste disposal, cooling, and recycling of elements.

The broad area of environmental Earth science can be very simply defined as the study of natural processes, and their interaction with each other and with humans. In this course we are concerned with the geological aspects of the environment and their effects on people. Throughout the course we will be emphasizing four basic concepts: Earth system sciences:

• geologic thresholds; and

• systems, models, and cycles within and among the major Earth’s reservoirs.

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The geoscience of natural hazards:

• prediction of processes, rates, and thresholds and use of feedback mechanisms; and

• practical, pragmatic approaches and solutions to perceived environmental and hazard problems.

Definition of environmental geoscience There has been much written and said about the precise meaning of the term environmental geoscience. No other branch of the Earth sciences has received such a detailed discussion and scrutiny about its name. The first point that needs to be remembered is environmental geoscience is not a new field of investigation, although certainly large scale public awareness of this branch of science “per se” did not come about until the 1960s. In the 1850s von Cotta wrote a textbook titled The Geology of the Present which included virtually all aspects of modern environmental geoscience. In Europe, anthropogenic sedimentation (or increased sedimentation due entirely to the onset of human activities) was recognized as a major problem for over 1,000 years. In 1826 von Grouner wrote a major treatise on the use of geoscientific principles and concepts in military science—a theme which would dominate the field of environmental geoscience for over 100 years!

Two major points need emphasizing here. The first is: environmental geoscience is a field that has grown out of a social need to broaden the application of Earth science. Particular emphasis is today being placed on problems associated with industrialized society’s use of the Earth and its resources. The basic tenet is that society can better manage the Earth’s natural resources if society (or at least its decision makers) knows something about the Earth. The second point is that with more pressure on the Earth to supply energy, material, food, recreation, etc., the more complexity that is built into the system. The more complex the system, the more vulnerable it is to disruption. As this vulnerability increases, the margin for error decreases, and the probability of disaster increases. These two basic themes will arise many times during this course.

Much of the controversy over use of the term environmental geoscience arose from a group of scientists who, in the early 1970s, convincingly argued that if geoscience is the study of the Earth, with specific emphasis on the physical environment, then the term environmental geoscience represents “terminological inexactitude.” In other words, they contended that environmental geoscience was, by definition, geoscience and, therefore, the combined terms were not required. They also contended that use of the term environmental geoscience implies that prior to the widespread use of the term beginning in the 1960s, geoscientists were not interested in the environment and were not “doing” environmental geoscience, which is clearly wrong. Thus, the term environmental geoscience, they insisted, should be dropped, a geoscientist, by definition, is an environmental geoscientist.


Today, however, most Earth scientists would recommend maintaining the term environmental geoscience. The reasons for this are:

• Not all geology is environmentally related.

• Often the geologic factor is not perceived by the general public as the single most basic component of the human environment. Thus, the term environmental geoscience reinforces, to the public, the attitude and posture of this branch of geoscience.

• The social orientation of environmental geoscience is only part of the science. Environmental geoscience is also equally concerned with the influence of human activities on the geologic environment.

• There are many other examples of terminological inexactitude (such as lunar geology, planetary geology), but these all seem to convey a meaning.

Thus, a definition of environmental geoscience is the application of Earth sciences to the benefit of humans and the biosphere; it is the integrated application of many branches of Earth science. Importantly, in this course we will be taking a very pragmatic viewpoint of environmental geoscience. Much of environmental geoscience today is aimed at attempting to replace “corrective” geoscience with “preventative” geoscience. Corrective geoscience implies treating the environmental problem or consequence of the hazard after the human activity has interfaced with it; preventative geoscience is the anticipation of the problem and the application of social, scientific, and technological means to avoid major negative consequences. Geoscience is excellent for this purpose because it is a “retrodictive” science, unlike engineering, biology, chemistry, etc. On the one hand it is basically an historical science, but on the other it is largely predictive.

Historical development of environmental Earth science Environmental Earth science is as old as our awareness of the physical world. The practise of environmental geoscience actually predates the development and formulation of geology as a science. Primitive peoples used basic environmental Earth science in the search for and exploitation of flint, chert, salt, and other natural resources. The Egyptians were probably the first group to formulate the concepts and ideas of environmental geoscience. Other classic “Old World” environmental geoscience applications and problem solving include: increased siltation of harbors in the Mediterranean nearly 3,000 years ago, and construction of major river diversions, sewers, and aqueducts by the ancient Romans.

In North America some of the earliest environmental geoscience efforts were related to military use of geoscience. In battles such as at Bunker Hill and Breeds Hill in 1775, and Cemetery Ridge and Gettysburg in the United States Civil War, geoscience (and knowledge of the geoscience) played a pivotal role in dictating success (or failure) of the military event. Military strategists of the 18th and 19th centuries were some of the foremost geomorphologists and terrain scientists of the era. Early, nonmilitary, published environmental

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geoscience efforts included classic works by G. R. Marsh, Physical Geography as Modified by Human Action (1860s), and the extensive exploration and mapping by J. W. Powell. In the 1870s Powell was the first to try to evaluate the carrying capacity of the land in western North America, a concept that still today dictates the “usefulness” of the land. Two of the most important advances in North America were the creation of the United States and Canadian National Parks systems and the United States Geological Survey (U.S.G.S.) and the Geological Survey of Canada (G.S.C.). The establishment of these government bodies and institutions advanced public awareness of the geologic environment and greatly advanced the science of geology (and environmental geoscience) in North America.

More recently, environmental geoscience studies have been intensified in the post-World War II era. In the mid-1960s, the Illinois Geological Survey emerged as a world leader in the application of geoscience to human impacts. The 1969 NEPA (National Environmental Policy Act) and associated legislation required that EIS (environmental impact statements) be generated on any significant event or action which will or may affect the quality of the human environment. Subsequent use and application of the NEPA-EIS style of legislation discovered many unanticipated problems; who is responsible for preparation (and costs) of EIS? What is considered significant? What should be included? But the EIS were found to be effective; over 200 Army Corps of Engineers (ACE) projects were ultimately cancelled. Some 10,000 EIS were generated in a 10 year period. Most important, however, is that the EIS brought geoscientists in direct contact with decision makers. Earth scientists and geological considerations were now intimately involved with planning. There are many examples which will be discussed at length in your textbook and later in these course notes that describe this new social function of geoscientists.

Modern environmental Earth science A major mission of environmental geoscience, then, is to develop information about the Earth, Earth processes, and human interactions with the Earth for public purposes. This must be done in a systematic manner, focussing on the problem(s) or perceived problem(s). Most important, the results of environmental geoscience must be useable to the educated layperson. In summary, environmental geoscience is “normal” Earth science activity that is directed and emphasizes practical applications and communication with the public.

Surprisingly, society has been very slow to accept geological conservation. The reason for this is, simply, that society has continued to grasp and hold a number of outdated or even incorrect concepts. Some of these “erroneous” concepts include the following:

• “Man is superior” attitude; the human activities take precedence over all else.


• “Nature is self-healing” attitude; although we can destroy some (many) aspects of the environment, this destruction is only temporary, and nature will renew in due course.

• The belief that “nature is cyclic”; all things progress through a series of stages, and humans have very little to do with controlling these stages (i.e., what happens is inevitable).

• The “now generation” philosophy; we live in the present and need not worry about future resources or environmental conditions.

• The “infinity” complex; the land and natural resources are so plentiful as to be nearly infinite, and even if they do become exhausted there are always new substitutes. Another version of this belief suggests that science is so intelligent and inventive that any mismanagement that may occur can be overcome by science to ensure survival.

Much of the philosophy of environmental geoscience stems from several oft-referred to “fundamental concepts.”

• We are dealing in nearly all cases with a closed system.

• The Earth is presently our only habitat, and resources are finite and limited.

• Uniformitarianism does apply, but uniformitarianism says nothing about rates!

• There always have been natural hazards. There is nothing to be done to stop most of them.

• Proper planning must consider both economics and aesthetics.

• In environmental geoscience we must try to consider the cumulative effects of an action.

• The geologic environment and the underlying geologic factors in environmental planning are the most basic and fundamental components to consider.

• Complexity is the norm in natural processes and systems. The implication of this is that many factors influence an action. This is in direct conflict with the simplicity of the law of parsimony, which contends that the “correct” answer lies with the least complex alternative.

• Every action will have a feedback effect on some other component of the Earth system. The classic example of this is groundwater use by a municipality. Over pumping of the groundwater results when withdrawal rates are greater than recharge rates. This leads to a lowered water table, which leads to collapse and subsidence of the surface of the land. The lowered water table also feeds back to the economics of the use of groundwater; drilling costs are higher because deeper drilling is required to reach the groundwater. In addition, in much of western Canada the deeper water is of poorer quality (higher salinity), which leads to salinization of the soils when used in agriculture. Salinization leads to loss of fertility, which

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causes a decrease in the carrying capacity of the land. Complex feedback loops like this exist in virtually every other type of environmental Earth science problem or hazard. We will be discussing many of these feedback “webs” later in the course.

• Threshold concept: A great many Earth systems have a critical point. If stress is applied that exceeds this critical point, the system changes rapidly often with extreme negative results. This threshold concept is not unusual and is found in many other natural science systems. In physics an example is critical mass; in engineering an example is bearing load; in geography and agriculture an example is the carrying capacity of the land; in rock mechanics and mining an example is yield strength of the material; in hydrodynamics examples are Froude numbers and Reynolds numbers.

• Environmental decisions involve and produce conflicts. These conflicts tie in closely with society’s basic ethics of resource management. There are essentially three “end-member” viewpoints of resource management. Often political parties, governments, administrations, and company managers are categorized as one or other of these types:

− utilitarian ethic (good environmental management means mastery over nature);

− conservation ethic (good environmental management means maximizing the use of the resources through time); and

− preservation ethic (basically that the only environmental management that is necessary is preservation; we should not make any basic changes or alterations in natural areas).

The conflict between preservationists and conservationists was very noticeable in the 19th century is the arguments between J. Muir (preservationist) and G. Pinchot (conservationist). Similar arguments are raging even today as exemplified by conflicts over water use in Alberta, dam construction in Saskatchewan, or oil and gas drilling in national parks and wild areas of the Rocky Mountains.

Sources of environmental problems Obviously, today there are many factors that can be cited in causing or contributing to environmental problems. Ultimately, the two most important causes of problems are population and urbanization.

Sometime in the latter part of 1999 (supposedly in October) the population of the Earth surpassed 6 billion. Although the population density of the Earth is only about 30 persons per km2, the problem is that the spatial distribution is very poor. Densities range from less than 2 persons per km2 to greater than 5,000 persons per km2. The present rate of growth of population is about 1.7% per year. Although short term prediction is very imprecise, it is abundantly clear that we are dealing with exponential growth. Thus, in virtually all aspects of environmental Earth sciences an understanding of the arithmetic of exponential growth is essential. Certainly this is the single most important concept in areas


such as oil and gas resource management, water resources, and base metal exploration and development. A growth rate of 1.7% per year means that growth is occurring at a constant or fixed percentage per unit of time. Thus, the time required for the quantity to double in size (i.e., increase by 100%) is fixed. This is referred to as the doubling time or T2. In very broad general terms, the doubling time can be approximated by T2 = 70/p; where p is the percentage increase per year. For example, in the case of population, T2 = 70/1.7 or 41 years. This means that approximately every 40 years the population of the Earth doubles. It is interesting to calculate the doubling times of the use/consumption of geologic resources such as oil, gas, coal, metals, etc. For example, our record of long term use of fossil fuels indicates an average increase in consumption of 7% per year. The repercussions of exponential growth are truly impressive.

Some generalizations of the arithmetic of exponential growth are:

• Exponential growth is characterized by a doubling of the quantity in a fixed period of time.

• Just a few doublings can generate huge quantities.

• The size (quantity) of the material after each doubling is always greater than (or possibly equal to) the sum or total quantity of the material before the doubling took place. Using our example of consumption of oil and gas, with a growth rate of 7% per year, T2 is 10 years. Thus, the total amount of oil and gas that we will need between 2000 and 2010 is greater than the total amount of oil and gas consumed from 1850 to 2000!

The implications of exponential growth to world population growth in a finite environment are obvious. The favorite story often cited in introductory geology and ecology textbooks is that of the bacteria in a bottle. Assume that bacteria grow by simple division; one bacterium becomes two; two become four; four become eight, etc. Also assume that the bacteria are growing at a rate of 1.16% per second (i.e., T2 = 60 seconds). You place one bacterium in a bottle at 11am and notice the bottle is completely full at 12 noon. If you were to map this growth you would notice the following: at 11:54 the bottle was 1/64 full; at 11:55 it was 1/32 full; at 11:56 it was 1/16 full; at 11:57 it was 1/8 full; at 11:58 it was 1/4 full; and finally at 11:59 it was 1/2 full. Only at two minutes before noon (11:58) you might realize what is going to happen and make a frantic effort to find more room (more bottles or more “resource’). At 11:59 you rejoice in finding three new empty bottles—a total of four times the space (i.e., resource) that you started with. How long will this resource bonanza satisfy your growing population? Just two more minutes! At 11:59 the first bottle is 1/2 full; at 12:00 noon the first bottle is full; at 12:01 both the first bottle and second bottle are full; at 12:02 all four bottles are full! It is most instructive to think about how close to noon we are with respect to natural resources.

In any type of discussion of growth in a finite environment (population growth, energy consumption growth, growth in the use of zinc, oil, etc.) it is essential to realize the options that are available (presuming, of course, that the resource is finite). In population dynamics, there are two basic options:

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• population crash; and • gradual approach to zero population growth.

Obviously, the first is not preferred by most of society. It would result in starvation, famine, and chaos until a new population level is attained.

Some population dynamics studies suggest that this could occur by 2030. The second is a preferable scenario assuming that:

• We have not already exceeded the threshold level with respect to essential resources.

• Humanity can or will slow its growth voluntarily.

• There are sufficient resources left to maintain the stabilized level of population (estimated to be 8-10 billion).

We will postpone a detailed discussion of the second most important factor/cause of environmental problems—urbanization—until later in the course except to mention here that urban populations are growing at a much faster rate than the overall growth. The problems associated with urbanization include encroachment into areas that are less desirable (geologically), and the concentration of human activity in relatively small areas that can initiate and accelerate many normally nonhazardous geologic processes.

Key concepts and terms to remember (Don’t forget to review the key word list in your textbook, too.)

ACE aesthetic quality carrying capacity conservation ethic corrective geoscience doubling time EIS environmental ethics exponential growth feedback GSC land ethic

military geoscience population crash preservation ethic preventative geoscience NEPA reductionism terminological inexactitude threshold U.S.G.S. utilitarian ethic zero population growth

Review questions (Be sure to work on the review questions in your textbook, too.) Note: Sample answers are given in the answers appendix.

1. Give two reasons why the term environmental geoscience should be kept.


2. Briefly explain the concept of threshold with regard to environmental geoscience. Give an example.

3 a. How long will it take for the world’s population to double if it is growing at a fixed rate of 2% per year. Show your work.

b. Explain the concept of doubling time.

4. Give two examples of “feedback” in environmental geoscience.

5. Society has been slow to accept geological conservation. List three possible reasons for this slow acceptance.

6. J. W. Powell was a famous geologist/explorer of western United States during the nineteenth century. In addition to exploring and mapping much of the Colorado River area, describe or identify one other important environmental geological role or task undertaken by Powell.

7. Describe an example (past or present) of environmental geoscience being used in military activities.

8. Why is geoscience a retrodictive science?

9. The size of the Earth’s human population directly affects the severity of any environmental problems. Explain this idea in the context of (a) resources, and (b) pollution.

10. Define uniformitarianism and discuss it in the context of environmental geoscience.

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Notes


Unit 2 Introduction to Earth System Science

Topics Introduction to Earth system science

Systems

Dynamic interactions

Cycles Carbon cycle Sulfur cycle The Earth’s energy relationships

Summary

Introduction In this unit we will focus on the concept that the Earth can be studied from a “systems” perspective. This means that, instead of studying individual parts of the Earth in isolation (i.e., the reductionism approach), we look at the whole Earth in the context of a series of interconnected systems. How these various parts are defined and how each system is related to others in the whole Earth system is the basis of Earth system science. This approach is clearly a helpful way to break down a large, complex conceptual problem into smaller, more easily studied parts without ignoring the linkages among the parts. The main concept to remember from this section is that emphasizing the links among Earth systems rather than just the specific systems or components of individual systems provides Earth researchers with an essential framework for understanding (and mitigating) virtually all environmental problems and hazards.

We will begin by defining and discussing a system and the systems approach to science and identifying the major environmental systems on Earth. We will then explore the very important concept of energy and the energy cycle, with emphasis on the Earth’s energy sources and budget and, of course, the interaction with human development. Finally, we will briefly introduce a few examples of feedback and change in the Earth’s environmental systems. Your textbook reading will supplement the course notes and also cover the aspect of the evolution of the overall Earth system—the origin of the universe, the solar system and its planets and the early history of the Earth.

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Learning objectives Earth system science provides much of the necessary background that, for example, plate tectonics provided for you in your introductory geoscience course. The ultimate goal of Earth system science is to provide an enhanced understanding and description, in a clear and coherent framework, of the interconnectedness and interrelationships of geoscientific and environmental processes, events, and features that shape our world. Clearly one short chapter or even an entire course will not make us experts on any aspects of Earth system science, but the systems approach to studying the Earth and our environment will provide you will some fundamental concepts and understanding of the major systems, cycles, fluxes and reservoirs which will be important in our more detailed discussion of geohazards during the coming weeks.

With this in mind, the goals of this unit are to:

• examine the concept of systems and understand why it is a powerful tool for deciphering how the Earth works;

• identify the major forces that drive selected Earth processes and briefly introduce key feedback mechanisms that either amplify or regulate them; and

• briefly introduce several important Earth system cycles and reservoirs.

By the end of this unit you should be able to:

• define a system;

• discuss the ways in which Earth operates like a system;

• identify the major environmental systems on Earth;

• summarize the origin of the universe and solar system and, in particular, the early history of the Earth;

• discuss the major differences between Earth and other planets;

• compare and contrast the various types of systems with respect to energy and matter flux;

• define a cycle and discuss how matter and energy cycle through Earth systems;

• provide examples of feedback and change in Earth’s environmental systems; and

• identify Earth’s major energy sources.


Learning activities 1. Read chapter 2 your textbook, and answer the review questions, thought

questions, and exercises on page 60.

2. Read the study notes and answer the review questions in your course manual.

Study notes Introduction to Earth system science In unit 1 we introduced the subjects of environmental Earth science and Earth system science. We also defined several basic goals and objectives of modern environmental Earth science and briefly described Earth system science as representing a new approach or paradigm to understanding how our planet works. Your textbook reading assignment for unit 1 summarized some of the major concepts involved in science and explored several of the major issues critical in the area of environmental Earth science. We are now ready to explore in more detail the fundamental concepts of Earth system science.

When the astronauts of the Apollo missions looked down on Earth they saw, within the span of just a few minutes, a blue planet with its swirling weather systems, the vastness of the Pacific Ocean, the brown wastes of the subtropical Sahara, and the peaks of the Himalaya shrouded in cloud. The finiteness of Earth and realization that this small planet permeated with water was our home was evident in their communications back to Earth. To many people the space program has, for the first time, emphasized that Earth is a single global system; on the one hand exceedingly complex but on the other extremely vulnerable. As your introductory course has already emphasized, the Earth has had a long evolution that resulted in a Garden of Eden which is potentially perfect for humankind. For all practical purposes it is the only natural environment which can serve as our home.

Many of us, scientists and nonscientists alike, are still amazed by the fact that a natural event, such as a volcanic eruption or the opening of a seaway, occurring at one location on Earth can result in complex and far-reaching effects throughout the whole Earth. Earth system science provides the necessary global perspective on phenomena that might not seem to be related but actually are.

Perhaps we should begin with a formal definition of this new paradigm of science. Earth system science focuses on the interconnections among the various Earth systems and the changes that occur in them over time. Earth system science views the whole Earth as a single, integrated system in which matter and energy are continuously cycled through numerous subsystems, including the lithosphere, pedosphere, hydrosphere, biosphere, and atmosphere. We humans are, of course, part of the whole Earth system, depending upon it for resources but also being able to affect the whole system.

Your textbook reading discusses one example of interconnectedness that occurred at about 30 million years ago when the Drake Passage opened between

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the southern tip of South America and the Antarctic continent. Many other similarly well-studied examples could be cited. For some time scientists have suspected that the formation of the Isthmus of Panama about 4 million years ago had something to do with the onset of the Earth’s most recent (and continuing!) Ice Age because of the coincidence in timing between the two. With our enhanced understanding of global climatic change and ocean-atmosphere interrelationships, we are now reasonably confident that the collision of the two continents (i.e., North and South America) resulted in the blockage of the warm equatorial water (which previously flowed along the equator) and the diversion of this warm water northward, via the Gulf Current, to the eastern coast of North America as well as southward along the eastern coast of South America. Once this blocking of the equatorial current took place, the warm water being diverted northward resulted in increased evaporation and, as a further feedback, in increased humidity and snowfall which, in turn, lead to more ice formation in the northern hemisphere. Both of these examples (the opening of the Drake Passage and the closing of the equatorial passage) also emphasize the importance of positive feedback on the Earth’s system; once ice formation began in the polar regions of each hemisphere, positive feedback caused by the large albedo of ice relative to land resulted in reflection of more solar radiation and further atmospheric cooling.

Although humans are simply one part of the whole, we must emphasize that, as cultural animals, with an ever increasingly complex social organization and technological capacity, we are making progressively larger demands on the environment. Although our technological advancement has also made us progressively less dependent on our immediate environment, we have, nonetheless, exploited and modified (and some would contend damaged and/or destroyed) components of the system well beyond that of any other animal. Indeed, one of the major driving forces behind the paradigm shift mentioned earlier is the contention that our (human) demands during the 20th century have begun to outstrip the capacity of the environment to absorb the disturbances which result from human activities. For example, the discovery of the ozone hole, the onset of late 20th century global warming, the realization and perception of desertification and acid precipitation on a regional scale, have all heightened our environmental awareness and added impetus to the integrated study of our natural environment.

Systems Thus, the basic, underlying reason for adopting an Earth system science approach to investigating environmental hazards and problems is that we are users (or potential users) of all components of the Earth system, so we need to know how it works. This understanding must be more than a fragmented, piecemeal approach. We must strive to create a comprehensive and integrated picture.

As discussed in your textbook, there are many different types of systems and many different meanings of this word. For example, if you are reading these notes at home, your home probably has a heating “system” (or perhaps an


airconditioning “system”). Depending on what time of day or night it is, you might suspend your reading and grab a snack or meal to satisfy your digestive “system.” Or you might use the public transport “system” to travel back and forth to work or school. What does this word mean? Does it have a common meaning or thread of meaning throughout each of these examples? Why is it used to describe such dissimilar things?

First of all, system in the above examples clearly refers to a collection of ‘things’ or sets of objects. Secondly, we recognize that each of these sets of objects are organized in some way. Within each system the components are connected with readily defined links between each component and “flow” or flux occurs from one component (object) to another via the links. Implied in the use of the term system in these examples is that each system functions in some way as a whole. The public transport system is not merely a collection of buses, taxis, rails and stations, but rather the objects within the system work together (more or less!) to achieve some purpose—getting you to work and back. Finally, the movement of material within each system requires some sort of impetus or motivation. In your home heating system, the driving force is obvious - either gas or oil or electricity. In other Earth systems, the motivation may not be quite so obvious. But each system does have a driving force that makes it work. In summary, the common characteristics of systems are:

• All systems have some internal organization or structure. • All systems function in some way. • There are relationships among the objects making up the system. • The function of each system implies there is transfer of material or flux. • The function also requires the presence of some driving force.

In the context of Earth system science, the systems we are concerned with are the atmosphere (air), the lithosphere (soils, sediments, and rocks), the hydrosphere (water), and the biosphere (life). These systems are composed of mixtures of organic (both living and dead) and inorganic (abiotic, nonliving) material. Most organic material making up the bodies of plants; animals; and materials derived from these organisms, such as oil, coal, and gas, is composed mainly of compounds of carbon (C) and hydrogen (H) atoms (i.e., hydrocarbons). Inorganic compounds form most of the Earth’s land, air, and water systems and are made up mainly of oxygen (O), silicon, (Si), and aluminum (Al).

Thus, we can see that there is nothing new or startling about the concept of a system. Chemists and physicists have applied the system concept to their laboratory experimentation and research for hundreds of years (e.g., thermodynamic systems). Applying the system concept to the entire Earth (or even to the Universe) is the important new perception that has initiated the paradigm shift described in unit 1. The movement of materials or energy (flux) from one part of the Earth’s system (reservoir) to another occurs in cycles. As you already know from your introductory geosciences course, the major Earth cycles are the rock, tectonic, hydrologic, geobiochemical, and energy cycles. Throughout the rest of this course we will be returning to components within

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each of these cycles: flooding (hydrologic cycle), earthquakes (tectonic cycle), landslides (rock cycle), etc.

Clearly our description of “system” above is still rather vague or arbitrary. But this is intentional and emphasizes one more important aspect; each system must be defined. Although a system is confined to a definite place in space, it still must be defined by identification of its boundaries. The boundaries of some systems can be readily observed and defined; the boundaries of a watershed or the walls of a test tube, for example. In other systems, the boundaries may be more poorly defined or a more intangible definition must be used, the boundary of a cloud for example.

However the boundary of a system is ultimately defined, it is the boundary that separates the system from the rest of the universe (i.e., the surroundings). Within the defined boundary, the system is composed of objects or elements. As we saw above, these elements are quite diverse—buses, trains etc., for public transit systems; furnace, ducts, thermostat, etc., for your home’s heating system. Each of these objects or elements has a set of attributes. It is these attributes that are ‘measurable’—things like size, number, temperature, colour, density, viscosity, etc. By measuring or evaluating the properties of the objects, we can define the state of the system.

There are several distinct types of systems that can be distinguished on the basis of the behaviour of the elements at the system boundary. An isolated system means there is no interaction with the surroundings. In real life, the only isolated systems that exist are those we make up in the laboratory. Nonetheless, the use of isolated systems was essential in defining our basic laws of thermodynamics. A closed system is one that is closed with respect to matter flux, but energy may still be transferred between the system and its surroundings. True closed systems are rare in Earth science. However, we often find it useful to treat large systems as closed in order to simplify the complex relationships among the objects in the system. Frequently we view the Earth as a whole as a closed system because relatively little matter enters or leaves over the short term. This perception of the Earth as approximating a closed system has a number of important implications for environmental Earth science:

• The amount of matter (including resources!) in the Earth system is fixed and finite;

• When changes are made in one part of the system, the results of those changes must eventually affect other parts of the system.

• Finally, open systems are those in which both energy and matter can cross the boundary and be exchanged with the surroundings. Virtually all environmental Earth systems that makeup the overall Earth system are open.


Dynamic interactions Many of Earth’s open systems are dynamic in that the energy can be used to cause changes in the objects within the system or in the relationship between the objects. These changes result in a change in the state of the system. The way in which the change is made is referred to as a process. When the process (or processes) results, ultimately, in a return to the initial state of the system, the process is cyclic, although nothing is implied about the time involved in this cycle.

From an environmental Earth science perspective, the significance of the interconnectedness of the various Earth systems is obvious. The major point, we now realize, is that whenever human activities produce changes in one part of the Earth system, the effects of these changes are eventually felt elsewhere. The acid rain ‘controversy’ which raged during the1970s and 1980s here in North America is a good example. Although industry (and, indeed, government!) was initially reluctant to accept the findings of academic researchers, it is now abundantly clear that when sulfur dioxide is produced by burning high-S coals in a coal-fired electricity generating plant in the Midwest, it can eventually lead to deforestation, loss of fisheries, and reduction of soil and aqueous fertility in northern Ontario and Quebec. Or when pesticides are applied to cotton fields in central India, breast milk of mothers halfway around the world can be affected.

Although considerable progress has been made in understanding the complex interaction and cycling within and flux through the Earth systems, we must realize Earth system science is still in its infancy, and many important links and mechanisms still elude us. For example, we have all heard of and probably understand the basic concepts behind the El Niño phenomenon (or, more properly, the El Niño-Southern Oscillation, ENSO). This anomalously warm ocean current, which occurs every few years off the west coast of South America, is characterized by weakening of the trade winds and suppression of upwelling cold ocean currents, and results in worldwide abnormalities in weather, such as droughts in western Canada and eastern Australia, flooding in parts of Africa and western United States. These features of ENSO have been reasonably well known to scientists for many years. What is still not known is what causes or triggers an El Niño event. The interactions among the myriad of processes in the atmosphere, hydrosphere, and lithosphere are so closely interrelated that we cannot yet pinpoint the exact causal mechanism.

As we summarized above, a key feature of systems is transfer of material. This then begs the question. Why are the various systems apparently stable? For example, if material such as sediment and dissolved ions in river water is constantly being shed from the continents and put into the Earth’s oceans, then why does the salinity and composition of the oceans not change with time? Indeed, this dilemma has been pondered by many scientists for hundreds of years prior to the latter part of the 20th century.

In the early part of this century, a physicist, John Joly, suggested that since the ocean is a reservoir into which all rivers flow and that this process of transfer of material from the continents to the oceans has remained constant over

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geological time, it should be possible to estimate the age of the Earth by a simple mass balance calculation. Joly reasoned that if he knew the rate of addition of a common dissolved component, such as sodium, and the amount of this dissolved component presently in the oceans, he could calculate when the addition of the ion began. It would then make sense that this time would be when the Earth’s crust cooled to a solid state and the ocean basins formed. From the work of oceanographers at that time, Joly knew the average depths, areas, densities, and volumes of the world’s oceans. He was, thus, able to estimate the mass of the ocean water at about 1.3 x 1018 tonnes. The total salinity (TDS) of the ocean was also well known then, ~3.5%, with sodium chloride (NaCl) making up 77.8% of this total salinity. In terms of molecular weights, Na+ comprises about 39% of NaCl, so simple arithmetic (39% x 77.8% x 3.5% x 1.3 x 1018 tonnes) allowed Joly to the estimate the total amount of sodium in the oceans as 1.4 x 1016 tonnes. The volume of water and the average salinity of most of the world’s major rivers were similarly known at the time Joly did his calculations, so he was able to estimate the annual influx of Na+ to the oceans from the rivers at about 1.4 x 108 tonnes. Thus, the age of the Earth, according to Joly’s reasoning above, could be determined simply by dividing the total mass of sodium in the reservoir (1.4 x 1016 tonnes) by the annual rate of incoming sodium (1.4 x 108 tonnes): 100 million years!

Clearly we all know this is incorrect. But the question is why is it incorrect? Although the reasoning and logic Joly applied is sound, his conclusion is incorrect because he failed to consider the fact that not only is sodium being transferred from the continents to the oceans, but it is also leaving the oceans. In fact, we now (80 years after Joly’s back-of-an-envelope type of calculations) know that nearly half of the sodium entering the oceans from the continents gets returned to the continents via the atmosphere as sea spray and precipitation. The other half is lost to the oceans as sedimentary deposits. So, if half of the Na+ returns to the continents as a component of rain and/or sea spray and the other half is deposited as sediments on the ocean floor, then the concentration of sodium in the oceans must be remaining the same through time. In fact, by examining marine sedimentary deposits, geologists now realize that the concentration (i.e., salinity) of the Earth’s oceans has been in approximate chemical equilibrium since very early in the Earth’s history. When a reservoir in a system is in a state of no change over time this is referred to as steady state. Calculating the ratio of the total amount of an object or element in the reservoir to the difference between the inflow and outflow results in a ‘residence time’ value.

Residence time = total amount of an object or element in the reservoir difference between inflow and outflow results

Joly succeeded in calculating not the total age of the oceans, but rather the residence time of one object/element within the ocean. Indeed, much to Joly’s credit, modern estimates of the residence time of Na+ in the oceans are very close to his value.


Cycles In your introductory geoscience course, you discovered several ways in which Earth is different from other planets of the Solar system. For example, among the terrestrial planets, only Earth, with its unique atmospheric and surface conditions, has abundant liquid water. An important implication of this atmosphere/hydrosphere/surface configuration is that the various components of these Earth systems are not static but are in a constant state of flux. Elements, compounds, mass, and energy are cycled and recycled through the atmosphere, hydrosphere, biosphere and lithosphere. In the remaining parts of this unit we will introduce several of these cycles - carbon, sulfur, and energy.

Carbon cycle Much of our deliberation and discussion during the rest of the course will center on specific interactions within the overall Earth system; however, we must be cautious not to forget the interrelatedness of these processes and cycles. Carbon, for example, is an element common to all of the Earth’s systems. The magnitude of carbon in the major reservoirs and their fluxes is summarized in Table 2.1. It is found in abundance in the biosphere where it is the fundamental building block in all molecules that make up living matter. It makes up about 50% of all living material and, in the form of carbon dioxide, it is necessary for plants to grow. It is even more abundant in the lithosphere, comprising a major part of coal, oil, gas, and carbonate rocks; in the atmosphere it is part of one of the essential gases that help to keep the planet warm enough for life to continue; it occurs both as inorganic and organic gases and particulate matter in the hydrosphere. It constantly cycles from one reservoir to another. The chief constituents of the carbon cycle are methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and organic matter (CH2O). The Earth’s natural production of methane is about 145 x 1012 moles/yr mainly by bacterial decomposition of organic material under reducing conditions. The reaction that forms this methane is: 2CH2O = CH4 + CO2. Although our knowledge of methane cycling is still poor, it appears that most of the CH4 comes from swamps, stagnant lakes, and flooded cultivated areas of the land. After the methane enters the atmosphere it is readily oxidized by OH- to carbon monoxide. There is also production of CO from degradation of leaves and other decaying chlorophyll plant matter. The production of CO in this manner is about 20 x 1012 moles/yr and is approximately the same as CO released into the atmosphere from fossil fuel burning. These two sources of carbon monoxide show a strong seasonality; CO from human production is maximum in the winter season versus late summer and fall for the organic matter decomposition source. Although CO has a relatively short residence time in the atmosphere, it is a serious local problem in highly urbanized areas. For example, during the 1980s Los Angeles routinely produced nearly 10 million kg of CO per day.

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Table 2.1 The Earth’s carbon cycle

Reservoir Concentration or annual flux

C in the atmosphere reservoir 55 x 1015 molesC in the lithosphere reservoir 100 x 1015 molesC in the biosphere 41 x 1015 molesC in the hydrosphere 326 x 1015 moles C flux: Atmosphere-biosphere 149 x 1012 molesC flux: Lithosphere-atmosphere 407 x 1012 molesC flux: Atmosphere-hydrosphere 4 x 1012 molesC flux: Lithosphere-hydrosphere 17 x 1012 molesC flux: Biosphere-hydrosphere 4 x 1012 moles

Ultimately, both the CH4 and CO that go into the atmosphere end up as CO2. The CO2 is photosynthesized into organic matter (CH2O) in plants which provide the organic material at the land surface that, in turn, gives rise to more CO and CH4. Clearly, the atmosphere is quite delicately balanced with respect to carbon dioxide; any significant change in the ratio of photosynthesis to respiration and organic matter decay could change CO2 levels several-fold in a short period of time. However, it is worth emphasizing that the total CO added to the atmosphere through the chain CH4-CO-CO2 and CO-CO2 from natural sources dwarfs that from fossil fuel combustion. In other words, we must be aware of the relative weighting of the influence of the various mechanisms on the C budget. If, for example, respiration and organic matter decay were somehow doubled, and photosynthesis remained constant, the carbon dioxide in the atmosphere would double in about a decade. Conversely, if organic matter production were doubled and decay/respiration were constant, the CO2 content of the atmosphere would be cut in half within a decade. Indeed, the contribution of photosynthesis and decay/ respiration to the carbon cycle is so much greater that that of natural weathering sources that it is unlikely atmosphere CO2 is significantly related to weathering processes.

Sulfur cycle Like the carbon cycle, the Earth’s sulfur cycle is closely related to geobiochemical processes. The S cycle involves:

• three major atmospheric compounds: (a) reduced gaseous forms of sulfur such as dimethlysulfide (DMS) and hydrogen sulfide (H2S), (b) sulfur dioxide (SO2) gas, and (c) sulfate (SO4) aerosol;

• aqueous sulfate; and


• three major compounds in the lithosphere: (a) reduced S minerals such as pyrite, (b) gypsum (CaSO4 2H2O) and anhydrite (CaSO4), and (c) organic matter.

A major site of interaction is at the lithosphere-atmosphere-hydrosphere boundary. Weathering of sedimentary rocks containing gypsum, anhydrite, and pyrite yields substantial amounts of sulfur. On a global scale the ratio of reduced S sources to oxidized sources is about 2:3. The gypsum and anhydrite weather by simple dissolution and the ions enter the hydrosphere as Ca2+ and SO4

2-. Pyrite and other reduced S mineral species consume oxygen to oxidize according to: 4FeS2 + 15O2 + 8H2O = 2Fe2O3 + 8H2SO4. The resultant sulfuric acid (H2SO4) reacts with other carbonate minerals and components within the soils and bedrock to generate carbon dioxide: CaCO3 + H2SO4 = Ca2+ + SO4

2- + CO2 + H2O. The generated carbon dioxide, in turn, would react with still more carbonate mineral material to yield more calcium and bicarbonate in solution: CaCO3 + CO2 + H2O = Ca2+ + HCO3 -. The point is that nearly all the S from the weathering of the lithosphere is ultimately released into the hydrosphere as dissolved sulfate. In contrast, the burning of fossil fuels results in a major addition of either H2S or SO2 to the atmosphere. Coal averages about 2% sulfur, oil about 0.3%, so it is easy to see how the global sulfur cycle can be dramatically perturbed by ever increasing fossil fuel use.

The Earth’s energy relationships (The energy cycle) One of the advantages of assuming a closed system Earth “model” is that we can endeavour to better understand the complex energy relationships of this closed system. We must always remember, however, that modeling the Earth as a closed system is a significant simplification when examining the system on a geological time basis. Meteorites, for example, have played a pivotal role in helping to significantly change the state of this “closed system.”

Energy crosses the Earth system boundary in several forms and the energy cycle is complex. The most significant input is that of solar radiant energy, but the Earth system also receives electromagnetic energy from other bodies in space and experiences gravitational energy associated with the interaction of the mass of the Earth and these other bodies. Only a tiny fraction—about 0.002%—of the total radiation emitted by the Sun forms the input to the Earth system, but this small fraction is sufficient to supply all the energy needed to drive the Earth’s external processes. The average amount of radiation received per unit area per unit time at the boundary of the Earth system (i.e., the edge of Earth’s atmosphere) is known as the solar constant and is 1370 W/m2 (watts per square metre of surface). Integrating this over the surface of the Earth gives a value of incoming solar energy of about 54,000 x 1020 J (joules). Table 2.2 lists other Earth system energy sources and reservoirs. The complete solar radiation spectrum consists of about 7% ultraviolet, 50% visible, and 43% infrared radiation.

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Table 2.2 Earth’s energy

Source or Reservoir Energy (1020 J)

Annual solar input 54,385.000

Total coal reserves 1,952.000

Total conventional oil reserves 179.000

Total conventional gas reserves 134.000

Annual North American energy consumption 1.000

Annual geothermal energy flux 0.030

Total energy released by 1976 T’ang Shan, China earthquake (240,000 deaths)

5,000.000

Total energy released by 1991 eruption of Mount Pinatubo

0.002

The output of energy from the Earth system is also in the form of electromagnetic radiation, but, in contrast to the mainly short wave incoming radiation, the emission radiant energy is characteristically longwave.

Within the Earth system there are important energy fluxes between the major elements (atmosphere, Earth surface, Earth interior). During its passage through space solar radiation loses very little energy. However, upon entering the atmosphere the radiation encounters gases, liquids, and ultimately solids, all of which are able to both absorb and reflect radiation. The various gases of the atmosphere absorb selective parts of the incoming radiation. For example, carbon dioxide (CO2) and water vapor (H2O) absorb mainly infrared wavelengths, whereas ozone (O3) absorbs ultraviolet radiation. In total, over 50% of the incoming solar radiation is lost by atmospheric absorption (17%), reflection (23%), and scattering (6%). Of the remaining 47% making its way to the surface of the Earth, another 7% is lost due to reflection. However, the Earth’s surface also receives some energy from the Earth’s interior as geothermal heat flow and some of this is output to the atmosphere as longwave radiation. Table 2.3 indicates the relative magnitude of these fluxes averaged over the surface of the Earth per year.


Table 2.3 Earth’s energy fluxes

Energy (joules per year)

Solar energy 5.6 x 1024

Geothermal energy as radiation 1.3 x 1021

Geothermal energy as volcanic emissions 5.3 x 1018

Earthquake energy 1.0 x 1018

Summary The global interconnectedness of air, water, rocks and life has now, at the dawn of the 21st century become the focus of much of modern environmental Earth science. Earth system science is the science that studies the whole Earth as a system of many interacting parts and focuses on the changes within and among those parts. A convenient way to think about the Earth as a system of interdependent parts is to consider it as four major reservoirs of material with flow of matter and energy among them.

A system is a portion of the universe that can be isolated from the rest for the purpose of observing and understanding changes. There are a number of different types of systems, but the Earth is generally thought to be a good approximation of a closed system in that it has boundaries that permit the exchange of energy but not matter. The Earth receives solar energy and re-radiates much of it back to space, but we receive and lose very little matter over the short-term. As humans existing in a closed system, we must realize the important implications:

• the amount of matter (resources) is fixed and finite; and

• when changes are made in one part of the closed system, the results of those changes will very likely affect other parts of the system.

Material and energy move from one part of the Earth system to another or within various major reservoirs or compartments. The four major reservoirs are:

• the atmosphere: a mixture of gases composed mainly of nitrogen, oxygen, argon, carbon dioxide, and water vapor;

• the hydrosphere: the totality of the Earth’s water, including oceans, lakes, streams, groundwater, snow, and ice;

• the biosphere: all of the Earth’s organisms as well as any undecomposed organic matter; and

• the geosphere: the solid Earth, composed mainly of rock (any naturally-formed, nonliving, firm, coherent, aggregate mass of solid matter) and

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regolith (the blanket of loose, uncemented rock particles that covers the solid Earth).

Movement within and from one of Earth’s reservoir to another is often cyclic. The major Earth cycles are the energy, hydrologic, rock, tectonic, and geobiochemical cycles. We have introduced several of these cycles in this section, and you are already familiar with several others from your introductory geoscience course.

The fundamental energy cycle encompasses the internal and external energy sources that drive the Earth system and all its other cycles. The major sources of input of energy are solar radiation, geothermal, and gravitational. The major losses of energy from the cycle are reflection and re-radiation.


carbon cycle carbon dioxide closed system boundary Drake Passage Earth system science energy cycle feedback flux geobiochemical cycle geothermal energy

isolated system Isthmus of Panama interaction open system reservoir residence time steady state solar energy sulfur cycle system


1. What are the most important Earth cycles relative to environmental Earth science?

2. What are the three most important sources of input into the energy cycle?

3. Why was the collision of North and South American continents about 4 million years ago important to us as environmental geoscientists?

4. The estimate for total annual photosynthesis is about 5,000 x 1012 moles of organic matter per year. For the reaction CO2 + H2O = H2O + O2, about 110 kcal of solar energy are required per mole of CH2O formed. What is the total annual solar energy used in photosynthesis?

5. Why is the natural flux of methane to the atmosphere so much greater from the land than from the ocean?


Unit 3 Techniques, Data, and Investigative Procedures

Topics Topographic maps

Geological maps

Soils maps

Hydrological and hydrogeological data

Remote sensing

Introduction This section describes the most important and most common types of data used by environmental geoscientists, outlines how this information is acquired, and provides the foundation for the application of these data and techniques in helping to anticipate environmental impacts or mitigate the effects of hazards discussed in later sections.

Learning objectives The acquisition and interpretation of data are fundamental components of an environmental geoscientist’s daily work. It is this quantitative geological data and qualitative environmental information that allows Earth scientists to predict the kind, degree, and location of potential hazards and permits the planners and decisions makers to undertake appropriate actions to mitigate the effects of the hazard. However, the mounting losses of mineral, water, and agricultural resources, as well as human life, suggest that this wealth of Earth science information is being neglected. The widespread failure of decision makers to make use of critical Earth science information and technology is due mainly to a lack of awareness. Thus, public communication of the environmental data and the Earth scientist’s interpretations, in a form directly usable by planners, architects, developers, legislators, and government officials, is of utmost importance.

By the end of this unit you should be able to: • summarize each major group of sources of environmental geoscience

information;

• rank the basic types of geoscientific data in terms of ease of access/availability and cost of acquisition;

• differentiate the various types of maps used in environmental geoscience, and illustrate the kinds of data that can be acquired or interpreted from each;

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• describe the source(s) of hydrological data in North America;

• discuss the nature and characteristics of EMR;

• describe the main types of remote sensing equipment and methods;

• evaluate the usefulness of orbital versus suborbital methods of remote sensing for various environmental geoscience problems;

• give examples of the use of orbital remote sensing in environmental geoscience;

• trace the developmental history of ERTS-Landsat satellites and technology;

• outline seismic refraction;

• construct a seismic time-depth plot; and

• calculate the depth to bedrock using seismic data.

Learning activities 1. Read the study notes and answer the review questions in your course manual.

2. Complete Assignment 1 and submit the completed assignment to the Distance Education Program Office.

Study notes Introduction One of the most basic concepts in the entire realm of Earth science is that we are always dealing with incomplete data. Thus, the environmental geoscientist is constantly faced with the problem of having to decide between: • Is the existing data base and level of knowledge sufficient to solve the

environmental problem at hand?

• Should more data be collected in order to “properly” understand the problem or hazard?

Unfortunately, it is often necessary to make this decision early in the evaluation of the problem; furthermore the decision is usually dictated almost entirely by economics rather than scientific necessity. Because of this overriding economic concern, it is essential in all cases that the environmental geoscientist exploit every possible source of pre-existing data. These sources consist mainly of the following: • government prepared and issued maps and reports; • hydrological and hydrogeological data; and • remote sensing data.

In nearly all areas of North America these sources of data can provide an abundance of environmental information. If, on the basis of examination and interpretation of these data, it is necessary to acquire more detailed site-specific


information, the usual techniques include geophysical surveys, geological drilling, and geological sampling programs. In addition to providing different types of data, these various sources of environmental information vary in terms of cost of acquisition. Maps are the least expensive source of information followed by: hydrological data, remote sensing, geophysics, and finally geological drilling and sampling.

Topographic maps As you probably learned in your introductory course in Earth science, a topographic map is a graphical representation of the Earth’s surface plotted to a given scale. The scales of topographic maps vary greatly; in most cases the environmental geoscientist is interested in relatively small areas of the Earth’s surface, so 1:20,000 and 1:50,000 scale maps are frequently used. More regional problems might call for smaller scale maps: 1:250,000 or even 1:1,000,000 maps. Similarly, the contour interval of topographic maps varies depending on the scale of the map and the amount of relief in the map area. Most of the 1:50,000 maps of western Canada have 25-foot contour intervals. In areas of very low topographic relief, such as in the Lake Agassiz basin of southern Manitoba, a 25-foot contour interval map would have very few contour lines on it. Thus, much smaller intervals are sometimes used. Around Winnipeg, for example, topographic maps with a 2-foot contour interval have been prepared! In contrast, in the Rocky Mountains of Alberta and British Columbia, 100 foot or 200 foot contour intervals are most common.

The contoured topographic map is the fundamental starting point or base for every field oriented Earth science investigation, whether the ultimate purpose of the work is to find uranium or to map the contamination plume from a landfill. This is because the maps can provide the geoscientist not only with the identification and location of streams, lakes, and landforms, but also can give preliminary ideas about the type of surface deposits and the geologic character and orientation (dip and strike) of the bedrock of the mapped area.

Geological maps A geological map shows the lithology and distribution of rock units that crop out at (or near) the surface of the Earth and provides an interpretation of the structural relationships of these units.

There are two basic types of geological maps: • bedrock geological maps that show the bedrock surface; and • surficial geological maps that illustrate the unconsolidated surficial

materials.

The purposes of both of these types of maps are to: • illustrate structural relationships; • provide interpretations of the origin and history of the units; • identify the main properties and lithologic character of the rocks; and • show the spatial relationships of the units.

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The most important feature of geological maps, and the key element that sets these maps apart from topographic and soils maps, is that the geological maps provide an interpretation.

Of the two types of geological maps, the surficial maps are most commonly in demand by the environmental geoscientist. This is because they provide detailed information about floodplain areas, landslide areas, surficial resources (such as sand and gravel), water resources and water hazards, depth to bedrock, the nature of the sediments, and any construction or habitation barriers.

A separate type of geological map is a geotechnical map. Geotechnical maps combine either bedrock geology or surficial geology with key engineering data. They are usually done on a very local scale, covering only small geographic areas. The geotechnical maps provide the environmental geoscientist with detailed information and analytical data concerning: • potential and past hazards in the area (e.g., landslides, floods, swelling soils); • resources (e.g., gravel, sand, sources of minerals); • hydrogeology; and • land use.

Soils maps In much of North America the most detailed maps available to the environmental geoscientist are those depicting the nature of the soils of the area. Because the type of soil that develops in an area is closely related to the type of parent material in which the soil is developed (as well as other factors such as the climate, length of time for soil development, relief, etc.), very important geologic information can be derived from soils maps. Often these maps include quantitative information on the texture, porosity, permeability, and erodibility of the surficial material, the thickness of soil, the depth to bedrock, the depth to the water table, the drainage conditions of the area, and the suitability for various types of agricultural, urban, and recreation activities. Many times detailed soils maps are superimposed on an aerial photograph of the area in addition to a topographic contour map.

Hydrological and hydrogeologic data In United States and Canada, this type of data is collected mainly by federal and provincial/state government agencies. In United States, the data are collected and published as U.S.G.S. (United States Geological Survey) Water Supply Papers; Environment Canada is responsible for documentation of the hydrological information in Canada. The type of data collected, the density of the sample points, and how often the sampling stations are examined vary considerably from region to region. In Canada, there are about 3,000 permanent streams that are monitored; this represents less than 0.1% of the total number of streams. In United States, about 0.5% of the streams are instrumented. The main types of data collected include: annual flow character and temporal discharge variations, flood discharge, drainage basin area, sediment load, and water quality.


In Canada, most of the provinces also have agencies that are responsible for monitoring the quality, movement, quantity, and use of subsurface waters. Because of the costs involved in drilling groundwater wells and installing instruments to monitor the long-term hydrodynamics of the flows, this type of coverage is much more sporadic than surface water hydrological data.

Remote sensing Remote sensing is a very diverse field in terms of technology and application. Most simply defined, remote sensing is the acquisition of information by making measurements at a distance without any physical contact. There are four basic types of data that are measured: • electromagnetic radiation (EMR); • gravity force field; • magnetic force field; and • mechanical vibrations (seismic, acoustic waves).

Although each of these types of data is useful to the environmental geoscientist, our main concern in this section will be with EMR. Some of the other types of data will be discussed later in the course.

Character of EMR Electromagnetic radiation is energy in the form of harmonic waves that is produced by motions of atoms and molecules. All substances produce EMR and do so in many different forms including: heat, light, radio, and radar. It is only when this energy interacts with matter that it can be detected. In order to be able to discuss and study these various forms of EMR, scientists have arbitrarily subdivided the radiation into a series of fields on the basis of the wavelength and frequency of the motion of the wave. There are two very important points to remember about EMR: first is that this subdivision of EMR is artificial; in fact, there is a continuum—a continuous spectrum—of frequencies and wavelengths. The second point is that the speed of the wave of EMR is constant: about 3 x 108 metres per second. Thus, the only properties that vary in EMR are the frequency (i.e., the number of cycles per second) and the wavelength (the distance from one peak to the next or from one trough to the next). Frequencies are usually expressed in terms of hertz (Table 3.1). Wavelengths are evaluated using distance (Table 3.2). These parameters of EMR are related according to:

Speed = frequency ∗ wavelength

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Table 3.1 EMR units of frequency

Unit Cycles/sec

Hertz (Hz) 1 Kilohertz (kHz) 103 Megahertz (MHz) 106 Gigahertz (GHz) 109

Table 3.2 EMR units of wavelength

Unit Equivalent metres

Kilometre (km) 103 Metre (m) 100 Centimetre (cm) 10-2 Millimetre (mm) 10-3 Micrometre (µm) 10-6 Nanometre (ηm) 10-9

The artificial division of EMR into regions is, in part, dictated by our technical ability to measure or detect the frequency and/or wavelength of the radiation. This artificial subdivision is shown in Table 3.3. All remote sensing devices are designed to operate in only a specified region of the spectrum. For example, a photographic sensor (such as a conventional camera) attached to an airplane is designed to record the EMR in the restricted region of 0.4 to 0.7 µm wavelength. The same airplane could also be equipped with a separate device to detect and record ultraviolet radiation (i.e., 0.004 to 0.4 µm), or infrared radiation (i.e., 0.7 to 3.5 µm), depending on the objective of the survey and the nature of the environmental problem. Thus, it is important that the environmental geoscientist is aware of the uses and limitations of each of the various EMR spectral regions.


Table 3.3 EMR spectral regions

Region Wavelength Wavelength

Radio > 80 cm Electromagnetic pulse detectors, scanners, antennas

Microwave

Infrared (IR)

1 mm - 80 cm 0.7 µm - 1 mm

Radar Cameras with IR film, optical- mechanical scanners

Visible

Ultraviolet (UV) 0.4 µm - 0.7 µm

0.03 µm - 0.4 µm

Normal film in cameras

Scanners with filtered photomultipliers, cameras with UV film

X-ray

Gamma ray 0.03 ηm - 0.03 µm

< 0.03 ηm

(rarely used in remote sensing)

(rarely used in remote sensing)

The frequency and intensity of electromagnetic radiation depend on the temperature, composition, and physical state of the emitting body. The higher the temperature of the radiating body, the shorter the wavelength of the EMR. For example, the sun, with a very high temperature, emits radiation with a peak at about 0.5µm. The Earth, a much cooler body in contrast to the sun, emits radiation with about 9.7 µm wavelength. This shift in wavelength also explains the so-called “greenhouse” effect; the short wavelength solar radiation is able to penetrate the atmosphere, clouds, etc., but the radiant energy being emitted by the relatively cool Earth, with a much longer wavelength, cannot penetrate the atmosphere and is reflected back to the Earth’s surface, thereby warming the Earth. Thus, the total amount of EMR striking an object (E) is equal to the sum of the reflected energy (R) plus the absorbed energy (A) plus the transmitted energy (T):

E = R + A + T

The ratio of reflected energy to total energy is termed reflectance; the ratio of absorbed energy to total energy is absorbance; and the ratio of transmitted energy to total energy is transmittance.

Uses of EMR in environmental geoscience Visible spectrum The uses of remote sensing are extremely varied. Within the realm of environmental geoscience, however, conventional (visible spectrum: 0.4 - 0.7 µm wavelength) aerial photography is by far the most widely used (and least expensive) remote sensing tool. Indeed, aerial photographs are now almost as

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widely used in geoscientific field work as topographic maps. Their popularity stems from the following considerations: • excellent spatial resolution and high “usable” information content; • low cost; and • variety of standard films including: UV, UV-visible, visible, and IR.

Of course, the photography must be done during daylight (by convention, most surveys are done between 10:00 and 14:00) and only when the weather is clear with little or no cloud cover. The standard black and white airphoto is excellent for examining the texture and composition of surficial sediments, and for establishing the large scale configurations or landforms that these sediments occupy at the surface of the Earth. Thus, having these data, the environmental geoscientist is able to interpret the character and response of the surface of the region to given impacts such as changes in drainage pattern, vegetation cover, hydrology, etc.

Most aerial photographs cover relatively small areas and are typically acquired at scales of 1:50,000 or larger. Most of western Canada has airphoto coverage of 1:20,000 scale. These many single photographs are frequently put together to form a photomosaic that covers a much more extensive area. Another commonly used technique of airphoto interpretation is to view the overlap area of two adjacent photos with a device called a stereoscope. The two photos form a “stereo pair,” with the overlapping area producing a three-dimensional image.

Infrared spectrum Infrared EMR is also frequently used by environmental geoscientists. Because IR has longer wavelengths than visible light, IR is not scattered or absorbed by the atmosphere as much as visible or UV radiation. The net result is that the resolution of an IR image is much better than visible light photographs. The first environmental use of IR was by the military to detect camouflage. Living vegetation strongly reflects IR radiation. However, camouflage made up of cut (i.e., dead) vegetation or cloth reflects much less intensely. More recently IR has been mainly applied to detection of plant growth, foliage conditions, and biomass studies in land use analysis. Environmental geoscientists are also using infrared radiation to monitor volcanic hazards, geothermal “hot spots,” and ice thicknesses.

Microwave spectrum Microwave radiation detectors and sensors are becoming more widely available. The single most important feature of using microwave EMR in remote sensing is that the very long wavelengths allow the radiation to penetrate clouds, dense vegetation, and even soils and surficial sediment. Radar (an acronym for radio detection and ranging) has been used to map old fluvial drainage channels beneath as much as 60m of sand in the Sahara Desert. Obviously, microwave EMR has tremendous potential in environmental monitoring because it can be used at night, it can acquire images through heavy cloud cover, and it can penetrate surficial materials.


Orbiting remote sensing satellites Beginning with the launch of ERTS (Earth Resources Technology Satellite) in 1972, unmanned orbiting remote sensing satellites have been extensively used by environmental geoscientists. Although technologies have evolved during the last two decades, the image formats acquired by the various generations of Landsat systems are completely compatible, giving rise to nearly twenty years of remote sensing data from the entire Earth collected approximately every six days. There have been five Landsats satellites: from 1972 to 1984 Landsats 1, 2, and 3 collected data from an altitude of 918km. A second generation of satellites, Landsats 4 and 5, orbit at about 700km altitude. In 1978, NASA launched an orbiting radar satellite system, Seasat, and in 1981 and 1984 conducted radar survey experiments using remote sensing equipment on the manned space shuttle missions (SIR-A and B). Spatial resolution of these radar imaging devices ranged from 25m2 to about 40m2, with image swath widths of 40 to 100km.

The imaging systems on the various generations of Landsats enable the satellites to detect EMR in four spectral bands: green (0.5-0.6 µm), red (0.6-0.7 µm), low IR (0.7-0.8 µm), and near IR (0.8-1.1 µm). The system scans a swath 185km wide normal to the orbital path. Although the imagery is continuous, the image strips are subdivided into scenes, with each scene covering a 185km by 185km area. This results in maximum resolution on the ground of about 50m2 (i.e., the largest thing that can be seen is 50m2). Although satellite imagery has been shown to be extremely valuable in resource evaluation, land use inventory, and natural hazard detection and monitoring, it should be pointed out that it is intended only to support, not replace, conventional suborbital devices. In fact, there are many instances in which Landsat type of imagery is not the most suitable type of remote sensing for the environmental geoscientist. While orbital detection devices offer a very cheap means of monitoring large geographic areas, the geoscientist often does not want to sacrifice the resolution for this added coverage.

In summary, the important features of EMR that make its use so attractive in environmental geoscience are:

• The “multi-concept”: multi-date (environmental geoscientists are able to monitor environmental impact with time); multi-band (a single remote sensing device, such as an airplane or satellite, is able to carry equipment that will detect several spectral bands, each having a different use); and multi-scale (EMR lends itself well to large areas, such as that covered by orbiting detectors, as well as very small areas, such as airphotos).

• The technology is now cost effective, particularly when applied to large areas that have to be monitored over an extended period of time, such as floodplains, volcanoes, or earthquake-prone regions.

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Summary of land use/land cover study techniques Topographic maps offer the least expensive means available to the environmental geoscientist to obtain information about the surface of the land, the form, and, in some cases, the genesis of the landscape. But most topographic maps offer little information on how the land is actually used. Thus, other types of maps must be added to the geoscientist’s arsenal; geologic maps, soils maps, airphotos and airphoto mosaics, and EMR remote sensing all offer different types of data and allow more complete interpretation. Beyond this, however, the practicing environmental geoscientist must often resort to a program of actual drilling and sampling in order to fully understand the potential impact of an environmental problem or hazard.

Other geophysical methods and tools The application of geophysical techniques by an environmental geoscientist is done only after all the less expensive sources of data are exhausted. In addition, because of the costs involved, geophysical surveys are carried out only on a local basis. Although all the main methods of geophysical exploration (i.e., seismic, gravity, electrical conductivity, and magnetics) are applicable to environmental problems, we will discuss here only the seismic method.

Seismic techniques are most widely used among geoscientists investigating shallow subsurface environmental problems. Determining the depth of overburden or the depth to a specific rock unit is frequently necessary. Basically, the seismic procedure involves applying a sudden pressure to the surface of the Earth and very accurately measuring the amount of time it takes for the seismic energy to travel to a series of detectors. This original pressure or disturbance most commonly is in the form of a small explosion, but in some situations the simple impact of a sledge hammer against a metal plate will be a sufficient source of energy. The sudden pressure causes a distortion of the particles of matter making up the Earth. The shapes of the particles are momentarily changed as the energy is felt by each particle. When the disturbing force is removed or has passed, the force of the interaction among the particles tends to restore the displaced particles to their original position. However, the inertia of the particles causes them to overshoot their rest positions, and, as a result, the rebounding forces cause each particle to oscillate about its rest position. This vibratory motion is transferred to nearby particles. This results in the seismic disturbance being propagated through the Earth as a wave, with the wave moving outward from the point of impact in much the same way a pebble tossed into a quiet pool of water causes waves on the surface of the pool. The velocity of the wave is controlled by the density and the elastic properties of the Earth material the wave is moving through according to:

( )density

1.333µKVelocity +=

where K is a constant for the Earth material that is related to the material’s ability to resist change in volume, and µ is the resistance of the Earth material


to a change in shape. These two constants are usually known or easily acquired in the laboratory. The bulk density of the Earth material is also easily determined or estimated. Thus, by having some knowledge of the type of material the wave is passing through, we can calculate an interval transit time, or how long the wave will take to travel a certain fixed distance. This calculation of the velocity of the wave can be checked by setting off the explosion and timing how long it takes the wave to travel across the surface of the Earth to some point away from the site of the explosion. In practice, the time of arrival of the wave at a series of points is determined, with each point being farther away from the explosion. Geophones are used to detect the minute movement of the Earth as the wave passes that point.

Keep in mind that the wave is three-dimensional; it is not only travelling laterally across the surface of the Earth to reach the geophones, it is also spreading out and downward in the form of an equidimensional arc. This arc will continue to travel outward at the given velocity until it hits material having a different density or different elastic properties. At this point, the velocity of the wave will change. In addition, however, the interface between the two materials of differing properties will cause the wave to refract and reflect. The refracted wave will then travel upward toward the surface at a velocity controlled by the upper Earth material. The geophone will, of course, also pick up the movement of the Earth caused by this reflected wave.

Shot Point Geophones SP A B C D E F

Figure 3.1 Geophones

Overburden Velocity = 4 Ft/sec

Bedrock Velocity = 5 Ft/sec

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Most often lower velocity material is overlying higher velocity material as shown in Figure 3.1. In this situation, let the upper material (low velocity V1) be unconsolidated clays and silts, whereas the underlying, higher velocity material (velocity V2) is well consolidated, dense bedrock. We set off our seismic explosion and measure the arrival times of the wave(s) at each of the geophones. The disturbance from the explosion will, of course, be recorded first at geophone A after a time controlled by the distance to the geophone and the velocity of the wave in the material (i.e., V1). Then the wave will be detected at geophone B, then C, and so on. As the rest of the wave continues to travel downward, it reaches the interface between unconsolidated material and bedrock. Some of the wave is refracted to continue downward into the bedrock; some of the wave will be reflected back up toward the surface, travelling at V1. Finally, some of the wave also travels laterally along the interface at a velocity of V2. While travelling laterally, there is continuous reflection (upward toward the surface).

Because V2 is greater than V1, there will be some point at which the reflected seismic wave will actually reach the surface geophone before the slower surface wave travelling directly from the explosion site. This is a critical observation to make because at all the geophones further away from this point the velocity of the first arrival will be controlled by a combination of V1 and V2, rather than just V1. By plotting the time of arrival of the first wave to affect each of the geophones on the vertical axis of a graph and the distance between each geophone on the horizontal axis, it is possible to identify this critical point by the change in the slope of the line connecting the points. Once this point, termed critical distance, is determined it is a simple matter to calculate the vertical depth to the interface (i.e., in this case the depth to bedrock) using:

5.0

12

12

VVVV

2distance criticalDepth ⎟⎟

⎠

⎞⎜⎜⎝

⎛+−

∗=


absorbance bedrock geological map contour interval critical distance emr erts frequency geologic map geotechnical map IR spectrum landsat microwave spectrum

multi-concept radar reflectance seismic reflection seismic refraction stereo pair surficial geologic map topographic map transmittance uv spectrum visible spectrum wavelength



1. Give one example of the use of a topographic map in environmental geoscience; be specific with respect to the problem being solved or the task being undertaken.

2. Rank the following in terms of cost for the environmental geoscientist (use 1 to indicate lowest cost, 5 to indicate highest cost):

Hydrological data Geophysical survey Topographic map Geological drilling Remote sensing

3. What is the relationship between electromagnetic radiation frequency, velocity, and wavelength?

4. List three factors that influence the frequency and intensity of EMR.

5. Why is UV EMR rarely used in environmental geoscience studies?

6. Describe one use for IR EMR in environmental geoscience studies.

7. What is the main advantage of the use of microwave EMR in environmental geoscience?

8. Describe the “multi” concept with respect to the use of EMR in environmental geoscience.

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Notes


Unit 4 The Earth’s Cycles I: The Lithosphere

Topics Introduction to the lithosphere

Geological framework of time

The crustal system

Chemistry and mineralogy of the lithosphere

The major rock types

The major components of the lithosphere system

Transfer of matter and energy

Summary

Introduction In this unit we will delve into some of the mechanisms and Earth system features that ultimately provide nearly all of the major relief on the surface of the Earth and all of our material resources—the lithosphere. Indeed, it is often easy to forget that virtually all of the topography we see at the Earth’s surface, all of the bathymetry of the oceans, the major configurations and morphology of the continents’ shorelines, and all the climatic conditions we experience are the result of the interaction of primary plate tectonic mechanisms. The major Earth features listed above are all produced or controlled by the tectonic processes acting largely in response to slight changes or differences in the lithosphere that are related to density or thickness of the crust.

Learning objectives The Earth has four major reservoirs: the lithosphere, atmosphere, hydrosphere, and ecosphere. The goal of this section is to understand the basics of the first of these—the lithosphere. We will investigate the concentric zones of the Earth and concentrate on the major processes by which the outmost thin skin of the Earth—the lithosphere—is formed and what factors control the movement of the material in the lithosphere.

After this section you should be able to: • summarize the importance of the lithosphere relative to environmental Earth

science and Earth surface science;

• explain the implications of a differentiated Earth structure;

• identify how geologists have created and calibrated the geological time scale;

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• summarize the various components of the Earth’s crustal system;

• explain how the crust of the Earth spreads;

• define the major processes affecting the lithosphere;

• discuss the abundance of the minerals and elements of the lithosphere;

• compare and contrast the major families of rocks;

• summarize the distribution of minerals and rock types at the surface of the Earth;

• define the major crustal types;

• provide examples of various plate margin types;

• discuss the environmental implications of the various plate margins; and

• describe the movement of material at the various plate margins.

Learning activities 1. Read chapter 3 in your textbook, and answer the review questions, thought

questions, and exercises on page 86-87.

2. Read chapter 4 in your textbook and answer the review questions, thought questions, and exercises on page 120-121.


Study notes Introduction to the lithosphere The surface of the Earth is constantly changing. As we learned in unit 2, material and energy are constantly being transferred from one place to another as well as being exchanged among the various reservoirs of the Earth. At the largest scale, the driving mechanism for these changes is, the Earth’s planetary energy. The Earth moves around the Sun gravitational forces causes the Earth to rotate on its axis. Gravitational interaction with the Moon and Sun also affects the water in the oceans and large lakes. Solar radiation drives atmospheric circulation, controls the Earth’s water budget, and provides the energy for ecosphere processes. These energy sources are all external.

The Earth’s surface is also significantly affected by processes from within the Earth. Heat generated in the interior is mainly from radioactive decay. This energy drives a deep convection system within the Earth. The thermal convection, in turn, is ultimately responsible for the large-scale topography of the Earth by controlling the movement of relatively cool and rigid lithospheric plates at the Earth’s surface. These plates collide and override each other to form ocean basins and deep ocean trenches, mid-ocean ridges, continental rifts, volcanic island arcs, continental margins, and mountains and plateaus.


On a smaller scale, the topography at sites of crustal extension (rifting) can be explained by thinning of the lithosphere or by the interaction of the base of the lithosphere with a hot plume of mantle material. Another important aspect for controlling topography in areas of crustal extension is subsidence coincidence with or following the rifting and the cooling of newly formed lithosphere as it moves away from the hotspot or mid-ocean ridges. In areas where the crust is being consumed or subducted, a wide variety of other mechanisms interact to control Earth surface features including volcanos, earthquakes, and mountain building. Thus, it is the internal energy of the Earth and the plate tectonics that are primarily responsible for the Earth’s topography at various scales.

The processes operating at the surface of the Earth can be divided into those that are due to processes originating externally to the Earth (exogenic) and those caused by processes within the Earth (endogenic). Exogenic processes include river, wind, currents, waves, tides, and glacial action. Endogenic processes include volcanic activity and earthquakes, and horizontal and vertical motions of the Earth’s surface caused by plate tectonics and mantle convection.

Geological framework of time Geologists have subdivided the history of the Earth into a type of calendar based upon the geological time scale. Your introductory course in geoscience has already introduced you to geological time; please take the opportunity to review and refresh your memory with regard to geological time by studying the assigned textbook reading. An understanding of geologic time is absolutely essential to gain a proper perspective on the cycling behavior of various components within the Earth system.

The construction of this geological time calendar was based on studies of the distribution of sedimentary rocks and the fossils they contain and upon determinations of the age of these materials. The relative ages of rocks are obtained by investigation of their relative vertical position in a thick sequence of rocks. For example, in an undisturbed sedimentary rock sequence, it makes sense that the oldest beds or strata are on the bottom and the youngest are on the top. Similarly, the lowermost sedimentary layers may contain fossils that lived much before the remains of organisms that are found in overlying layers. But this type of straightforward reasoning only provides geoscientists with the relative age of the units. The absolute age of the rocks can be obtained by using dating methods that involve the radioactive decay of elements found in minerals in these rocks. Thus, to construct the geological time scale, both the relative and absolute ages of the rocks and the fossils contained within them must be studied.

The time scale is subdivided into a series of time periods of varying length. Most divisions of the geologic time scale mark major environmental changes and associated important paleobiological changes with new species development. Biological change, as recorded in the sedimentary strata, generally occurs slowly. However, rapid extinctions of many groups of organisms can occur. These extinctions are recorded in the rocks by the absence of fossil remains of some organisms in younger rocks. Extinctions mark the

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boundaries of the major subdivisions of the geologic time scale. It is important to realize that these extinctions and resultant biological changes are caused by natural environmental change, such as changes in temperature and precipitation patterns, atmospheric composition, and sea level. The causes of these environmental changes, in turn, are variable and include such factors as the amount of solar radiation reaching the Earth, the distribution of radiation at the surface, changes in the intensity of volcanic eruptions, fluctuations in the rate of movement of the lithospheric plates, and impacts of extraterrestrial bodies.

The crustal system In your introductory geoscience course you considered the overall structure of the Earth and examined the various sources of Earth’s internal energy. As you recall, the Earth’s internal structure is one of concentric shells, which differ in chemical and mineralogical composition and in physical properties. The structure has evolved from an original uniform agglomeration of particles through the action of the gravitational process referred to as differentiation. The least dense materials became segregated in the thin outer layer of the crust, and the more dense iron and nickel settled toward the center of the Earth to form the core. On the basis of mineralogy, a distinction is made between crust, mantle and core, with each layer separated by a major boundary. The crust is very thin in comparison with the other layers and comprises only 1.5% of the Earth’s total volume. However, it is this thin layer and its surface characteristics with which most of environmental Earth science is concerned.

The bulk of the Earth’s mass is composed of mantle material which has a density between that of the crust and the core. Within the mantle there is a boundary at a depth of about 50km which separates the more rigid crust and upper mantle, that together makeup the lithosphere, from the less rigid (more viscous) lower mantle. This viscous or plastic zone of the mantle extends down to about 250km and is termed the asthenosphere. Below this is the mesosphere and ultimately the core.

The outer boundary of the lithosphere, the crust, forms a very complex interface with the atmosphere, hydrosphere, and ecosphere. The lithosphere is an open system with exchanges of matter as well as energy across both the outer and inner boundaries

Chemistry and mineralogy of the lithosphere The “objects” of the crustal system are the chemical elements of which the minerals and rocks of the lithosphere are composed. Table 4.1 shows the elemental composition of the crust for the most abundant elements as a percentage by weight and volume. Oxygen is by far the most abundant. Importantly, there is much more oxygen in the solid crust than occurs in the atmosphere. This crustal oxygen is firmly bonded to other elements of the lithosphere that form the major rock-forming minerals. These minerals, in turn, combine in typical mineral assemblages to form the major rock types of the lithosphere. The second most abundant element in the lithosphere is silicon; it is no surprise that most of the oxygen is in combination with silica to form silicate


minerals. The manner in which these mineral structures form and their relationship to the processes operating in the crustal system are beyond our scope for this course. For the moment, suffice it to say that they occur in particular combinations and amounts in response to the conditions under which they formed, and it is these mineral assemblages we recognize as rock types.

Table 4.1 Elemental composition in percentages of the Earth’s crust

Element Percent by volume

Percent by weight

Oxygen 93.700 46.6 Silicon 0.900 27.7 Aluminum 0.500 8.1 Iron 0.400 5.0 Magnesium 0.300 2.1 Calcium 1.000 3.6 Sodium 1.300 2.8 Potassium 1.800 2.6 Others (trace elements) 0.001 1.4

Major rock types The rocks of the Earth’s crust can be grouped into three major classes according to their mode of origin:

• Igneous rocks are formed by the cooling and crystallizing of molten rock or magma or by mineral fluids derived from the mantle. They form either as intrusive rocks, where the magma is injected into existing crustal material, or as extrusive rocks, which are formed by ejection of material at the Earth’s surface. Intrusive igneous rocks form at depth and they crystallize slowly, forming large crystals and a coarse-grained rock. The extrusive rocks cool rapidly at the Earth’s surface and are generally fine-textured. Thus, igneous rocks are generally composed of interlocking crystals of primary rock minerals. The mineral composition may vary considerably depending on the particular minerals present. A distinction is frequently made between acidic (more than 60% SiO2) and basic (relatively low SiO2 content) rocks. The mode of formation of igneous rocks means that there are few voids within the rock, and the low porosity usually means a high mechanical strength.

• Sedimentary rocks are formed by the weathering and breaking down of previously existing rocks. The sediments are transported across land masses, either as detrital grains or as ions in solution, and the bulk of them are carried to the oceans where they accumulate on the continental margins. Along the way, of course, smaller or more isolated sediment ‘traps’ exist on the continental surfaces, for example, in lakes and river valleys. The detrital

58

grains comprising clastic sedimentary rocks are usually composed of the most resistant minerals, such as quartz. Other sedimentary rocks are formed by the accumulation of organic materials (e.g., coal) or by chemical precipitation of ions in solution (e.g., evaporites, carbonates). These loose unconsolidated sediments are transformed into rocks by the process of lithification. Lithification is the compression and compaction of the sediments and the extrusion of water and is usually accompanied by cementation, through the precipitation of various inorganic compounds in the pore space. Sedimentary rocks are typically stratified with variations in composition and texture resulting from changes in depositional conditions or sources of sediment.

• Metamorphic rocks are formed by the alteration of any previously existing rocks. Clearly, conditions of high temperature and high pressure are essential for metamorphic rock formation. These conditions may develop in the crust resulting in both mechanical deformation and chemical recombination of the elements in the rock-forming minerals. Many types of metamorphic rocks exist depending on the original minerals present and the type of metamorphism. A great variety of both mineralogy and texture is possible within the metamorphic rock group. Typical examples are slates and schists, representing lower and higher grades of metamorphism, respectively.

Of the rocks composing the lithosphere, over 95% (by volume) are of igneous origin and consist of primary silicate minerals. Metamorphic and sedimentary rocks, which can be thought of as having been derived from these same primary silicate minerals of igneous rock, make up the remaining 5%. Although igneous rocks are dominant in terms of volume, the sedimentary rocks, such as shale, sandstone, and carbonates, are exposed over most of the surface of the Earth (Tables 4.2 and 4.3). These sedimentary rocks occur as a thin veneer, with igneous rocks occupying only about 18% of the Earth’s surface area.

Table 4.2 Proportion of major minerals exposed on the land surface

Mineral Percent of Earth’s surface

Feldspars 30

Quartz 28

Clay minerals 18

Carbonate minerals 9

Iron oxide minerals 4

Others 11


Table 4.3 Proportion of major rock types exposed on the land surface

Rock type Percent of Earth’s surface

Shale 52

Sandstone 15

Granite 15

Carbonate rocks 7

Basalt 3

Others 8

In addition to their chemical and mineralogical properties, igneous, metamorphic, and sedimentary rocks all possess important physical and mechanical attributes, which control their response to processes operating within the crustal system and at the interface with the atmosphere and hydrosphere.

Major components of the lithosphere system On the basis of rock type and mineralogy, the lithosphere is divided into three large overall structural units: • continental crust; • oceanic crust; and • upper mantle.

Two of these, the continental crust and the oceanic crust, have particular relevance for us as environmental Earth scientists. The continental crust is composed of granitic rocks rich in Si and Al with an average density of 2.8. These rocks form a discontinuous outer layer of the planet and underlie the continents. Each continent has a core of ancient Precambrian rocks of either metamorphic or igneous origin. These continuous masses are very stable and are termed cratons. The long-term stability of the cratons is a property that environmental geoscienists are presently attempting to exploit as they search for possible sites for the disposal of nuclear waste. The rocks of these core cratons (or shields) are the oldest exposed rocks at the Earth’s surface, often with ages in excess of 2 billion years. In North America this ancient core is represented by the Laurentian or Precambrian Shield, whereas in Europe it is the Fenno-Scandian Shield.

The cratons or shields are separated from one another by zones of more mobile portions of crust. These zones, which may be hundreds of kilometres across, are known as orogens. They are more readily deformed by crustal pressure and are the sites of intense seismic activity, folding, and faulting. This deformation process is termed orogeny. The rocks in these mobile zones can include sedimentary rocks that are deformed into fold mountains, as well as metamorphosed and intruded by igneous rocks. These areas are exceedingly

60

complex in terms of geologic structure, but on the large Earth system scale they can be readily identified as forming strongly linear patterns at the Earth’s surface. Although old fold mountains likely have been reduced to low elevations, the mobile zones of more recent deformation make up the areas of the highest relief and elevation on Earth. The average elevation of the land is about 900m above sea level, but this is strongly influenced by these orogenic zones because over 70% of the continental surfaces lie well below 900m elevation.

The oceanic crust differs mineralogically from the continental crust. It is basaltic in composition, consisting of more basic (lower SiO2) minerals and has a mean density of 3.0. The oceanic crust is generally structurally simple and relatively young. Indeed, nowhere is the oceanic crust older than about 225 million years. This relative youth of the oceanic crust is emphasized by the fact that no sediments older than the Jurassic period rest upon it. The topography of the floor of the ocean is much more subdued than that of the continents and generally does not have the local variations of relief that characterize the continents. Submarine elevations drop off sharply around the margins of the continents so that 85% of the ocean floor occurs between 3km and 6km below sea level.

There are, however, two features of the ocean floor that are quite distinctive: the mid-ocean ridges and the deep submarine trenches. The mid-ocean ridges form an interconnecting chain circling the entire globe for over 60,000km. Although not always found in the central position of the ocean basin, these ridges range from 500 and 1,000km in width and reach heights of up to 3km. One characteristic feature of these ridges is that they are always associated with high geothermal gradients or heat flows. Deep submarine trenches or troughs are also strongly linear features but comprise a more restricted area. They are located close to the margins of the ocean basins and are particularly obvious around the Pacific Ocean where they attain depths of 11km. The ocean trenches are zones of crustal instability as indicated by their coincidence with deep earthquakes.

Transfer of matter and energy The overall structural units of the lithosphere described above form a large-scale functioning system that is responsible for the formation and destruction of crustal material and for the break up and distribution of the continents. The processes responsible for the transfer of material on such a large scale operate slowly (on the human timescale) but nonetheless, are clearly responsible for configuration changes at the Earth’s surface which have a direct impact on virtually every aspect of human activities and habitation.

The key feature of the operation of the lithosphere system is the process of sea-floor spreading. New basaltic crustal material from the mantle is intruded into the crust along the central rift of a mid-ocean ridge. There it solidifies to form new crustal rock. As this new material is added to the crust, existing crust is pushed aside in both directions in order to accommodate it. Thus, the high heat flow of the ocean ridges is explained by the presence of hot mantle material beneath the ridge. The pattern of progressively older crustal material (and


sedimentary material covering it) away from the ridge axis is explained by the lateral movement of crustal material in each direction.

This continuous formation of new crustal material at the mid-ocean ridges and the transfer away by sea-floor spreading means that the entire ocean floor is in motion. It also explains the relative youth of the oceanic crust. It is possible to decipher the speed of sea-floor spreading by dating rocks at various distances from the mid-ocean ridge axes; the North Atlantic is spreading apart at an average of 2cm/yr whereas the Pacific is more active, with a spreading rate of about 4-5cm/yr.

Since the Earth is not expanding, the creation of new crustal material implies that there is a balancing process by which crustal material is destroyed at the same rate. This is subduction, and it occurs beneath the ocean trenches. Here ocean crust slides down along a fault zone (the Benioff zone) at an angle of about 45o beneath the adjacent section of continental crust. It is this faulting and shearing of the descending plate that creates friction and sets up most of the world’s earthquakes. The seismic instability of these zones is also reflected in the abundance of volcanic and geothermal activity at the Earth’s surface above the Benioff zone.

In summary then, distributed across the surface of the Earth there are zones where new crustal material is being created and rising to the surface and other zones where it is descending toward the mantle and being consumed. The lateral movement of crustal material across the surface by sea-floor spreading is balanced by the movement in the opposite direction at depth within the mantle. Thus, it is intuitive that there is likely a large-scale convection cell or cyclic movement of material within the mantle. In fact, a series of convective cells, all driven by the Earth’s internal energy, exist within the asthenosphere. However, the pattern of convective cells is irregular and is not constant through geologic time. Older rises and sinks die out and new ones become active very similar to the action you can observe within a saucepan of gently simmering, viscous custard on your stove.

The surface of the Earth comprises a series of 6 major plates (and several minor ones), each carrying a continent. There are several types of plate margins: • rises where crustal material is generated, (constructive or divergent margins);

• sinks where crustal material is consumed (destructive or convergent margins); and

• transform margins where two plates slide laterally past one another.

Events occurring at plate margins have a very significant effect on the form of the Earth’s surface and, of course, on the humans occupying the surface environment. A constructive margin developing beneath a continent causes rifting and eventual fragmentation of that continent. At destructive margins, where there is a convergent movement of surface crust, significant mountains are developed. Three basic types of destructive margin can be identified:

• Where two oceanic plates meet, one may slide down beneath the other, giving rise to deep-seated volcanic activity. Such eruptions may cause

62

volcanic accumulations which break the ocean surface to form island arcs such as the Aleutian Islands.

• At a margin in which a continental plate is meeting an oceanic plate a major mountain chain may develop because of the large amount of detrital sediment that is available from the weathering and erosion of the continent. This sediment is piled up against the advancing continental plate to form major linear mountain chains such as those we see in the Andes and Cordillera of North America.

• The third type of destructive margin, the meeting of two continents, also results in major mountain chains. For example, the northward movement of India into continental Asia resulted in the massive uplift of former marine and continental sediments to form the Himalayas.

The operation of the Earth’s lithospheric system has enormous implications for the distribution of continents, the relief across the Earth’s surface, and ultimately the climatic and hydrological systems that help to dictate the boundary condition of the ecosphere.

It is this lateral transfer of continents that has moved them to present positions and is continuing to move them still further. Continents become fragmented and rejoined as the pattern of convective cells and the position of rises and sinks change through geologic time. There have been times during the geological record when all the continental crust was combined to form one or two supercontinents, such as at the close of the Paleozoic era. At other times, such as the present, continental crust is fragmented, and the continents become scattered. In addition to these major lateral movements, the operation of the lithospheric system also affects vertical movements of crustal material. Such transfers of matter involve changes in the potential gravitational energy of these materials. For example, mountain building activity transfers rocks and sediments to higher elevations, thereby increasing their potential energy. These vertical movements, however, are compensated by isostatic reactions. The continents, with their lower density, tend to float on the more dense lower crust. This permits them to rise or fall according to whether mass is being added to or removed from them very similar to the way an ice cube (density of 0.9) will bob in water (density 1.0) when light downward pressure is exerted on it. These isostatic movements also involve major transfers of gravitational potential energy.

Summary An understanding of the lithosphere is an integral part of our Earth system science. The lithosphere is being constantly renewed and rearranged through plate tectonics. The land surface is, as a result, being continuously acted upon by mechanisms and processes triggered by this lithospheric change.

It should now be obvious to you why it is relevant (indeed, essential!) to study the physical structure and chemical composition of the Earth as part of environmental Earth science and to achieve a proper understanding of the lithosphere. Some of the reasons include:


• The structure of the Earth and its internal processes have much to do with shaping our landscape and causing hazardous natural processes. For example, internal Earth processes cause earthquakes and volcanic eruptions, shape mountains and ocean basins, and dictate the distribution of climatic zones on the surface of the Earth.

• Humans are also dependent on the material of the solid Earth for resources. Although we will not be discussing this aspect directly within the context of this course, the internal processes of the Earth are essential in providing these resources for human society.

• Finally, you should realize now that the materials of the Earth have distinct physical and chemical properties; we need to understand these materials and their properties before we can adequately mitigate the impact of natural hazards.


absolute time asthenosphere continental crust core crust density differentiation geologic time scale hot spot igneous rock lithosphere mantle

mesophere metamorphic rock mid-ocean ridge oceanic crust plate plate tectonics relative time rift sedimentary rock spreading subduction trench


1. What is the lithosphere?

2. What is plate tectonics?

3. What is a subduction zone?

4. What kind of rock comprises most of the Earth’s crust?

5. What are the three major rock families?

6. What are the major types of convergent plate margins?

7. What is a divergent margin?

64

8. What is a transform margin?

9. What are the main differences between oceanic crust and continental crust?


Unit 5 Endogenic Geologic Hazards: Earthquakes

Topics Introduction to geologic hazards

Overview of geologic hazard mitigation

Earthquakes

Tsunamis

Summary of mitigation of hazards associated with earthquakes

Earthquake prediction

Introduction Geologic and hydrologic hazards have affected the lives of many North Americans. The dramatic display of urban damage sustained in the San Francisco area by a moderate earthquake on 17 October 1989 (televised to millions of onlookers expecting to watch a World Series baseball game) demonstrates that we cannot underestimate the potential impact or suddenness of natural hazards. This section will first set the stage for our discussion of natural hazards over the next several weeks by briefly describing the range of natural hazards and identifying some of the broad actions that environmental geoscientists can suggest to decision makers in an effort to mitigate the effects of these hazards. We will then launch into our detailed description of the major types of hazards, beginning with earthquakes and problems associated with ground shaking, surface faulting, and substrate failure. Subsequent sections will consider the other endogenic and exogenic hazards, such as volcanic eruptions, flooding, landslides, and coastal hazards.

Learning objectives Much of environmental geoscience today can be broadly classified as “hazard” geoscience—the investigation of processes that have potential for harmful impacts on people. The single most important point to realize in hazard geoscience is that the process itself is usually not a geologic hazard. Indeed, many times the process is greatly beneficial to human activity, such as in the case of volcanoes and floods. It is only when this process (or some result of the process) poses a threat to society that we can speak of it as a geologic hazard.

66

There are many kinds of hazards, both natural and induced by humans. The first two goals of this section are to:

• provide an overview of the range of processes that can be identified as natural and/or technological hazards; and

• cite, in general terms, some of the major kinds of actions that environmental geoscientists can suggest be implemented by society to help mitigate the harmful impact of the hazard.

The third major goal is to explore one of the these endogenic natural hazards—the earthquake.

By the end of this section you should be able to:

• summarize the major genetic groupings of hazards;

• differentiate between natural and technological hazards, and give examples of each;

• discuss hazard prediction;

• identify the role that hazards play in North America in terms of monetary impact;

• explain how society perceives hazard severity;

• specify the major government agencies that are responsible for hazard prediction in North America;

• distinguish the various types of seismic waves, their velocities, and wave forms;

• describe the impacts of the various seismic waves on buildings at the surface of the Earth;

• calculate the epicenter of a given seismic event;

• outline the difference between earthquake magnitude and earthquake intensity, and discuss how each is evaluated;

• compute the magnitude of a given earthquake event;

• determine the intensity of a given earthquake event;

• explain human’s role in initiating earthquakes;

• show why taller buildings are more susceptible to certain kinds of damage due to ground shaking than smaller buildings;

• relate hazards due to ground shaking to substrate type;

• classify the types of faulting associated with earthquakes;

• describe ground failure and the types of hazard it produces;

• calculate the magnitude of tsunami hazards in coastal areas;


• compare and contrast the various types of earthquake prediction models, warning systems, and procedures; and

• illustrate the impact of an actual earthquake warning on a large urban center.

Learning activities 1. Read chapter 5 in your textbook and answer the review questions, thought

questions, and exercises on pages 156-157. Note: this is also the textbook reading assignment for the next section on Volcanic Hazards.

2. Read the study notes and answer the review questions in your course manual. 3. Complete assignment 2 and return it to the Distance Education Program

Office.

Study notes Introduction to geologic hazards Although this is a course in environmental Earth science, and not strictly geologic hazards because of the economic importance of geologic hazards as they impinge on humans, it is appropriate to devote some discussion and time to this topic. Geologic hazards, such as earthquakes and volcanoes, are the most dramatic, awesome, and spectacular displays of natural processes. Their occurrence often leads to significant economic losses as well as loss of life and property. Many of these geologic hazards can influence very large regions of the Earth.

A geologic hazard is most simply defined as a geologic condition, process, or event that poses a threat to the safety or welfare of people or the activities and economy of a group of people. Many factors enter into such a definition. Some Earth scientists consider a key feature of a hazard to be its short duration (e.g., it is an “event”) as opposed to damage caused by longer term processes. Many engineers and geoscientists also refer to hazards in the terms we discussed in the first section: it is a geologic process that can produce significant loss of life or property when the critical threshold is exceeded.

Whichever definition we ultimately adopt, it is very important to realize that the process in itself is usually not hazardous. Natural geologic events or processes quite often have very positive effects. For example, river flooding is a natural sedimentological/hydrological response of fluvial systems. The floodplain of a river is appropriately named! It is the flat land adjacent to a river which is periodically inundated by high water levels. Riverine flooding only becomes a geologic hazard when people attempt to live on the floodplain. Such is the case in nearly every geologic hazard known; it is only when this process interacts in a negative way with people does the term geologic hazard apply. Thus, as we have already learned in section one, environmental geoscience is very much a “social science.”

There are many ways of classifying and subdividing geologic hazards. One way of thinking about hazards is to examine the origin of the process; if the hazard

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arises from deep seated, internal Earth processes, the term endogenic hazard applies. This category includes ground shaking, surface faulting and other earthquake-induced ground failures, volcanoes, some types of landslides and subsidence, and, in many cases, tsunamis or “tidal” waves. In contrast, exogenic hazards originate from or develop in association with processes that occur at or near the surface of the Earth. Examples of these hazards are: river and coastal flooding, some types of landslides, compaction and karst-related subsidence, and nonearthquake-related ground failures.

Another way of looking at hazards is to assign the event or problem to either natural causes (e.g., volcanoes, most earthquakes) or to the direct outgrowth of some human activity or action (e.g., flooding due to failure of a dam, land subsidence due to mining or groundwater extraction). Thus, the terms natural hazard and technological hazard can be often found in popular literature such as magazine articles and newspaper reports. Because a geologic process in itself is not hazardous, this subdivision into natural versus technological hazards is somewhat ambiguous and arbitrary; the terms are rarely used by practising Earth scientists.

Society has realized the importance of identifying high risk areas and potentially hazardous geologic situations for many years. However, in North America it has only been within the past several decades that serious efforts have been made to abate the losses due to these hazards. The single most important advance to come out of these international efforts is the realization that although in most cases the hazard cannot be completely eliminated, the risk from the hazard can be evaluated, quantified, and, thus, mapped. This ability to map geologic hazards in terms of the risk they create for society has improved steadily in recent years as a result of the advancement of our understanding of certain geologic processes and, most importantly, the communication of scientists and government policy makers. However, it is still up to society to evaluate this risk assessment in terms of the overall priorities of the region.

The first step in assessing this risk is to realize what risk is and how it is calculated. In terms of environmental geoscience, risk is very simply the product of two other quantitative factors: • the probability or the chances of occurrence of some specified event within

a given time frame and area; and • costs incurred by the event.

Thus, the product of these two factors is expressed in units of monetary value per time.

In general, once a hazardous event is underway, there is little that can be done to stop it. Or expressed another way, the key to successful risk assessment is prediction. Society’s dilemma arises in trying to decide how much of its


resources should be devoted to the prediction of each hazard that may be affecting an area. This prediction hinges on several conditions that must be met:

• There must be an adequate source of current and historical information about the hazard in the area.

• A system of monitoring, data gathering, and data analysis must be in place well in advance of the potential event occurrence.

• Earth scientists must sufficiently understand the threshold levels of the phenomena, as well as the characteristics leading up to the threshold.

These three conditions will be outlined in detail in future discussions of specific hazards.

Society’s perception of the impact of geologic hazards varies greatly. Which hazard is the “worst” depends on the criteria of evaluation: worst in suddenness or lack of warning? worst in size of area affected? worst in dollar value of losses? and so on. For example, certainly many people would rank earthquake hazards high on their list of worst events due to the limited success of prediction and the often large areas affected. However, it is surprising to find that the world’s worst hazard, in terms of monetary losses, is river and coastline flooding. Equally surprising is the fact that in North America losses of between 4 billion dollars and 12 billion dollars per year are sustained from expansive soils and freeze-thaw phenomena, amounts greater than the total losses by all other geologic hazards!

Overview of geologic hazard mitigation In 1998, the dollar-value loss attributed to natural hazards was approximately 3% of the gross national products of United States and Canada. Just nine years before, in 1989, the loss was about 2% of the GNP, and a decade before it was only about 1% of the GNP. What has caused this dramatic increase? The answer is simple: people and urbanization. There are ever increasing numbers of people living, working, and commuting in high risk areas. Associated with this increased number of people is the development of urban centers, which provide concentrated sources of high monetary losses in the event of a geologic hazard occurring.

As we learned in the first section, the ultimate purpose of environmental geoscience and the goal of an environmental geoscientist is to suggest action involving Earth science considerations that planners and decision makers can take to reduce losses from geologic hazards. To accomplish this, it is imperative that the environmental geoscientist assist the decision makers in two ways by:

• helping them to understand the consequences of allowing urbanization or other concentrated human activities in high risk areas; and

• helping the planners to understand the processes causing the hazard such that future losses can be avoided. Without this understanding of the underlying processes, it is virtually impossible to devise methods of reducing losses.

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The specific action and the specific types of planning that are required by society will be discussed in more detail in future sections. However, in general terms, the broad categories of action that can be taken to mitigate hazard impact include:

• modifying the cause(s) of the hazardous event through human intervention;

• decreasing the vulnerability of society to the event; and

• distributing the losses caused by the event over a wider population base.

In many cases complete and absolute avoidance of the geologic event or process is the only way to assure protection from the hazard. For example, although prediction of volcanic eruptions has improved dramatically in the past several decades, avoidance of a volcanic area is clearly the only known mitigating action. In newly developed subdivisions or in urban areas that are undergoing redevelopment, land use zoning is a very effective means of mitigation of hazard effects. The zoning should endeavor to do one or both of two things:

• prohibit certain types of structures or activities that may be susceptible to damage or injury in the high risk area; and

• reduce the density of certain uses or structures in the area affected by the hazard.

Obviously, engineering and architectural design plays a major role in dictating what level of hazard a building will withstand. Indeed, in some types of “technological” hazards, such as flooding due to dam failure, sound engineering design is essential in order to prevent the hazard from even occurring. Unfortunately, the great expense of hazard-proof design usually makes such construction economically unattractive. In situations in which local governments might be reluctant to impose zoning or construction restrictions, the losses due to the effects of a specific hazard can be distributed more widely through specific hazard insurance. Examples include flood insurance and earthquake insurance.

Earthquakes On the 5th of February, 1663, about half-past five o’clock in the evening, a great rushing noise was heard throughout the whole extent of Canada. This noise caused the people to run out of their houses into the streets, as if their habitations had been on fire; but instead of flames or smoke, they were surprised to see the walls reeling backwards and forward, and the stones moving, as if they were detached from each other. The bells sounded by the repeated shocks. The roofs of the buildings bent down, first on one side and then on the other. The timbers, rafters, and planks, cracked. The earth trembled violently, and caused the stakes of the palisades and paling to dance, in a manner that would have been incredible had we not actually seen it in many places. It was at this moment every one ran out of doors. Then were to be seen animals flying in every direction; children crying and screaming in the streets; men and women, seized with affright, stood horror-struck with the dreadful scene before them, unable to move, and ignorant where to fly for refuge from the tottering walls and trembling earth, which threatened every instant to crush them to death, or sink them into a profound and


immeasureable abyss.... for the earthquake ceased not, but continued at short intervals, with a certain undulating impulse, resembling the waves of the ocean; and the same qualmish sensations, or sickness at the stomach was felt during the shocks as is experienced in a vessel at sea.

The violence of the earthquake was greatest in the forest, where it appeared as if there was a battle raging between the trees; for not only their branches were destroyed, but even their trunks are said to have been detached from their places and dashed against each other with inconceivable violence and confusion —so much so, that the Indians, in their figurative manner of speaking, declared that all the forests were drunk. The war also seemed to be carried on between the mountains, some of which were torn from their beds and thrown upon others leaving immense chasms in the places from whence they had issued, and the very trees with which they were covered sunk down, leaving only their tops above the surface of the earth; others were completely overturned.

(R.M. Martin, History of Upper and Lower Canada [London: J. Mortimer, 1836], 171-173.)

Introduction The above quotation, from a manuscript written by a Jesuit priest living near Quebec City in 1663, is not an out of the ordinary description of a large earthquake. What is surprising to many people is that it occurred in Canada. In fact, this oft-forgotten seismic event ranks as one of the most intense earthquakes on the North American continent. Both eastern and western Canada have areas of very high or severe earthquake potential.

Earthquakes are short-lived geologic events that occur when the threshold of an elastically strained rock is exceeded. These factors make earthquakes a particularly bad geologic hazard:

• They are generally unpredictable both within time and space.

• They are capable of inflicting more damage and loss of life in a shorter period of time than any other hazard.

• The earthquake event usually triggers a wide variety of other geologic hazards, such as landslides, subsidence, and flooding.

The hazards associated with earthquakes consist of ground shaking, surface faulting, ground failure and subsidence, and the generation of tsunamis. Despite the occurrence of a number of well-publicized earthquakes, including the Loma Prieta (“World Series”) earthquake of October 1989, the large San Fernando, California, earthquake in 1971, and the “Good Friday” earthquake in Alaska in 1964, North America has sustained a remarkably low level of dollar and life losses due to earthquakes. Indeed, environmental geoscientists and seismologists are fond of pointing out that historically the greatest amounts of damage in North America have been brought about by moderate sized earthquakes, rather than the large quakes. This is very simply because the moderate earthquakes, which can still cause immense damage and disruption in an urban center, occur much more frequently than do the larger quakes. In

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California, for example, a “moderate” seismic event occurs every three years, whereas a large quake can be expected only once every 150 years.

Earthquake mechanisms Geologists define earthquakes simply as a motion or trembling of the Earth caused by the sudden release of accumulated strain. The motion is propagated through the Earth in the form of waves. Thus, one of the more important points to remember about earthquake hazards is that the severity of the quake is controlled by two quite different factors:

• the nature of the way the wave is propagated through the Earth materials; and

• the nature and geometry of the original rupture or fault that caused the wave.

Today geoscientists realize that, although earthquakes can occur anywhere on Earth, many of the major quakes are in some way related to plate tectonics. As you recall from the previous unit on the lithophere, the “solid” surface of the Earth is, in fact, broken into several large, 50 to 60km thick plates. These crustal plates are mobile, moving slowly and more or less continuously over the Earth’s interior. In some places the plates are separating and new crustal material is being added, such as at the mid-Atlantic ridge. In other areas, the plates are being pushed together creating regions in which one plate is riding up over the top of the other. These areas of plate collision are called subduction zones. Finally, in some places the plates are being shoved laterally past one another along vertical boundaries called strike-slip faults, as exemplified by the well known San Andreas fault in California.

Over 95% of all earthquakes recorded are associated with one of these plate boundary settings. The type of movement and the rates at which the plates are moving vary greatly from less than a millimetre per year to velocities measured in tens of centimetres per year. Sometimes this movement takes place smoothly and continuously, such as along parts of the San Andreas fault near Hayward, California. This type of movement is called aseismic creep or fault creep. In other places, as the plates move, strain accumulates at the boundary between the two plates until the critical threshold is reached and exceeded, causing a rapid displacement of the blocks of Earth. This displacement is called seismic slip and is usually associated with earthquakes. The specific type of displacement and the geometry of the fault are controlled by the nature of the plate to plate contact. In subduction zones, very deep movement often occurs, giving rise to a deep focus for the earthquake. In transform fault areas, where the plates are moving laterally past one another, shallower focus depths are common.

The strain energy released by the earthquake sets the seismic waves in motion. The waves radiate outward in all directions from the focus. The waves travelling through the interior of the Earth are called body waves. Once the energy reaches the surface, a different type of wave moves along the surface of the Earth. There are two kinds of body waves and two kinds of surface waves. The surface waves (Rayleigh and Love waves) are the slowest moving but are


also the waves with the strongest ground motion or amplitude. The body waves (primary or P wave and secondary or S wave) have higher velocities than the surface waves. The P wave can be propagated through both liquid and solid material and has a faster velocity than that of the S wave. The S wave can only move through solid material. These different velocities and movements can be used by environmental geoscientists to determine the location of the epicenter of the earthquake just as you will do in the assignment for this section.

Human-induced earthquakes Although modern science can demonstrate that nearly all earthquakes are “natural” in the sense that they are related to movement on faults at plate boundaries, it has also been shown that human activity is capable of initiating earthquakes and that this ability to cause seismic events has increased significantly in the last 50 years. One of the most common ways of “artificially generating” earthquakes is to build a dam and impound a large reservoir behind it. The generation of seismic activity by this means was first observed in reservoirs in Greece during the 1930s; since then there have been over 40 examples of reservoir-induced earthquakes. One of the best documented cases is the occurrence of some 600 earthquakes that took place after the Colorado River was dammed by Hoover Dam and Lake Mead was created. In this case, there was even a good correlation between the size of the lake as it was filling and the number of quakes. Although only about 1% of the world’s artificial reservoirs show an increase in seismic activity after formation, there is a striking correlation between the size of reservoir and earthquake generation: the bigger the body of water, the greater the chance of seismic activity. Approximately 10% of the world’s reservoirs that are up to 90m deep show increased seismic activity, whereas 20% of the reservoirs that are 140m deep are seismically active.

It has also been shown that humans are capable of generating earthquakes by pumping fluids into the subsurface. This discovery was made by accident in the 1960s outside of Denver, Colorado. The story of the Rocky Mountain Arsenal disposal well is now a classic episode in environmental geoscience detective work. There had not been a significant earthquake in the Denver area since the late nineteenth century. However, beginning in early 1962 the population was subjected to an almost daily occurrence of small earthquakes. A local geologist traced the epicenters of the quakes to an area just north of Denver and the foci to a linear trending zone that was centered on a 3,000m deep well on United States Army property. The well was drilled by the Army to dispose of poisonous liquid wastes into the deep subsurface; approximately 22 million litres of liquid were being injected into the subsurface per month. It was soon realized that there was a direct correlation between the amount of fluid injected per month and the number of earthquakes that occurred in Denver per month. When the injection well was shut down for several months, the earthquake frequency dropped to nearly zero. Conversely, when the injection started again, earthquake frequency rose dramatically. Armed with this excellent statistical data base, which extended over a period of four years, the U.S.G.S. undertook a series of experiments in an oil field in Colorado during 1969 to 1973. They

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found that by altering the pressure of the injected fluids being pumped into the subsurface, earthquakes could be turned on and off.

The Rocky Mountain Arsenal earthquakes and the earthquakes produced by the formation of large reservoirs have led geoscientists to the startling proposal that natural seismic activity on faults located at or near plate margins may possibly be controlled by fluid injection. If we view the shearing strength of the rocks as the sum of the intrinsic strength of the rock and the friction times the normal stress across a fault plane according to:

τ = τo + µ σn

(where τ = shearing strength, τo = intrinsic strength, µ = coefficient of friction, σn= normal stress across the failure plane), then the effect of increasing the pore pressure in the subsurface by injecting fluid is to reduce the σn value. This results in a lowered shearing strength of the rock according to:

τ = τo + µ (σn - P)

where P is the net pressure difference after injection.

Thus, if it is indeed possible to artificially trigger earthquakes by changing the subsurface pore pressure, then at a plate boundary it should be possible to initiate many small but harmless earthquakes rather than waiting for the accumulated stress to build up to be released in a single large, destructive quake. Application of this theory has not yet been attempted in urbanized areas.

Earthquake effects In many respects, earthquakes rank as the number one geologic hazard. In the last thousand years more people have lost their lives and more property damage has been caused by earthquakes than by any other geologic hazard. Although an average of only 10,000 people are killed each year by earthquakes, in some years this total is very much higher (e.g., greater than 1 million people died in 1976 as a result of earthquakes).

There are a great many effects of seismic activity. In general the impact of an earthquake on an area is determined by a combination of the following factors: • amount of energy released; • the frequency, orientation, and duration of the ground movements; • the distance from the epicenter; • the physical properties of the substrate, including both bedrock and surficial

deposits; and • the building design.

The negative impact of the seismic event on humans can be subdivided into two broad groups of effects:

• changes to the land and destruction of property and life that are the direct result of the Earth’s vibration; and


• indirect effects that are ultimately caused by the earthquake but are not the direct result of it.

Ground shaking and ground vibrations Ground shaking at the time of the seismic event causes some of the most spectacular damage associated with earthquakes. In the worst case, there can be partial to complete collapse of virtually all structures in a relatively large area. Approximately 75% of the total amount of damage caused by the 1971 earthquake in San Fernando is attributed to ground shaking. It is estimated by California state geologists that if there was a repeat of the infamous 1906 San Francisco earthquake today, about 25 billion dollars of damage would be sustained (despite our enhanced knowledge of building design and earthquake mitigation techniques).

Buildings and structures vibrate due to ground shaking at different frequencies. The frequency of vibration of the structure is controlled by three factors: • the shape and dimensions of the building; • the nature of the substrate; and • the amount of energy applied.

Earthquake engineering attempts to mitigate the loss or damage to a building by construction of a building that is either solid and stiff enough to resist disintegration or elastic enough to “give” rather than collapse during the shaking event. The response of a building subjected to earthquake shaking is determined by its natural period of oscillation or vibration. In other words, the period of vibration of a building is a function of the building itself. This period of vibration can be easily calculated according to:

T = 0.05 H W0.5

where T is the period measured in seconds, H is the building height in feet, and W is the building width in the direction of the seismic motion. This equation basically says that tall buildings have greater periods of vibration thus are more likely to sustain damage due to ground movement than short buildings. Unfortunately, this ability to vibrate is sometimes more difficult to predict and quantify than implied from this simple relationship. For example, if a seismic wave causes a building to vibrate, and this vibration continues and is at the same frequency as a subsequent seismic wave hitting the building, the structure movements will reinforce one another causing much more vibration, and, therefore, damage, than originally anticipated.

As the building oscillates, it will undergo drift. Drift is defined as the maximum deflection from the vertical at the top of a building during shaking. This drift will, of course, produce distortion of interior walls, conduits, and windows. It may be severe enough to actually cause beams to buckle, bend, and fail; structural columns to compress and ultimately collapse; and walls to slide relative to each other in a direction parallel to their points of contact. When the building undergoes drift, structures whose outlines are regular and symmetrical

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experience much less damage than buildings that are characterized by more irregular and nonsymmetrical architectural designs.

The construction characteristics of the building itself play a major role in dictating its response to earthquake ground shaking. As a very general rule, wooden buildings fare much better in earthquakes than do buildings constructed of rigid, unflexing brick, or concrete. This aspect of construction materials can be dramatically emphasized by comparing two earthquakes of similar intensity and magnitude: one in northeastern Iran (Dasht-e Bayaz) in 1968, in which over 12,000 people were killed, and one a few years earlier in Taiwan in which only 100 were killed. Although the population densities of the two regions affected were similar, most of the Iranians lived in adobe huts (i.e., rigid construction material), whereas most of the Taiwanese population occupied bamboo or wooden huts.

Finally, the nature of the bedrock and surficial materials is extremely important. Again this fact can be dramatically demonstrated by looking at the conditions surrounding the 1985 Mexico City earthquake in which over 10,000 people were killed and more than 1200 multilevel buildings destroyed. Mexico City was actually several hundred kilometres away from the epicenter of the earthquake. Regions much closer to the focus sustained little or no damage because the earthquake waves were dampened by the bedrock type. In contrast, the substrate at Mexico City, which consists mainly of unconsolidated water-saturated lake clays, actually caused an increase in the seismic wave amplitude and also decreased the velocity of the waves such that the impact of the ground shaking was felt for a much longer time. The material most susceptible to this type of wave amplitude increase and velocity decrease is unconsolidated human-made fill deposits, followed (in terms of decreasing severity) by: unconsolidated natural fill (i.e., alluvium), semi-consolidated human-made fill, and finally, bedrock. Within the bedrock realm, massive igneous and thickly bedded sedimentary rocks are the safest from wave amplitude magnification, followed by less-compact layered or folded crystalline or sedimentary rock, dry argillaceous rock, poorly-consolidated or highly-interbedded sedimentary rock, unconsolidated sediment with a deep water table, unconsolidated sediment with a near-surface groundwater table, and, finally, fractured or faulted rock. The last three groups pose a very high seismic risk.

Nothing can be done to eliminate the ground shaking hazard associated with even minor earthquakes, but the environmental geoscientist can suggest ways of reducing the risk or, alternatively, decreasing the losses in high risk areas:

• In areas of new urbanization avoid the areas that will be most susceptible to severe shaking, prohibit certain types of structures, and control the density of high rise buildings.

• In old, well-established urban areas, engineering retrofit and redesign can usually make the structures somewhat more stable and resistant to shaking. Alternatively, reducing the use of the buildings that would be most susceptible to damage may be suggested. Finally, complete removal is sometimes the only possible way of mitigation.


Ground cracking and surface faulting Your textbook reviews the various types of fault movement under tensional and compressional forces. Obviously, fault movement of several metres can cause severe damage and even destruction of structures located directly on the fracture. Usually, however, there are very few deaths or injuries directly related to surface faulting, and often only a small percentage of the total damage is attributed to this type of hazard. This is because the actual fault line is a relatively small proportion (in terms of area) of the total region affected by the earthquake. Nonetheless, railroads, highways, tunnels, bridges, and gas/water lines can be easily severed where they cross the fracture.

It is very difficult to reduce the losses associated with ground cracking. The two main problems are that:

• The geoscientist must be able to predict very accurately where the fault will occur.

• Not all active faults are damaging at the surface.

If the fault has been active in the last 10,000 years, geologists can usually identify the trace of the fault at the surface with sufficient accuracy to be useful for engineering and construction design. The surface trace of faults which have not been active for some time, however, are very difficult to predict.

Ground failure In contrast to ground cracking, earthquake induced ground failures often cause a large proportion of the total damage and loss of life sustained in the seismic event. For example, in the 1964 Alaska earthquake, ground failure accounted for an estimated 60% of the total damage. In a 1920 earthquake in northern China, ground failure and soil-flows killed about 200,000 people.

Ground shaking causes a disturbance of the original arrangement of the grains in unconsolidated and poorly cemented sediment. This reorientation results in an arrangement in which the grains are more closely packed together than before. This closer packing also means that there is less porosity in the sediment. If the material that is being rearranged is dry (i.e., above the groundwater table) the decrease in porosity results simply in a net settlement or subsidence of the earth’s surface. However, if the rearrangement takes place below the watertable, the pore water prevents a rapid decrease in porosity and, instead, results in temporarily increased pore pressures by transferring the overlying load from grain-to-grain contact to the pore water. This is known as liquefaction. It is the transformation of material which normally behaves as a solid into material which behaves temporarily as a liquid. Liquid, of course, has very little strength, so structures built on top of the material undergoing liquefaction quite literally sink into the Earth.

There are three main types of earthquake induced ground failures. The first is simple lateral spread of large blocks of Earth due to liquefaction of material beneath the blocks. This movement can occur on even very gentle slopes with much less than a degree of inclination. Although this movement can be quite

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destructive when it occurs in an urban environment, it is usually comparatively small and rarely catastrophic. The second type of ground failure is the much larger flow failure or landslide. We will discuss the landslide geologic hazards later in the course, but a very common triggering mechanism for landslides is the ground movement and shaking associated with earthquakes. Flow failures of this type can be very large, covering several square kilometres of surface area and involving several cubic kilometres of sediment. In the Alaska Good Friday earthquake, a submarine landslide carried away much of the port of Valdez. The final type of ground failure hazard is the loss of bearing strength identified above. When the sediment loses bearing strength, large and spectacular deformation can occur at the surface.

The only available recourse for mitigating the losses due to lateral spreading and flow failures is avoidance zoning. Fortunately, the type of substrate conditions most conducive to flow failure and spreading are easily recognizable, thereby making accurate prediction possible. Loss of bearing strength of the sediment undergoing liquefaction can be compensated for by engineering design and implementation of such things as subsurface drainage conduits (tiles), grouting, cementing, and building foundations in bedrock rather than sediment.

Tsunamis Tsunamis are large water waves which are often generated by earthquakes or caused by landslides that are, in turn, triggered by the seismic activity. Earthquakes are, of course, not the only cause of tsunamis; other mechanisms, such as volcanic eruptions, or large, non-earthquake related landslides, are also capable of generating tsunamis. Tsunamis are sometimes referred to as tidal waves. This is a misnomer because tidal action of the moon and sun has very little to do with the wave. A more correct term to use, if you do not like the Japanese expression, is a seismic sea wave.

A tsunami is caused by the sudden vertical movement of a large area of seafloor. Historically, tsunamis have affected mainly lands bordering the Pacific Ocean. There have been very few tsunamis reported in the Atlantic area. The reason for this geographic distribution is probably twofold:

• In general, the Atlantic region experiences far fewer earthquakes relative to the Pacific area.

• The type of faulting in the Atlantic region generates mainly non-vertical Earth movements.

Although tsunamis affect mainly the Pacific area, their impact can be very widespread and catastrophic. The Hawaiian Islands have recorded about 90% of all known tsunamis and have experienced many of the most destructive events.

Unlike the popular concept of a “tidal wave” as a huge wall of water looming up and washing over ocean-going vessels (as portrayed in the movie The Poseidon Adventure), tsunamis are actually very difficult to detect in the open ocean. They characteristically have heights of only about one foot and have


extremely long wave lengths (up to 1,000 km). The speed of the tsunami can be very high. The velocity of the wave in the deep open ocean is equal to the square root of the product of water depth and acceleration due to gravity according to:

Velocity = (g h)0.5

where g = 9.8m/second and h = water depth in m.

However, as the wave approaches shallower water it slows dramatically. This slowing of the wave’s velocity results in a large increase in wave height (in much the same way “normal” waves at the beach increase in height in the shallow waters of the shoreface). Because of the tremendous wave lengths, the energy cannot be readily dissipated in the surf like normal waves.

Thus, tsunamis pose a very special environmental problem. First, they are very difficult to identify in deep water, thereby making prediction nearly impossible. Second, they can very rapidly cross great distances of ocean. Finally, they are able to affect areas far removed from the original seismic event that caused them. The actual effect of a tsunami on the coastal area of the land is a function of several factors:

• the nature of the original disturbance (e.g., a dip slip fault has more vertical displacement than a strike slip fault);

• the distance from the original disturbance; and

• the bathymetry of the coastline immediately offshore from the area in which the wave will impinge.

There is no practical way of preventing tsunamis although there are several engineering techniques that strive to lessen the physical impact on the coastline. These techniques fall into two groups:

• structures, such as artificial reefs and offshore barriers, that dissipate the energy of the wave; and

• reinforcement of the buildings and structures in the nearshore and runup areas of the coast in order to withstand the lateral forces applied by the wave.

The main way of avoiding loss of life is by a warning system. There has been a tsunami warning system in place in the Pacific region since 1948. However, the great velocity of the waves often precludes the possibility of evacuation even if the tsunami is detected. Furthermore, the ultimate effect on the coastline does not usually have a direct relationship to the size or magnitude of the earthquake.

Summary of mitigation of hazards associated with earthquakes These types of action can be undertaken by society in order to lessen or remove the impact of hazards associated with earthquakes:

• Community preparation: Planning for evacuation and disaster relief in communities in high risk areas can greatly reduce loss of life and property

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damage due to secondary earthquake hazards. Also keeping the public informed and aware of the risks to which they are exposed is necessary.

• Land use management: This is the simplest, most direct way of reducing losses due to earthquakes. Essentially, do not live or work in high risk areas. Problems arise with this, of course, in old, well established urban centers such as Los Angeles, San Francisco, and Vancouver. Refusal of rebuilding and renovation permits, and refusal to allow construction of “critical” structures in high risk areas are possible alternatives.

• Earthquake engineering and design: Most deaths attributed to earthquakes are usually the result of the collapse of human-made structures. It is now technologically feasible to design and construct buildings to withstand all but the strongest earthquakes (up to magnitude 8). Even so, this design and construction is very expensive, thus resulting in a situation in which most cities, even in earthquake prone areas, are “undercoded.” For example, most of the modern buildings that were destroyed in the 1971 San Fernando earthquake exceeded that city’s building code, which was one of the strictest in the country. Similarly, a 1967 earthquake in Caracas, Venezuela, resulted in the total collapse of five high rise office and apartment buildings and major structural damage to over 500 other buildings of ten to thirty stories high. Caracas at that time had the most rigid building code in the western hemisphere after California. At best, the general recommendations for cities that exist in high risk areas are:

• Make sure there is proper spacing between high rise buildings so that when the buildings do vibrate, they do not hit one another.

• Remove all loose or hanging ornamentation.

• Encourage square or rectangular architectural designs rather than more complex patterns that will suffer more damage.

Earthquake prediction It has only been within the last decade that earthquake prediction and control have been seriously considered by scientists as a way of reducing earthquake risk. Despite significant advances in the last five years, earthquake control is not yet practicable and implementation of extensive prediction programs in North America is many years away.

There are two main approaches to earthquake prediction:

• long term analysis of the overall seismicity and recurrence rates of earthquakes in a region; and

• short term study of precursor events.

The long term approach gives insight into the overall pattern of earthquakes both spatially and in terms of time. However, this long term perspective lacks the specific information that is needed by planners to provide definitive risk statements, alter building codes, etc. The short term “precursor” approach strives to supply specific information on the time and location of an impending


quake. The problem with this precursor method is that the phenomena that are being monitored (for example, animal behavior) can be affected by factors other than earthquakes.

Long term approach — seismicity gaps Prior to the mid-1960s a considerable effort went into identifying seismicity gaps, or areas within an overall seismically active area that have abnormally low numbers of earthquakes. The interpretation of these gaps at the time was that they were areas in which the strain was being released by slow, continuous movement rather than allowing a build-up to the threshold point and having a large displacement. It is now realized that this low seismic activity may be associated with areas of the fault system that are locked. Strain is continuing to accumulate and ultimately will be released in a major earthquake. Thus, rather than being the place of lowest risk, the seismicity gaps are the most hazardous.

Long term approach — recurrence rates The identification of seismicity gaps tells us nothing about how often earthquakes would occur or what the magnitude of the event would likely be when it did occur. In contrast, a purely statistical analysis of historical records in an area often shows a relationship between the number and magnitude of earthquakes that have occurred. Using North American data, this relationship is expressed as:

log N = a - (b M)

where N is the number of seismic events in the historical record, a and b are constants, and M is the magnitude of the quake.

Using this historical/statistical approach, earthquakes of magnitude 3 to 4 can be expected to occur at a rate of 49,000 per year in North America, events of between 6 and 7 at a rate of 120 per year, and earthquakes of magnitude 8 to 9 once per year. The problem with this approach is that it is entirely a function of the reliability and length of historical record. Deductions made from a long record with many earthquakes is obviously more valid and shows a better statistical relationship than a historically short record with few earthquakes.

Short term approach — precursor events There are many types of precursor events that have been used in an attempt to predict the occurrence of an earthquake. A precursor event is a phenomenon that occurs in a characteristic way before an earthquake. Thus, close monitoring of this phenomenon may reveal an impending quake. Over the past two decades there have been nearly 300 instances in which earthquakes have been “successfully” predicted by this method. The phenomena monitored range from microchanges in the ground elevation to the behavior of pet dogs and cats. The most consistently successful efforts have used the slight tilting and deformation of the ground to predict the earthquake. Although the monitoring of animal behavior to predict impending earthquakes has a long history in China, the key event that brought about renewed investigations into precursor phenomena was an earthquake in northern Russia in 1946. It happened that immediately prior to

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this quake, the region around where the seismic event was to take place was undergoing extensive geophysical monitoring for a different purpose. Thus, by accident all the necessary instrumentation was in place and data was being collected just prior to and during the earthquake. The first thing that was noticed is that for several months there was a gradual decrease in the velocity of the compressional waves (and, therefore, a decrease in the ratio of the P wave to the S wave). Then, just before the earthquake occurred, the P wave velocity and ratio of P to S returned to normal. Subsequent observations of these seismic wave velocities immediately before earthquakes has shown that not only does the return to normal signify the impended event, but that the duration of the anomaly (i.e., the low ratio period) was directly correlated with the intensity of the earthquake. This can be expressed quantitatively as:

log T = 0.8 M - 1.92

where T is the length of time (in days) of the anomaly and M is the magnitude of the quake.

Studies by Earth scientists on these precursor phenomena have resulted in several theories to account for the empirical and observational data. The most widely accepted of these theories is the dilatancy-diffusion model. As the strain in the bedrock accumulates near the future focus of an earthquake, small cracks and voids develop in the rock. This microporosity makes the rock less rigid and more compressible, thereby decreasing the velocity of a P wave travelling through the material. This newly created microporosity allows groundwater to gradually move into the rock, filling the new pore space, re-saturating the rock, and increasing the pore pressure. Because the pores and pore throats are so small, the flow of this water takes some time to saturate the entire rock. Ultimately, however, all the small cracks and voids are filled with groundwater, which decreases the rock’s compressibility and returns the P wave velocity (or Vp/Vs ratio) back to normal. In this new state, the rock is weakened because of the higher pore pressure and the presence of groundwater filled pores, thereby allowing the earthquake to occur. The empirical relationship between the Vp/Vs anomaly time and the quake magnitude can be explained by: the larger the volume of rock undergoing strain, the longer it takes to re-saturate the microporosity, thus, the longer the Vp/Vs anomaly time.

Although most scientists today accept that earthquake prediction using precursor events is still some way off, there are many different avenues undergoing testing and research. One of the more promising efforts is monitoring the electrical resistivity of the subsurface rocks. Electrical resistivity is the inverse of conductivity. Most Earth materials are electrical insulators, not conductors, thus, they have a very high resistivity. However, when pore space is added to a rock, and this pore space is filled with water, the pore-water system allows electrical current to pass through the rock, thereby lowering the resistivity. If the dilatancy-diffusion theory of earthquakes discussed above is true, then monitoring of the resistivity should show a gradual decrease before the seismic event. Similarly, the development of the new porosity in the subsurface formations should decrease the water levels in the overlying groundwater systems. Russian scientists have also suggested that the radon


concentrations in the groundwater should increase before the earthquake due to the formation of newly exposed bedrock (by the opening of the microporosity) from which the Ra can be leached.

Historically, the most successful precursor events have been in the area of monitoring ground level changes. As strain accumulates, there is often a bulging in the surface of the land where the earthquake will occur. Although there have been over 70 occurrences of successful earthquake predictions by monitoring this landscape change, an interesting example is the well studied Palmdale bulge in California. Here, an area of 84,000km2 started to uplift in the early 1960s. By the mid-1970s the area had developed a 45cm bulge which lead to numerous predictions of an impending earthquake. However, since that time, the area has subsided about 20cm, and there has not been a significant seismic event.

In summary, true earthquake prediction over the short term is still not possible in most areas, despite intensive on-going research. To be useful, the prediction scheme must provide the time, place, and magnitude of the impending quake, and must be reliable and consistent, with few false alarms. The most successful earthquake prediction system is that of mainland China. Since 1966 the Chinese have successfully predicted eleven major earthquakes, undoubtedly saving the lives of hundreds of thousands of people. Unfortunately, despite this success, their system has lacked the important element of consistency. For example, in December 1974, an earthquake was predicted in the Haicheng area of China. Complete evacuation of several of the major urban centers took place, but there was no quake. Three months later another event was predicted in the same area; again evacuation took place, but an earthquake did occur. Six months later, yet another major earthquake occurred, this time with no prediction. Over 650,000 people were killed.

The social problems associated with earthquake prediction are severe.

• Predictions can give the public a false sense of security. In areas where predictions have been made with even a poor degree of accuracy, the urban development of the region has actually progressed at an accelerated rate because the public feels “protected.”

• There are many significant negative economic aspects associated with a false prediction. In scenarios worked out for southern California, an erroneous prediction would actually “cost” society more than the damage if a moderate quake did occur. For example, if a credible official or government office were to issue a statement that there would be a high probability of an earthquake occurring at some specified urban area in southern California sometime within the next several months, several things would happen immediately. There would be an immediate stop in the sale of earthquake insurance in that area. This would result in a decline in the number of mortgages lent, which would result in a net decrease in the economic base of the area. Similarly, property values would decline rapidly and most new construction would stop, both of which would lead to increased unemployment, a decrease in sales income for the region and tax

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income for the local government, a net reduction in services, and, ultimately, to people moving out of the area. An interesting implication of this from an environmental geoscience viewpoint is that these factors would feed back on one another, resulting in a lowered population, and, therefore, a lowered earthquake hazard risk for the area!

• Geologists and geophysicists who generate false earthquake warnings face legal prosecution in some states. In China scientists and public officials tend to be immune from lawsuits. However, this is not the case in North America. Several states, including California and Texas, have threatened to sue geologists who make false public predictions.


body wave catastrophe continental drift convection cell convergent plate boundary creep dilatancy diffusion model dilatancy instability model divergent plate boundary earthquake elastic rebound eplcenter fault fluid injection focus frequency hazard hypocenter

intensity

liquefaction lithosphere magnitude modified Mercalli scale plate tectonics precursor event/phenomena P-wave recurrence interval Richter scale risk selsmic gap seismic wave seismograph seafloor spreading subduction zone surface wave S-wave transform fault tsunami


1. What is the period of vibration of a building 100 feet high and 100 feet wide that is undergoing an earthquake of magnitude 6? of magnitude 8?

2. List three “groups” of hazards associated with earthquakes, and indicate which is usually the least hazardous in terms of loss of life.


3. Describe the motion of the two types of seismic body waves.

4. In North America, which type of earthquake poses the greatest threat: large, moderate, or small? Explain your reasoning.

5. The Rocky Mountain Arsenal subsurface waste disposal episode dramatically showed that humans have the ability to induce and turn off earthquakes. Using both words and equations, show why it is possible to induce this type of seismic activity.

6. List two ways of mitigating the earthquake hazards due to ground shaking in old, well-established urban areas.

7. In Canada, where is the area(s) of highest earthquake risk?

8. What is liquefaction?

9. What is the empirical relationship between the number of earthquakes and the magnitude of the earthquakes in a region?

10. An earthquake occurred in the Aleutians at 6 a.m. (Winnipeg time). When (Winnipeg time) will the tsunami generated by this quake hit Hilo, Hawaii? (Assume a uniform water depth of 5,000 m; show your work).

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Notes


Unit 6 Endogenic Geologic Hazards: Volcanoes

Topics Introduction

Volcanoes and volcanic activity

Hazards from volcanic eruptions

Volcanic hazard prediction, mitigation, and evaluation

Introduction Until only a relatively short time ago, environmental geoscientists considered most of Canada and continental United States to be free from volcanic hazards. Volcanoes were something more akin to tropical islands or the Mediterranean region. While it was certainly recognized that western North America had its share of dormant volcanoes, there was hardly any concern about active volcanic hazards. However, the dramatic eruption of Mount St. Helens in southwestern Washington on 7 March 1980, brought society (as well as the geologic community) out of this rather complacent attitude. In retrospect, there were actually many warning signs and indicators of increased volcanic activity in the Pacific Northwest: melting of glacial ice on nearby Mount Baker and Mt. Rainier; and increased geothermal activity in the Long Valley caldera, California, to name just a few.

On a global scale, the past several decades have provided examples of nearly every kind of volcanic hazard that society has to cope with. These recent experiences have shown that, although little can be done to prevent either volcanic activity or many of the hazardous geologic processes associated with volcanoes, warning and evacuation as well as proper land use planning in urban areas potentially affected by volcanoes can save lives and mitigate the loss of property.

Learning objectives As you already know, volcanoes, like many earthquakes, are closely associated with the interaction of lithospheric plates. Our overall goal in this unit is twofold: to outline, classify, and describe the various volcanic processes; and to apply these fundamental concepts to discussing the main types of specific hazards that affect human life and property and what possible solutions can be suggested.

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By the end of this unit you should be able to: • summarize the distribution of volcanoes in the world;

• describe the physical and chemical processes involved in volcanic eruptions;

• differentiate among the types of volcanoes;

• list the major types of hazards associated with volcanoes, and discuss their short-term versus long-term impacts;

• summarize the hazard potential of the Cascades/Pacific northwest;

• categorize the types of pyroclastic activity;

• explain caldera formation, and identify the potential for this type of hazard in North America;

• trace the history of volcanic activity at Mt. St. Helens; and

• cite examples of successful prediction and mitigation techniques.

Learning activities 1. Continue reading chapter 5 in your textbook and answering the review

questions, thought questions, and exercises on pages 156-157. 2. Read the study notes and answer the review questions in your course

manual.

Study notes Introduction

I am walking toward the only light I can see. I can hear the mountain rumble. At this very moment, I have to say, Honest to God, I believe I am dead.

(Television photographer at the base of Mount St. Helens, Washington, 18 May 1980)

Volcanic hazards occur infrequently and historically in North America have accounted for relatively low annual losses of property and life. However, eruptions can occur very suddenly and without warning. In the immediate vicinity of an eruption the economic impact can be significant. For example, the 1980 eruptions of Mount St. Helens in southwestern Washington, although killing only 70 people, caused some 3 billion dollars damage and property loss. Indeed, volcanic hazards pose particularly difficult problems due, in part, to the very fact that eruptions occur so infrequently, yet they have the potential for total disruption of the normal functioning of society. The effects of large volcanic eruptions in the past have caused major weather and regional climate changes, have been responsible for the destruction of cities and the abrupt collapse of entire civilizations, and have even been held accountable for changing the course of human history.


Volcanoes, possibly more so than any other geologic hazard, have a rich and colorful selection of legends, folklore, and myths. Even the term volcano, named after Volcano Island in the Mediterranean Sea (which was home of Vulcan, the blacksmith of the Roman god Jupiter), conjures up images of both worship and death/destruction. Every elementary school child knows about the eruption of Mt. Vesuvius in 79 A.D. and the annihilation of Pompeii.

On a world-wide scale, the past twenty years have provided examples of most of the hazards that can be expected from volcanic eruptions. In the 1960s, geologists eagerly watched (and filmed for the first time) the creation of a new island in the middle of the Atlantic Ocean: Surtsey. Several years later, in 1973, the nearby island of Heimaey, which was the principle center of Iceland’s commercial fishing fleet, was under siege by a variety of volcanic hazards, including lava flows, ash flows, poisonous gases, and explosively ejected material. The late 1970s saw the renewed volcanic activities in the Pacific Northwest: Mt. Baker, Mt. Rainier, and, in 1980, the eruptions of Mt. St. Helens. The early 1980s was also a time when, as now, geologists focused their attention toward a large area in eastern California known as the Long Valley caldera. Increased geothermal and seismic activity in the area led the U.S.G.S. to issue a formal warning of possible eruptions; fortunately an eruption did not take place. But the geographic extent of the affected area was (and still is) predicted to be much larger than that of Mt. St. Helens. The explosive eruption of El Chichon in southern Mexico in 1982 was not only noteworthy because of the large number of people killed, but it put into the atmosphere the single largest mass of volcanic debris this century. It is interesting to note that the atmospheric pollution from this one volcanic eruption has many times surpassed all of the artificial (man-made) atmospheric pollution of the past eighty years!

Volcanoes and volcanic activity Distribution Volcanoes are not distributed randomly across the surface of the Earth but rather, like earthquakes, are intimately associated with plate boundaries. Only approximately 2% of the surface of the Earth is of volcanic origin. About 80% of the 850 modern volcanoes in the world are concentrated in the “ring of fire” which surrounds the Pacific Ocean basin. Another 15% of the world’s volcanoes are found in the Mid-Atlantic Ridge area. The largest volcano in the world is Mauna Loa, which has a net height of some 8km. However, the largest eruption in historic times, in terms of amount of magma extruded, occurred in 1783 in Iceland where over 12 km3 of lava surfaced. In the not so distant geologic past much larger eruptions have occurred. The Long Valley-Mono Lake caldera last erupted about 700,000 years ago with some 600km3 of pyroclastic debris; the eruptions in the Yellowstone area of Wyoming-Montana about 600,000 years ago generated over 1,000 km3 of ejected material. There have been about 6,000 eruptions recorded in history.

As we learned in unit 4, in zones where plates are converging, such as along the western coasts of North and South America, volcanoes form in response to the subduction of the plate and ultimate melting of the lithospheric material in the

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deep subsurface. The magma that is produced can exploit a weakness in the overlying rocks, such as a fracture or a fault, and move upward. In contrast, in zones of divergence, such as along the Mid-Atlantic Ridge, upwelling currents in the asthenosphere create linear areas of extension in which new crustal material is being added to the plate.

Causes and processes When the magma extrudes at the surface, a landform is created. This landform is a volcano. Thus, a volcano represents the intersection or net result of two opposing groups of processes or forces:

• the endogenic processes that cause the build up and eruption of the magma; and

• the exogenic processes taking place at the surface which erode, transport, and redistribute the volcanic material.

The processes involved in magma generation and volcanic eruption are complex, and many are still poorly understood. Essentially, two rather obvious things have to be accomplished. First of all, the rock has to be melted to create the magma. Because of the temperatures and pressures required we know that this process is a very deep subsurface process. Secondly, the magma has to move upward rapidly enough to avoid cooling and resolidifying and must meet the near surface environment. As simple as these two concepts are, they play pivotal roles in controlling the composition, temperature, type of magma, and the timing and nature of the volcanic eruption. Geologists categorize the types of magma on the basis of the silica (or SiO2) content.

Type of magma Silica (SiO2) content

Basaltic magma Andesitic magma Dacitic magma Rhyolitic magma

less than 50% 50 - 55% 55 - 65% greater than 65%

Because magma is simply a very hot liquid, it can be discussed in terms of liquid properties. Viscosity is one of the key liquid properties of magma that controls its character when the magma arrives at the surface of the Earth. Although many physicochemical parameters help to determine the viscosity of a magma, one of the most important is the silica content. Magmas having low viscosity tend to flow very easily at the surface and form lava flows and lava fountains. This type of magma is usually basaltic or andesitic. In contrast, magmas with higher SiO2 content, such as dacitic or rhyolitic magmas, are much more viscous (i.e., very resistant to flow) and tend to form domes and spires at the surface. The composition of the magma is a function of the composition of the original rock that was melted to form the magma. In general, magmas from mid-ocean spreading centers tend to be derived from very deep seated mantle material and are basaltic. Magmas generated in association with


subduction zones, however, are derived from much shallower crustal rocks and are composed of andesites and dacites. Finally, magmas that form in continental areas (“hot spots” such as Yellowstone and Long Valley) are normally rhyolitic or dacitic. While the original rock type affects the composition of the magma, the composition of an erupting magma can change during the life of the eruption as different parts of the magma chamber are tapped by the volcano.

Because a volcano is a landform, it can be classified on the basis of its geomorphology. There are several basic forms of volcanoes. The shield volcano is characterized by very broadly sloping sides and is composed of many thin lava flows. It is usually associated with basaltic magmas. The islands of Hawaii are examples of shield volcanoes. Composite volcanoes, in contrast, have much steeper slopes and are characterized by much more ejected material (tephra) and fewer flows. Composite volcanoes (sometimes termed stratovolcanoes) in North America include Mt. Baker, Mt. Rainier, and Mt. St. Helens.

Hazards from volcanic eruptions Although each type of volcanic eruption can be dangerous and result in large losses of property, shield volcanoes, in general, are rarely life threatening. The main hazards associated with a shield volcano are loss of property due to burial under a flow. The risk to people caused by hazards associated with composite volcanoes is much higher, and ranges from volcanic ash falls and hot avalanches to floods and mudflows. The specific hazard risk varies greatly depending on the local geology, the climate, and the geography of the region.

Lava flows The popularized movie-version concept of a river of molten rock pouring from a volcanic vent and over-running everything in its path is actually quite a rare occurrence. In reality, lava flows can range from very slow moving (1 metre/day) to fast (up to velocities of 3 metres/second), but many travel at speeds no faster than a person can walk. The fastest part of the flow is located at the source. Flows are almost always restricted to low spots and valleys, and most cover distances of less than 10km. Single events are measured usually in terms of only a few square kilometres, but multiple eruptions over a period of years can give rise to lava fields many hundreds of square kilometres in area. While all lava flows, by definition, are composed of low viscosity magma (basaltic), the most fluid lava is termed pahoehoe, and the more rubbly and viscous flow is termed aa. Because they are easily identified and predicted, loss of life due to flows is rare. However, developed land and structures in the way of the flow are subject to complete burial and total destruction. Lava flows from Mt. Etna in 1928 buried the city of Mascalli, Italy. Similarly, in 1960, the town of Kapoho, Hawaii, was destroyed by flows from Kilauea. Although a lava flow cannot usually be stopped or diverted, the inhabitants of the village of Vestmannaeyjar on the Icelandic island of Heimaey successfully prevented a 20m high wall of lava from engulfing the town by spraying it with water, thereby cooling it and stopping the flow.

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Magma domes As the silica content of the magma increases, the extruded material becomes more viscous such that it may not flow at all but rather pile up at the vent. This can create temporary steep sided domes and spires, which are then susceptible to collapse and avalanching. The main eruption of Mt. Pelee in Martinique in 1902, in which 29,000 people lost their lives, was preceded by the erection of a 300m high lava spine.

Explosively ejected material Shield volcanoes are often characterized by nonexplosive eruptions in that the volcanic activity is not violent. In contrast, explosive eruptions pose a great many more hazards to the surrounding population. A large amount of fragmental material is commonly produced, both from the magma and from the disintegration of the country rock by the force of the explosion itself. This fragmental material, which can range from large boulders the size of houses to small silt-sized ash, is collectively given the name tephra or pyroclastic material.

In the subsurface, the magma, which also contains dissolved gases and liquids, is in equilibrium with the surrounding pressure and temperature conditions. As the magma begins to rise toward the surface, however, it is subject to lower pressure. Thus, any gases that might be in the magma try to escape in much the same way the dissolved carbon dioxide in a soft drink bottle will bubble out when you remove the cap (i.e., lower the pressure). The faster the pressure is removed, the more suddenly the gas expands and the more forceful the escape. In addition, the more viscous the magma is, the greater ability it has to retain the gases under a relatively low pressure differential. Thus, in very viscous SiO2-rich magmas, very little of the gas is allowed to escape until it does so (explosively) at the surface.

The risk of being hit by the larger fragments of tephra (bombs and blocks) is relatively small because these generally fall back directly onto or near the volcano. However, the finer ash (material smaller than 4mm in diameter) and lapilli (material of 4 to 32mm in diameter) can be distributed very widely. This ash forms the most severe hazard of volcanic eruptions. This fine grained tephra can endanger lives and damage property at considerable distances from the actual eruption. It effects visibility and, in some cases, peoples’ ability to breathe. In large volumes it can collapse the roofs of houses and make soils sterile. Probably the biggest problem associated with ash falls is that they are not confined, but rather their distribution is controlled by the prevailing winds. In fact, large explosive volcanic eruptions have the potential to significantly alter weather patterns over the short term and even change climate on a global basis. For example, it has been estimated that some 80,000 people starved over a two year period following the 1815 eruption of Tambora in Indonesia — the direct result of crop failures due to lower temperatures because of the large amount of dust in the atmosphere. The eruption of Krakatoa in 1883 is estimated to have caused a lowering of the average temperature of the Earth of about 0.5°C; while this may not seem significant, a lowering of only 2°C is


generally considered by climatologists to be of sufficient magnitude to start an ice-age.

Nuees ardentes (glowing avalanches) The pyroclastic debris that is shot upward into the atmosphere from the volcanic vent is buoyed up by a combination of very high temperatures and the continued release of gas. The cloud can be subdivided into two parts: • a lower gas-thrust zone; and • an upper convective-thrust portion.

As the cloud of pyroclastic debris rises it cools, with some of the material falling immediately back to Earth in the form of a density current. This density current is composed of a mixture of the volcanic ash, pulverized rock, and the still very hot gases. The pyroclastic debris flow that rushed down the flanks of Mont Pelee travelled at speeds as high as 160km/hr and was not necessarily confined to the topographic valley.

Geologists consider three types of glowing avalanches, subdivided on the basis of the geometry of the explosion:

• A Soufriere type is a simple fall back of ash and gas from the tephra cloud.

• A Merapi type is a density current created by the collapse of a large dome or magma spire.

• A Pelee type of nuees ardente is an eruption from a low angle vent or a sideways eruption.

As a generalization, these flows can overcome any topographic barrier within an area bounded by a line drawn at a 30° angle connecting the gas-thrust/convective-thrust contact of the tephra cloud with the ground. Beyond this 30° angle line, which is termed the energy line, the nuees ardentes are still a hazard, but they will not overtop hills and will be confined to valleys and low spots.

Gases A great variety of gaseous material is emitted from a typical volcanic eruption. Much of this material is simply water vapor and carbon dioxide. Some of the gases given off are extremely poisonous, and, therefore, pose considerable hazard to human as well as plant and animal life in the vicinity of the eruption. The most common deadly gases are hydrogen sulfide, carbon monoxide, sulfur dioxide, ammonia, and methane. In fact, a violent eruption does not have to occur to make the emission of these gases a hazard. For example, Lake Nios in Cameroon occupies a volcanic crater. Gases emanating from the crater gradually built up in the bottom waters of the lake. Upon mixing of the lake waters due to a landslide/earthquake, the toxic gases were released to the atmosphere, and, because they were considerably heavier than the surrounding atmosphere, flowed down the side of the volcano, causing the death of more than 2,000 people and many farm animals in the valleys below.

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Lahars A lahar, or a volcanic mudflow, is one of the most common and most destructive hazards associated with eruptions. In the 1980 Mt. St. Helens eruption, lahars caused considerable damage along the entire 50km length of Toutle River. The causes of volcanic mudflows are quite varied: they can be triggered by an eruption into a crater lake, by the breaking of a dam, by rapid and massive melting of ice and snow on the mountain, or simply by heavy rainfall on the freshly deposited volcanic ash.

Volcanic hazard prediction, mitigation, and evaluation With the renewed volcanic activity in the Pacific Northwest during the late 1970s and early 1980s, prediction, evaluation, and mitigation of hazards due to volcanic events have been given considerable attention in North America. Each specific volcano and volcanic area has its own unique set of hazards and risks. This variability is controlled not only by the specifics of the volcano (i.e., type of magma, type of eruption, type of volcano, prevailing wind direction, etc.), but also by the social and economic conditions of the settlements being affected by the hazards. For example, the eruptions of both Mount St. Helens (1980) and El Chichon (1982) were roughly similar in terms of physical magnitude, type of eruption, and so on. However, the effects of the two events were dramatically different. Because of the settlement patterns and largely agricultural economic base of the people in southern Mexico, the eruption of El Chichon was much more devastating in human terms than that of St. Helens. While this is more or less true for all geologic hazards, the great discrepancy in terms of impact between these two very similar geologic events underlines the fact that hazard identification and prediction is only the first step in environmental geoscience.

Volcanic regions, much like floodplains of rivers, are very attractive areas for human settlement and use. The volcanic ash and weathered lava often form the basis for excellent fertile soils, even in dry climates. Geothermal energy associated with the volcanic region is often exploited for power and home/industry heating. Tephra can be used as a source of aggregate and many other industrial purposes. Finally, volcanic areas supply some of the world’s most scenic terrain.

In most cases, simple identification of the volcanic region or area is not a problem. However, identification and prediction of the hazards and risk factor from the volcano are major problems. The most important component of this hazard identification is simple mapping. Ideally, a volcanic hazard map should show the areas of maximum risk, identify the types of hazards in the various areas, and assess the frequency or probability of the volcanic hazard. However, even once this mapping is completed, and responsible officials are aware of the risks, there are actually relatively few actions, short of complete evacuation, that are effective in mitigation of the hazards. Many of the adjustments that society can make for other geologic hazards cannot be employed against volcanic hazards. In most regions, volcanic events are so rare that land use zoning, avoidance, or insurance techniques are not feasible. Except in relatively slow moving lava flows, most engineering techniques are not practical. Society


can, however, prepare for an impending eruption by formulating emergency plans for evacuation and disaster relief.


active volcano ash andesitic magma basaltic magma caldera cinder cone composite volcano dormant volcano extinct volcano energy line fissure eruption hot spot lahar lapilli

magma magma dome magma spire Merapi type nuee ardente Pelee type phreatic eruption pyroclastics ring of fire rhyolitic magma shield volcano Soufriere type stratovolcano tephra viscosity


1. What is the Icelandic island of Heimaey famous for with regard to volcanic hazards?

2. Most of the damage and loss of life in St. Pierre, Martinique, associated with the 1902 eruption of Mt. Pelee was due to what?

3. Compare and contrast shield volcanoes, composite volcanoes, and volcanic domes from the standpoint of type of volcanic activity and magmatic composition, and give an example of each type.

4. List three ways in which a pending volcanic eruption can be predicted.

5. List two public service functions that volcanoes provide.

6. On Figure 6.1, identify the regions and features shown by the letters A, B, C, and D. What is the significance of the feature labeled A?

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7. a. What is Long Valley, California (in terms of environmental geoscience)?

b. Why is it a problem of particular concern (i.e., what is unique about this type of geologic feature/hazard)?

c. What has been happening there during the past decade?

d. When and where was the last feature like this active in North America?

8. Describe the Lake Nios disaster.

9. Summarize and briefly discuss the events during the spring of 1980 at Mt. St. Helens.

Figure 6.1


Unit 7 The Earth’s Cycles II: The Hydrosphere and the Atmosphere

Topics The hydrosphere

The water cycle Water reservoirs Oceans The cryosphere Other water reservoirs

The atmosphere Stratosphere Troposphere

Concluding remarks

Introduction The lithosphere, as we discovered in unit 4, interacts extensively with the atmosphere and the hydrosphere. These interactions have produced an environment that allowed life to develop and organic evolution to take place. In turn, the development of the ecosphere, together with the hydrosphere and atmosphere, has produced an Earth surface environmental system (i.e., the exogenic system) that has been relatively stable, but changeable, throughout Phanerozoic time. The stability of this system is maintained mainly by physical and geobiochemical processes, fluxes, and feedbacks that involve all the major environmental reservoirs of the near-surface of Earth. Continuing from unit 4, we will now introduce and briefly discuss the air and water environment of the planet as part of this interactive exogenic system.

Learning objectives The mechanisms of control and the specific links by which energy and matter flow through and are interchanged between the hydrosphere and the atmosphere are exceedingly complex; Earth scientists have only in the past few decades begun to realize the significance of these mechanisms and their Earth system implications. A comprehensive treatment of this vast subject is beyond our scope for this course. Our goal here is simply to introduce several of the major cycles and links by which material and energy are cycled through Earth’s hydrological and atmospheric reservoirs. Indeed, this global system can be viewed as a highly interactive entity at a variety of scales. We will concentrate on the hydrologic and atmospheric processes operating at a global scale. Later

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discussions will investigate various components of these large systems at a smaller scale (e.g., river flooding, coastal processes etc.).

After completing this unit you should be able to:

• explain how water is cycled through each of the reservoirs;

• list and describe the significance of each of the major storage areas in the hydrologic cycle;

• summarize the composition of the ocean;

• discuss the factors that control surface layer circulation in the oceans;

• describe and discuss the deep circulation system of the oceans;

• explain the importance of the cryosphere;

• differentiate among the various processes affecting heat transfer within and through the atmosphere;

• list the major components of the atmosphere in terms of relative abundance;

• discuss the structure of the various layers of the atmosphere; and

• comment on the aspect of residence time of CO2 in the atmosphere and hydrosphere.

Learning activities 1. Begin reading chapters 7, 8, 9, and 10 in your textbook and begin working

on answering the review questions, thought questions, and exercises at the end of each of these chapters. This reading assignment will extend over the next four units.


Study notes The hydrosphere The planet has had a hydrosphere for more than four billion years. Throughout this time, the size and shape of the ocean basins and ocean circulation patterns have dramatically changed because of the controlling factors of the lithosphere (i.e., seafloor spreading and continental drift) which, as we learned in unit 4, rearrange the configuration of land and ocean. Likewise there have been continuous changes in the composition of the atmosphere, including its water vapor and carbon dioxide content. All these changes affect the temperature and climates of Earth and indirectly affect the volume of the cryosphere (ice). As a result, sea level has risen and fallen during Earth’s history.


The water cycle Water circulating through the lithosphere, atmosphere, and ecosphere is part of a continuous hydrologic cycle. It is a dynamic system, with water stored in many places at any one time. The water cycle involves the transfer of water in various forms (liquid, vapor, and solid) through the land, air, and aqueous environments.

As we discussed in unit 4, both matter and energy are involved in the transfer within this open system. Solar energy heats the ocean and land surfaces and causes water to evaporate. The water vapor enters the atmosphere and circulates with the air. Over the Earth’s land surface, precipitation is more or less balanced by evaporation plus runoff, but for the ocean the situation is different. Most of the water evaporated from the ocean returns there directly; however, a small amount (about 8% of that evaporated) is carried by wind over the continents where it precipitates. Warm air together with water vapor rise in the atmosphere and cooler air descends. The farther from the Earth’s surface the warm air travels, the cooler it becomes. Cooling causes the water vapor within the air to condense and to fall back to the Earth’s surface as rain, snow, or ice. When the precipitation reaches the land surface, it can be evaporated directly back into the atmosphere, or it can run off in the form of streams, rivers or diffuse flow, or it can be absorbed into the near-surface Earth material, or finally it can stay more or less in place as frozen snow or ice. Plants requiring water can also absorb it, retaining some of the water in the organic tissue. The rest is returned back to the atmosphere through transpiration.

Water reservoirs Water on Earth is found in the reservoirs of oceans, glaciers (cryosphere), groundwater, lakes, soils, atmosphere, and rivers. Nearly all of this water (~99%) is located in the oceans and glaciers, and only a very small proportion is found in groundwater, lakes, soils, rivers, and atmosphere on the planet Table 7.1).

Table 7.1 Distribution of water in the Earth system

Reservoir Percent of total Volume (106 km3)

Ocean 97.30000 1,370.0000

Cryosphere 2.10000 29.0000

Groundwater 0.70000 9.5000

Lakes 0.10000 0.0100

Soils 0.00500 0.0650

Atmosphere 0.00100 0.0130

Rivers 0.00010 0.0017

Biosphere 0.00004 0.0006 Total 100.00000 1,409.0000

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Water is continually being moved from one of these reservoirs to another as part of the hydrologic cycle. Although the total amount of water in the different reservoirs remains nearly constant with time over the short term, it can change for various reasons over the longer term time scale. These changes have profound effects on the Earth’s total system. For example, we know from examining sedimentary rocks that the temperature of Earth can fluctuate on time scales varying from yearly to centuries to thousands to millions of years. Therefore, glaciers have undoubtedly both decreased and increased in size as a result of regional and global climatic change. A consequence of these fluctuations in the cryosphere is that the amount of water in each reservoir of the hydrologic cycle has changed over geologic time. On the time scale of tens of thousands of years, growth of continental glaciers took water from the ocean and sea level fell; the water reservoir of the cryosphere grew at the expense of the seawater reservoir of the oceans. The opposite was true when the glaciers receded.

Oceans Because of their overwhelming importance (both in terms of volume and Earth processes) let us take a closer look at a few aspects of the structure, composition, and circulation of the oceans. About 1.4 billion km3, or 97% of the water on Earth, is in the oceans. The oceans cover 71% of the surface of Earth (~60% of the area of the northern hemisphere is covered by water; 80% of the southern hemisphere is covered. The oceans reach a maximum depth of about 11km in the Marianas Trench associated with the subduction zone in the western Pacific Ocean; the average depth is 3.8km. The oceans receive their heat mainly from the sun. The heat is derived mainly from solar radiation striking the oceans in the equatorial region, and it is distributed around the planet by ocean currents.

Seawater is composed mainly of sodium and chloride ions (Table 7.2), although, in detail, it is an exceedingly complex solution with salts of nearly every naturally occurring element present. All atmospheric gases are found dissolved in seawater. As we noted in earlier units, carbon dioxide is an environmentally important gas because of its production through human activities. The present-day concentration of carbon dioxide in the atmosphere is approaching 360ppm, whereas in the ocean, 1 kg of seawater contains about 25mg of dissolved inorganic carbon (~25ppm). As we discussed in unit 2, some of this carbon is in the form of dissolved CO2 gas. The carbon dioxide in the ocean and the atmosphere can rapidly exchange with one another, leading to a situation in which the concentration levels in the lower atmosphere and in waters near the sea surface are nearly in equilibrium. When carbon dioxide enters the atmosphere from the burning of fossil fuels, some of this carbon dioxide can subsequently dissolve and be stored in seawater.


Table 7.2 Major dissolved components of seawater.

Ion Concentration (g/kg) Percentage

Chloride 19.353 55.290

Sodium 10.760 30.740

Sulfate 2.712 7.750

Magnesium 1.294 3.700

Calcium 0.413 1.180

Potassium 0.387 1.110

Bicarbonate 0.142 0.410

Bromine 0.060 0.190

Carbonate 0.016 0.050

Strontium 0.008 0.020

Silica 0.006 0.020

Boron 0.004 0.010

Fluoride 0.001 0.003

Overall, the oceans can be thought of as a two-layer system, with a thin, warm and less dense surface layer on top of a much thicker, colder and more dense deep layer. The boundary between the two layers is the thermocline, a zone of rapid decrease in temperature with increasing depth. The upper layer is usually well mixed by winds blowing across the sea surface, whereas the deep ocean is a region of relatively slow moving currents. This stable layering of the ocean makes the transfer of substances like gases and other dissolved materials between the two layers a slow process.

The atmospheric winds are the main driving force for surface ocean currents. The winds blow across the water and literally drag it along. The direction of wind currents is influenced by the rotational spin of the Earth (Coriolis effect; see below). There is little exchange of air masses between hemispheres. The Coriolis effect and the present-day continental coastal boundaries cause the surface currents of the world oceans in both hemispheres to flow in a generally circular pattern. Winds blowing from easterly directions in the ‘trade wind’ belts of the two hemispheres force water toward the equator. Here, the water flows converge and then move westward as the North and South Equatorial Currents. On the western sides of the oceans, the equatorial water flows separate and move north and south into the northern and southern hemispheres. The continents on the western boundaries of the oceans form barriers to the motion of the surface currents and help deflect these large flows of warm water northward and southward.

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As these surface ocean currents move poleward, they carry relatively warm and salty tropical water to the colder polar regions of both hemispheres. In the prevailing westerly wind belts (midlatitudes of each hemisphere), eastward current flow is helped along by these winds. This surface circulation is partially responsible for the transfer of heat from lower to higher latitudes. To complete the circle, waters return equatorward from the polar regions along the eastern margins of the ocean basins. The return of the cooler waters is not in strong flows, like the Gulf Stream, but rather is more spread out.

Clearly, any change in established currents, either air or water, can have an extreme impact on regional and global climate. The El Niño-Southern Oscillation (ENSO) in the Pacific Ocean, which we introduced in unit 2, and the resultant intrusion of warm and low salt content water along the South American west coast is a good example. Normally, cool currents flow north along the northwest coastline of Peru and Chile. However, about every three to seven years, an El Niño event occurs that lasts up to several years. During this time the warm waters of the western Pacific extend far to the east and abut against the coast of central South America. The warming of the coastal waters interferes with the upwelling nutrient-rich waters, and nutrients do not reach the shallow, euphotic zone of the ocean. Phytoplankton productivity slows, and the zooplankton that feed on the plants virtually disappear. The local fishing industry suffers, the staple food supply of the local bird populations is lost, and the birds die off or leave the area. Atmospheric/oceanic changes accompanying ENSO events can affect circulation patterns and weather around the world and are responsible for droughts in Africa and heavy rains and flooding in Asia as well as in North and South America.

The deep currents of the oceans are not directly affected by winds but are instead driven by changes in temperature and salinity. Seawater becomes denser as its salinity increases. This causes more saline water to sink below less saline (less dense) water. Similarly, warm water of the same salinity is less dense than cold water and will rise relative to the cooler, heavier water. The deep currents of the oceans are driven by these basic thermohaline density relationships and, in contrast to surface currents, do flow between hemispheres. The flow is similar to that of a conveyor belt. Cold atmospheric conditions in northern Canada cool the waters of the North Atlantic and initiate the movement. Dense, cold, salty water sinks in a region just east of Greenland in the Greenland and Norwegian Seas and flows southward at great depths into the South Atlantic. There it meets and mixes with cold and even denser, northward flowing water from the Weddell Sea off Antarctica. This deep water flow then turns east around the tip of Africa and, continuing its deep passage, heads toward the Pacific Ocean. It flows into the Pacific along the western margin of the basin where it is deflected eastward by the presence of the Asian continent and the Earth’s rotation. As this current moves through the Pacific Ocean, however, the water is gradually warmed, becomes less dense, and, therefore, slowly rises toward the surface. The warm surface waters then move south across the Pacific Basin and turn west toward the South Atlantic and finally northward into the North Atlantic to complete the cycle. This cycling of the deep ocean current takes about a thousand years. Presently, this conveyor belt circulation pattern is


driven to some extent by an imbalance between the evaporative loss of water from the Atlantic and its gain by precipitation and continental runoff.

The Cryosphere Another major reservoir of water is the cryosphere. However, the mass of water in this reservoir is small compared to that in the oceans, with only about 2% of the water on Earth residing in snow, ice, and frozen ground. Glaciers cover approximately 10% of the land area of the Earth today. There are many types of glaciers such as valley, mountain-top ice cap, ice field, ice sheets, and ice caps. Glaciers form when moist air allows snowfall in the winter to accumulate, leading to the formation of ice. The ice can build up to great thicknesses and eventually a mountain or continental glacier can develop. These thick ice accumulations flow downhill under the influence of gravity. The rate of flow is generally slow (cm/yr) but can be measured in tens of m per year. In the northern hemisphere, glaciers are responsible for tremendous amounts of erosion.

Ninety-five percent of all glacial ice is found in the polar regions in the form of ice sheets. These are the largest glaciers on Earth and spread out over the continents. Ice shelves that float on the oceans may be attached to continental glaciers. In the past, ice sheets covered large portions of North America and Europe, but the only ice sheets left today are in Greenland (up to 3km thick) and Antarctica (3.6km thick). With the discussion about global warming during the latter part of the 20th century it is interesting to speculate about the melting of these ice sheets. If the Greenland and Antarctica ice sheets were to melt, sea level would rise by about 60 metres. Melting of all the rest of the ice on the planet would increase sea level an additional 6 metres.

In contrast to the Antarctic ice sheet, the Arctic Ocean in the northern hemisphere is covered by only a thin layer of sea ice. Sea ice differs from glacial ice in that it is frozen seawater and, of course, it does not move downhill. When it melts or when it forms, sea level is not affected.

Other water reservoirs Only about 0.7% of the Earth’s water occurs as groundwater. Groundwater flows very slowly in the subsurface. It enters the subsurface through soils and permeable sedimentary rocks at the surface then seeps into the ground under gravity. If this water in the subsurface is in rock or sediment strata through which it can pass easily the unit is said to be an aquifer. Wells may be drilled into these aquifers, and the water can be recovered by pumping or by free flow as in an artesian well. For example, water entering rock strata at the surface in Alberta can be obtained below ground over 1,000km away in Manitoba and Saskatchewan by drilling. In Hawaii, an extensive groundwater lens provides water for the island of Oahu. Groundwater is a major source of water for human consumption even though it comprises a small percentage of the hydrosphere. It is a natural resource that may be rapidly depleted and easily contaminated by the waste products of human society. Not all groundwater is suitable for human use, however, because the groundwater is also capable of leaching elements

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from the soil and rock that it passes through thereby increasing salinity during its flow.

The small remaining portion of the water in the Earth system is found in lakes (0.01%), soils (0.005%), atmosphere (0.001%), rivers (0.0001%), and the biosphere (0.00004%). These water reservoirs, although small in size, are an integral part of the hydrologic cycle, and their waters are generally relatively fresh, although variable in chemical composition. Rain contains dissolved salts and gases only at the parts per million level, whereas the average river water has about 0.01% of dissolved salts, mainly as dissolved calcium and carbonates. Soil waters have salt concentrations like those of rivers, but the waters of lakes are quite variable in composition, ranging from freshwater like the Great Lakes to salty brines like the Great Salt Lake and the Aral Sea. Rivers are the major source of water as well as dissolved and solid materials for the ocean. The atmosphere acts as a conveyor of water from the ocean back to the land. The soils collect water from the atmosphere as rainfall and are temporary repositories for water that will make its way into deeper groundwaters. Large lakes, like the Great Lakes, are temporary storage basins for waters on their way to the oceans and moderate regional climate. As with groundwaters, soil, lake, and river waters are easily modified by human activities and contaminated by effluents from industrial, agricultural, and household sources.

The atmosphere The Earth’s atmosphere extends approximately 500km above the surface of the planet. The atmosphere has been evolving since Earth was formed. The air is a mixture of gases constituting air and is currently composed of about 78% nitrogen, 21% oxygen, and 0.03% carbon dioxide, with a number of trace gases (Table 7.3).

Table 7.3 Composition of dry air

Constituent Percentage

Nitrogen 78.0840000

Oxygen 20.9470000

Argon 0.9340000

Carbon dioxide 0.0360000

Neon 0.0018180

Helium 0.0005240

Methane 0.0001700

Krypton 0.0001140

Hydrogen 0.0000530

Nitrous oxide 0.0000310


Constituent Percentage

Xenon 0.0000087

Ozone trace to 0.0008000

Carbon monoxide trace to 0.0000250

Sulfur dioxide trace to 0.0000100

Nitrogen dioxide trace to 0.0000020

Ammonia trace to 0.0000003

The water vapor content of the atmosphere varies from several percent in warm, humid environments to tens of parts per million in cold, dry conditions. The trace gases, although small in concentration, are important. For example, ozone shields surface life from detrimental UV radiation and contributes to the natural greenhouse effect that moderates climate.

The atmosphere is the most rapidly changing of the three physical systems, but it is a well-balanced system and one of the most dynamic. Residence times are very small relative to the other systems. The atmosphere, along with the ocean, is a heat distribution system that distributes incoming solar radiation throughout the globe and drives the climate system of the planet.

Stratosphere The stratosphere extends from about 12 to 48km above Earth. Harmful incoming ultraviolet radiation from the sun is absorbed by ozone in the top two-thirds of the stratosphere between approximately 24 and 48km above Earth’s surface. This region is referred to as the ozone layer, which is so important to the protection of life on the planet. When ultraviolet light from the sun reaches oxygen molecules in the stratosphere, the molecular oxygen (O2) is broken down into free oxygen (O) atoms. The necessary energy for this reaction is supplied by the absorption of solar ultraviolet radiation. The oxygen atoms unite with other oxygen molecules to produce ozone (O3).

The over-production of ozone in the natural atomospheric system is kept in check by the destruction of ozone by other gases, such as nitrous oxide (N2O), that are emitted from Earth’s surface. If this destruction did not occur, ozone would continue to build up in the stratosphere and would soon overwhelm atmospheric composition. Before human intervention in the natural system, the production and destruction of ozone were in balance.

The absorption of ultraviolet light by ozone produces a warming of the stratosphere, such that its temperature is above that of the troposphere immediately below it. The warmer stratosphere creates a stable situation with less dense air on top of more dense air and acts as a lid, trapping the cooler weather-generating system of the planet in the troposphere below. Materials that do reach the stratosphere, like gases and aerosol particles from volcanic eruptions, nuclear explosions, or high-flying airplane exhausts, may stay trapped in this layer for years and affect the temperature of Earth.

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Troposphere The lowermost atmospheric region, the troposphere, extends from about 12km above Earth down to its surface. Here, the weather system of clouds, surface winds, and water vapor circulates the planet. The temperature of the troposphere is higher than would be expected for a planet of this size. This is because of the influence of greenhouse gases. When short-wavelength solar radiation that is not intercepted by the outer atmosphere or the ozone layer penetrates to the surface of the planet, it is re-radiated back as energy of a longer wavelength. Carbon dioxide and other greenhouse gases absorb and trap this longer wavelength infrared radiation, leading to a natural warming of Earth’s surface and the lower atmosphere.

The amount of carbon dioxide residing in the atmosphere affects the amount of heat retained in the atmosphere, and this heat retention, in turn, affects the climate of Earth. The more carbon dioxide, the warmer the climate. Nitrous oxide, water vapor, methane, and other gases have effects similar to those of carbon dioxide in controlling the amount of heat retained in the atmosphere. This overall process has been called the natural greenhouse effect, although the trapping of heat within the glass enclosure of a greenhouse actually is due to a different process. Without greenhouse gases, the planetary troposphere would be about -20oC - some 35oC cooler than its present average temperature of 15oC.

The troposphere is the well-mixed region of the atmosphere. It is a turbulent region because it is mainly heated by the sun’s energy at its base, the Earth’s surface, thereby creating an unstable density contrast. In the lower troposphere, the air temperature is generally warmer near the surface and cooler above. The additional warming near the surface can make the lower layers of the troposphere less dense than overlying air. This leads to vertical convection in which lighter air rises and denser air sinks. Also, the distribution of heat on the surface of Earth is not uniform. Land masses and their elevations and oceans distribute heat differently and interact with and interfere with air circulation patterns. In addition, the equatorial belt is closer to the sun and receives more radiation per unit of area than is received at the polar regions. This causes warm equatorial air to rise and move toward the poles and the cooler air from the poles to move toward the equator. The warm, moist, rising air contains much water vapor that condenses and forms large cumulus clouds extending up into the high troposphere.

The final component affecting the large scale circulation of the atmosphere is a phenomenon produced by the rotating Earth. The Coriolis effect modifies what would otherwise be a very simple wind circulation pattern. The effect is due to the rotation of the planet and causes air or water at the surface to veer to the right of the direction of the force that is causing the motion in the northern hemisphere and to the left in the southern hemisphere. Thus, instead of simply travelling directly north-south toward the equator, the trade winds approach the equator form the northeast and the southeast.


Concluding remarks Processes in the atmosphere and hydrosphere are interconnected. The ocean and atmosphere redistribute the heat received at the planetary surface. Warm air and water are transported toward the poles from the equatorial region by atmospheric and oceanic circulations. As part of this process, heat and water are exchanged between the two large fluid reservoirs of Earth. Cool air and water are returned toward the equator by low-level atmospheric winds and surface ocean currents. The winds drive the surface currents of the ocean, and in turn, heat exchange between the ocean and atmosphere helps generate the wind systems of Earth.

Because of the very short residence times, the atmosphere is an easily perturbed Earth system. Many gases in the atmosphere are found only in trace concentrations. These trace gases have sources at Earth’s surface. It must be realized that natural variations in the gas fluxes associated with these sources can lead to changes in atmospheric composition on a relatively short time scale. Similarly, fluxes of these trace gases to the atmosphere from human activities can also rapidly modify the chemical composition of the atmosphere. Such changes in atmospheric composition can result in climatic change.

The water cycle of the Earth is complicated. Water may be salty or fresh; it may be warm or cold; it may be of high density or low density; and it may be clear or opaque. Each of these factors affects the movement of the water and its influence on the Earth system.

During one residence time of a substance in a reservoir, the material may be mixed repeatedly. For example, small lakes may be completely mixed many times in 100 years by currents generated by winds blowing across their surfaces. However, in 40,000 years the ocean would be mixed only about 25 times. Surface water systems are very susceptible to contamination by chemicals derived from human activities; the impact of contamination depends to a major degree on the residence time of the system. Atmospheric chemical changes and climatic change can also affect the properties of surface waters by modifying their temperature, circulation, and chemistry.


Atmosphere carbon dioxide coriolis effect cryosphere deep circulation enso equatorial current gulf current

greenhouse effect groundwater hydrologic cycle ocean ozone stratosphere troposphere

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1. Why is it said that the troposphere is unstable and the stratosphere stable?

2. Why do temperatures in the troposphere decrease upward?

3. Why do temperatures in the stratosphere increase upward?

4. The total precipitation over Earth’s surface is 496,000km3 per year. The amount of water vapor in the atmosphere is 13,000km3 . If there was no replenishment of water vapor in the atmosphere, how long would it take to remove all the water from the atmosphere?

5. Dust is susceptible to being rained out of the atmosphere. From your answer to question 4, would you expect dust to be evenly mixed throughout the atmosphere?

6. What are the four major gases in the atmosphere? List three trace gases that have Earth’s surface as their source for the atmosphere.

7. Discuss briefly the pattern of the trade winds, their origin, and their relationship to ocean currents.

8. Why is the long-wave, infrared radiation that is re-radiated from Earth’s surface so important to climate?

9. What are the seven most abundant elements in seawater?

10. Average seawater is primarily a sodium chloride solution. Seawater contains about 35 grams of salt per kilogram of seawater, of which about 19 grams are chloride ion. What is the weight percent of chloride in seawater salt?

11. Several major shipping lanes cross the North Atlantic Ocean. If ships accidentally or deliberately discharge oil or plastics in the ocean, how would you expect these materials to circulate?

12. What is the conveyor belt circulation pattern of the world’s oceans?


Unit 8 Floods

Topics Historical aspects of flood hazard

Definition of flood hazard

The fluvial setting

Causes of floods

Controls of flooding

Geologic factors in flood analysis and control

Flood analysis

Flood prevention and mitigation of losses

Introduction Historically, floods have been one of the most destructive natural geologic hazards in North America. Although much is known about the causes and controls of floods, risk due to flood hazard has actually increased dramatically on a global scale. In the decade between 1970 and 1980, flooding killed an average of 50,000 people per year. The study of flooding and flood analysis is not a new subject. For many years, however, this study has been fragmented and treated in many different ways by different groups of physical and social scientists. Our approach in this section will be to bring together these often quite different views and perceptions of floods so that application of the scientific concepts and data can be done in a uniform and holistic manner.

Learning objectives Floods are the most widely distributed and one of the most destructive geologic hazards facing society. Each year for the past decade damage due to floods in North America alone cost more than one billion dollars. Some of these floods are the result of spectacular or unusual events such as dam failures or hurricanes. Most, however, are the result of normal and predictable natural functioning of streams and coastal waters.


• outline the various physical components of a river system;

• assess the impact of floods on society over the past century;

• define a flood from an engineering perspective, and discuss how this use of the term might be different than society’s general perception of a flood;

• identify the various causes of floods;

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• describe, in general terms, how human activities can increase flood hazard;

• summarize the role of urbanization in flooding;

• review the pros and cons of channelization with respect to flood hazards;

• describe the main problems arising out of dam and reservoir construction to control floods;

• discuss how various geologic aspects of the drainage basin affect flood magnitude and intensity;

• construct and use a rating curve;

• interpret a unit hydrograph;

• calculate the recurrence interval of a flood;

• plot a flood frequency curve and extrapolate this curve to predict a “hundred-year” flood; and

• differentiate upstream watershed management control strategies from downstream techniques of flood control.

Learning activities 1. Continue reading chapters 7, 8, 9, and10 in your textbook (with emphasis

for this section on pages 191-229) and working on the review questions, thought questions, and exercises at the end of each of those chapters.


3. Complete assignment 3 and send it to the Distance Education Program Office.

Study notes Historical aspects of flood hazard

It is stupid to sleep in the flood plain.

(Mayor of Rapid City, South Dakota, after the 1972 flash flood which killed 242 people and caused 200 million dollars damage quoted in J. E. Costa, & V. R. Baker, Surficial Geography [New York: John Wiley & Sons, 1981], 390.)

Like earthquakes and volcanoes, flooding is a natural and recurrent geologic event. River flooding is one of the oldest geologic hazards to affect people and, today, is among the most costly in terms of property loss. Humans have always been attracted to floodplains for several obvious reasons: a constant supply of fresh water and often food, a convenient mechanism and avenue for waste disposal, a means of transport, the presence of fertile soils, and large expanses of relatively flat terrain and “usable” land. As a result of this historic competition between humans and the river to occupy a floodplain, there is a long list of catastrophic events involving humans and floods.

Floods have been a major concern for humans throughout history. The ancient civilizations built floods into their religion and legends in much the same way


they did volcanic eruptions and earthquakes. Probably the earliest recorded flood was that documented in the Bible; the entire “known world” (i.e., the Tigris-Euphrates River valley) was covered with water for nearly six months in about 3000 B.C. More recently in 1887, more than 800,000 people died in a single flood event along the Hwang Ho River in central China.

However, human concern with floods has not always been from a negative standpoint. The early Egyptians depended on the annual flooding of the Nile River to help irrigate the land and supply nutrients to the soils. In the 1500s the explorer De Soto used the flooding on the Mississippi River to help with his navigation of the lower part of the river system. Today, however, flooding is generally associated with loss of property and life. Until the 1989 Los Prieta earthquake, the single most destructive natural hazard in North America was the flooding in the East Coast area of United States caused by Hurricane Agnes in 1972. Although only about 100 deaths were directly attributed to this series of floods, the event caused an estimated four billion dollars in damage. Another flood associated with a hurricane that hit the Texas coast in 1900 was responsible for North America’s most severe flood in terms of fatalities when over 6,000 people were killed in Galvaston. The most costly flood ever to hit North America in terms of property loss was the extensive flooding of the Upper Mississippi River drainage basin during the summer of 1993 that caused over 12 billion dollars in damage.

One of the major points to be learned from the historical perspective of flood analysis is that the flood hazard is the most ubiquitous geologic hazard; it annually affects more people than all other hazards combined. In North America, about twenty million people in over 40,000 communities are adversely affected by flooding. The average death toll in North America due to floods is only 200 per year, but the annual dollar value loss is about four billion dollars. Of the “declared” natural disasters in North America in the last 50 years, over 85% have been flood related.

Although widespread floods can be caused by entirely natural causes, today the probability of humans being adversely affected by floods and the risk due to flood hazard is much greater than in the past. As we learned in unit 1, this is because of the combined factors of increased population living in flood-prone areas (i.e., floodplains) and the negative feedback mechanisms of other human activities such as river control, agricultural practices, weather/climate modification, and landscape changes.

Definition of flood hazard I asked a man building a new apartment building on the Boulder flood plain if he thought there was any risk of flood. He replied he had no risk at all. I asked him if he knew about the flood of 1894. He said, of course, he knew about it, the flood of 1894 had come up chest-high where he was standing. How then, I said, could there be no risk. Well, he said, there will be no risk, because he would sell the building within six months.

(G. F. White, “Prospering with Uncertainty,” in Floods and Droughts, in The Proceedings of the Second International Symposium on Hydrology, edited by E.

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F. Schultz, V. A. Koetzer, and K. Mahmood [Ft Collins, CO: Water Resources Publications, 1973], 14.)

A flood is very simply and obviously defined as any abnormally high water level. Within a river context, a high streamflow that overtops or threatens to overtop the natural and/or artificial banks of the channel is a flood. River floods can also be defined in hydraulic or engineering terms: a stream is considered to be in flood stage if the flow is above a certain predetermined level or datum. This datum is usually defined on the basis of potential damage to human or environmental conditions such that the flow above this datum will cause damage to a structure or contribute to hazardous environmental conditions for human occupation in the absence of protective works. Similarly, in coastal regions, flood-prone areas are often “artificially” defined on the basis of elevation with respect to historical levels of high tides, storm run-up, maximum tsunami levels, etc.

Even within the backdrop of massive destruction and devastation discussed above, floods have several positive aspects. Most importantly, floods are usually more predictable than other natural hazards. Consequently, the science of flood analysis and prediction is much more accurate than for other hazards. Also, in striking contrast to many other features such as earthquakes and volcanic eruptions, large floods are usually rather gradual events, taking place over several days or even weeks, rather than a few moments. There are, of course, important exceptions to these generalizations; flash flooding caused by unusual meteorological events (such as the 1972 Rapid City disaster) are difficult to quantitatively assess beforehand; downstream flooding caused by failure of a dam (e.g., Vaiont dam, Italy, or Teton dam, Idaho) is neither predictable nor easily quantified in terms of potential damage.

Unfortunately, despite these distinct predictability advantages, the scientific and political world of flood analysis, prediction, and control is awash with controversy. These controversies center around several main points:

• multi-government conflicts involving who or which agency of the government is responsible for flood prediction and/or prevention;

• basic scientific “philosophy” regarding the merit of one large flood control structure (dam) versus many small structures;

• the inevitable negative feedback mechanisms and effects of flood control structures on both downstream areas and areas above the dam; and

• economic assessments of the benefits of floods and maintaining a balance between the dollar value losses incurred by flooding versus the benefits gained by society in allowing the floods to occur.

The fluvial setting Discharge of water in rivers varies greatly both spatially and with time. Most rivers occupy a distinct channel bounded on each side by slightly raised ridges termed levees. The levees slope gradually away from the channel and grade laterally into an adjacent flat, level plain called a floodplain. The floodplain


extends to the steeply sloping valley walls of the fluvial complex. In this type of river, which is characteristic of most of the fluvial systems of central and eastern North America and Europe, natural flood stages typically occur an average of once every 2.5 years. One of the most critical concepts to be remembered about the geologic hazard of river flooding is that periodic overtopping of the natural levees by the flow is an important natural feature of the fluvial processes of rivers. Indeed, as we will discuss later, preventing this natural flooding by various “river training” schemes (e.g., artificial levees, channel straightening) can actually lead to much more severe flood problems farther downstream.

Much of the classic hydrological and hydrogeological work that has been done on rivers over the past 50 years has been directed toward the types of fluvial systems which are located in a characteristically “north-temperate,” relatively humid climate, such as occurs in eastern North America. Indeed, rivers in other climatic regimes are so poorly studied that there is a danger that the practicing environmental geoscientist might accept the concepts and principles of humid fluvial systems as representative of all rivers. This is clearly wrong; hydrologic conditions and flood fluctuations on rivers located in, for example, the arid western part of the continent or in rugged mountainous terrain are considerably different from those of the eastern North America or Europe region. It must be re-emphasized that the concepts of flood cycles and prediction discussed below are, in general, applicable only to rivers located in typical north-temperate climatic regimes.

Causes of floods Although the origins of floods are extremely varied, the causes of flooding can be grouped very simply into: • normal geologic causes; and • human-induced causes.

Within the normal geologic realm, the most common cause of flooding is simply excess precipitation. Rainfall over a relatively short period of time can be of such magnitude that the storage capacity of the river channel is exceeded, and the flow tops the natural levees resulting in a flood. Similarly, in the northern regions of North America, flood probability is greatly increased when heavy snowfall, which has accumulated over the winter, is combined with unseasonably warm spring weather. The flood situation is further enhanced in rivers that are flowing northward from relatively warm climatic regimes into areas in which the ground may still be frozen. The frozen ground is impervious to infiltration thus preventing any seepage loss of the melt-off flood water. The periodic flooding on the Red River in North Dakota, Minnesota, and Manitoba is a good example of this situation.

River ice can temporarily partially block the normal flow of water resulting in an impoundment behind an ice dam. Obviously, this ice dam is temporary; when the threshold is reached or when the dam is broken, downstream flooding will take place. The Saskatchewan and Red Deer rivers in Alberta frequently

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are subjected to this type of flood event. Likewise, landslides falling or flowing into a river valley can block the river and impound water behind the slide material. Eventually, the landslide barrier will be overtopped or broken resulting in a downstream flood. In 1925 a landslide in the Gros Ventre River valley south of Yellowstone National Park blocked the river, creating a 50km2 lake behind the dam. When the landslide was finally topped two years later, rapid erosion of the dam took place resulting in very high water levels in the river valley below the dam. Glaciers can cause flooding in much the same way. Indeed, some of the most spectacular flood events observed occur in association with glacial ice blocking a river valley. These types of floods, termed jokulhlaups, are particularly common in Alaska and Iceland. One of the most famous is the jokulhlaup associated with the catastrophic draining of Lake George in June-July of each year on the Knik River of western Alaska.

In addition to these “natural” causes of floods, many types of human activities can also lead to increased flood hazard or, in some cases, even initiate a flood itself. One of the most common ways in which man can increase the flood potential of a river is by occupying the floodplain. Not only does human occupation of floodplains increase the flood hazard risk by putting more people in direct contact with the river and its natural flood cycles, but the construction of buildings, parking lots, streets, and roads associated with this occupation makes the ground less permeable. Thus, precipitation from storms and snow melt is less likely to percolate into the soil and will more likely run off directly into the fluvial channel. The 400% increase in annual property losses and loss of lives attributed to floods over the past four decades is directly related to this increased urbanization of floodplain areas. In North America, about 20% of all urban areas are flood-prone, however, over 50% of all flood-prone areas are urbanized riverine floodplains.

Likewise, deforestation of a river’s drainage basin will similarly lead to increased runoff and increased levels of flow in the river during floods. There is a direct correlation between the frequency and size of floods in a drainage basin and the amount of deforestation in the drainage basin; the more stripping of vegetation from previously forested slopes that occurs, the larger the size of the floods and the more frequently the floods occur. In much the same way, mining activities in a drainage basin have been shown to increase the peak flows during floods.

Human occupation of the area near a river also increases the flood hazard by channelization or confinement of the river. Within an urban area it is, of course, desirable to have the river stay in the confines of its channel. This is usually accomplished by raising the level of the levees above the maximum elevation of the flood. However, this produces higher flood levels in downstream areas. In addition, because the river will continue to aggrade its channel and levee system, it will be necessary to continue to raise the levels of the artificial levees. Over the long term, this will result in the river being perched considerably above the surrounding urbanized floodplain as is the case in many of the cities on the lower reaches of the Mississippi River. In this situation, even a minor failure of one of the artificial levees during a flood can be devastating.


The subject of dam construction and geoengineering related to fluvial hydrology will be discussed in more detail in a later section. Because of either faulty construction or poor understanding of geologic conditions, dam failures are an additional cause of floods. This is ironic because dams are also one of the most common ways of controlling natural flooding on a river and in mitigating the losses and damage due to periodic high water levels. Dam failure flood hazard is particularly problematic because it is unpredictable.

Controls of flooding Over the past several centuries of flood control in North America the government has nearly always played a major role. Some of the earliest control efforts involved attempts to curb the periodic flooding of the lower Mississippi River. As early as 1700 the local city government of New Orleans constructed several kilometres of artificial levees, some as high as 6m above the level of the river. Beginning in the mid- and late-1800s and continuing through the 1930s, the United States Army Corp of Engineers, under the direction of the Mississippi River Commission, undertook construction of some of the most elaborate schemes of flood control structure complexes up to that time on the lower Mississippi River. In the 1930s the Tennessee Valley Authority (TVA) was created. This government body, charged with maintenance of water resources for a large drainage basin in eastern United States, became a model for much of the rest of North America and the world.

The most important aspect related to the control of floods is flood forecasting. The key elements in forecasting are:

• assessment of the meteorological factors, such as rainfall intensity and duration, snow melt, and evapotranspiration; and

• evaluation of the physical characteristics of the drainage basin, such as channel storage, infiltration characteristics, slope, and geomorphology of the basin.

Control strategies are grouped into two basic types:

• Transient strategies attempt to monitor, understand, predict, and possibly even control some meteorological factor, such as amount and type of precipitation, and location and quantity of snowmelt.

• Permanent control strategies involve adjusting some key physical feature or characteristic of the basin. This is usually done by construction of dams, diversions, channels, etc.

One of the major difficulties in flood control is the feedback concept: “everything affects everything else.” It is essential that the various feedback mechanisms be properly worked out before attempting any type of flood control scheme. Unfortunately, there are many examples in which this type of pre-control investigation has not been carried out resulting in severe and, in some cases, catastrophic consequences.

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Geologic factors in flood analysis and control Floods caused by storm and/or snowmelt runoff are significantly affected by a number of important geologic considerations of the drainage basin.

Land use of the basin As discussed above and reviewed in your text, there is a striking increase in water yield, or the amount of precipitation/snowmelt making its way to the channel, when the drainage basin is changed from a largely forested/vegetated area to an urbanized one.

Surficial material The type of surficial material making up the drainage basin has a direct and obvious impact on the amount of runoff that occurs in a storm event. Impermeable material, such as consolidated bedrock or clay, does not allow the runoff to percolate into the shallow subsurface thereby augmenting the flooding. Porous and permeable material, such as sand or fractured bedrock, permits infiltration and temporary storage of much of the runoff generated by the storm or melt. In general, the most important features of the substrate that affect the infiltration characteristics of the terrain are: • permeability; • thickness; • moisture content (or position of the groundwater table); and • amount of animal or root bioturbation.

Size of basin For any given storm or melting event, there is an inverse relationship between the area of the drainage basin and the magnitude or size of the flood. In other words, there is a decrease in flood magnitude as the drainage basin size increases. This is due to the fact that in a larger basin it takes a longer time for the flood to reach a given point. With time, there is a greater chance of decreasing the magnitude by temporary storage, evaporation, or simple dissipation.

Basin slope and orientation The physical orientation and the geomorphology of the basin are key elements in dictating the size of a flood in a small drainage basin. In general, basins characterized by steeper slopes have higher floods than those with less steep slopes because the high slopes increase the flow rate and prohibit infiltration. Similarly, in the northern hemisphere, north facing slopes remain frozen longer in the spring, thereby decreasing the infiltration capability of the soils.


Flood analysis Flood magnitude Of the many physical characteristics of floods and drainage basins that must be quantified for magnitude, the most important is the volume of water. The total flood runoff volume controls: • the area of the flood; • the duration of the flood; • the peak flow during the flood; and • the lag time, or the time between the rainfall event and the flood at a given

point in the drainage basin.

River flows are generally measured in volume of water passing a certain point per unit of time. Units are normally cubic metres per second (m3/sec) in Canada or cubic feet per second (usually abbreviated cfs) in United States (1 cfs = 0.0283 m3/sec). Thus, the discharge of water (Q) at a given point in the stream is equal to the average velocity of the flow (V) multiplied by the cross sectional area of the stream (average width, W, times average depth D):

VDWQ ∗∗=

By measuring the stream discharge at a single point several times during the year and plotting this data on a graph, a rating curve is generated. A rating curve simply shows the stream discharge (at a certain point) versus the level or elevation of the stream. Once a rating curve has been prepared for a stream (or for several locations along a stream), a gauging device is installed at each point. This gauge provides a continuous recording of water level in the stream. Thus, the seasonal and annual variation in discharge of the stream can be easily determined by simply reading the surface elevation of the stream and plotting it on the rating curve.

A stream hydrograph is prepared by plotting the variation in discharge of the stream versus time. The shape of the resulting curve on this hydrograph is very informative in terms of flood analysis. The shape of the curve is controlled by a number of factors: • the temporal and spatial distribution of the storm water input; • the physical characteristics of the drainage basin; and • the lag time.

Figure 8.1 shows a hydrograph curve recording an identical flood event in two different drainage basins. The hydrograph for Drainage Basin A shows a very high flood peak discharge, a relatively short duration, and a low lag time. In contrast, the hydrograph for Drainage Basin B shows a much longer lag time, a lower peak flood flow, but a much longer duration. These two curves are representative of drainage basins with strikingly different physical characteristics. The basin represented by A has high relief, little permanent vegetation, and a low “normal” or base flow for the streams. B is from a basin with relatively low relief, heavy vegetation cover, and high base flows.

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This type of graphical representation of flood magnitudes emphasizes the fact that a number of parameters must be evaluated in order to achieve magnitude assessment. Within many small, north-temperate drainage basins, attempts have been made to better evaluate these parameters and thereby quantitatively determine the magnitude of a given flood. For example, in northeastern United States it has been shown that the mean annual flood is related to drainage basin area, slope, and several other geomorphological and hydrometeorological factors according to:

Q = 0.4 + logA + 0.3 logS + 0.3 logSS + 0.4 logF + 0.8 logO

where Q is the mean annual flood discharge, A is the drainage basin area, S is the channel slope, SS is the surface storage area in the drainage basin, F is the number of below-freezing days in January, and O is an “orographic” factor. In England a similar statistical analysis shows the following mathematical relationship:

AStR12.05AQ 2.920.77 ∗∗=

where R is the maximum rainfall and St is the number of streams in the drainage basin. While there are numerous such equations, unfortunately, none of these empirical relationships are valid outside of the region for which they were devised.

Figure 8.1


Flood frequency Closely associated with flood magnitude analysis is the estimation and prediction of how often a flood of a certain magnitude will occur in a basin. This is termed flood frequency analysis. The point to this type of analysis is that floods are bound to happen. If it can be determined with some degree of accuracy how often or with what probability a certain level of flooding will occur, then society can better prepare for the event. This is much the same as earthquake frequency analysis that we discussed earlier. The principles are similar: large floods occur rarely; small floods occur frequently. Consequently, the probability of a large flood occurring at any given time is low, and the probability of a small flood occurring is large. This probability (P) can also be expressed as a factor called a recurrence interval (RI). Recurrence intervals are sometimes referred to as return periods. Importantly, a recurrence interval (or return period) for a flood of a certain magnitude is expressed in years and is calculated according to:

P1RI =

Recurrence intervals are the backbone of nearly all modern floodplain zoning and the design of flood control structures. For example, in designing the size and dimensions of a flood culvert for a highway, if the structure is designed for too small of a flood, the initial monetary investment will probably be low, but the periodic maintenance costs and damage costs will be high. In contrast, if the culvert is overdesigned, the annual maintenance costs will be low, but the initial installation costs will be very high. Obviously, the highway engineer working closely with the environmental geoscientist or geohydrologist must find an optimal position somewhere between these two extremes.

It is very important that the environmental geoscientist realizes and accepts the fact that this type of frequency analysis is based upon probabilities. Although the probability of a river experiencing a flood of a specific magnitude in a specified number of years in the future can be precisely evaluated, the exact year of the flood cannot be determined. The probability of a flood of a given magnitude (or of a given recurrence interval) occurring in the next “n” number of years can be easily calculated using the formula:

n

RI111q ⎥⎦

⎤⎢⎣⎡ −−=

where q is the probability (in percent), RI is the recurrence interval (in years), and n is the next number of years. Thus, a flood having a 1% probability of occurring in any given year (i.e., a recurrence interval of 100 years, commonly termed a “100-year flood”) has approximately a 10% chance (probability) of occurring at least once in the next 10-year period, a 22% probability of occurring in the next 25 years, a 39% probability of occurring within the next 50 years, and a 63% chance of occurring within the next 100 years. (It would be worth your time to verify these numbers by actually performing the calculations.) It is important not to confuse these probabilities of occurrence

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with actual flood occurrences. The chances of occurrence of a flood of a given magnitude are the same each year; whether the flood will occur, of course, depends on the drainage basin factors and meteorological conditions discussed above. Table 8.1 summarizes the interrelationships between Recurrence Interval and the chances (i.e., probability) of a flood of a given size or magnitude occurring in a certain number of years.

Table 8.1 Probability (%) of a flood of given RI occurring in the next x years

Probability (%) of a flood of given RI occurring in the next x years

RI (yrs)

next 100 yr

next 50 yr

Next 25 yr

next 10 yr

next 1 yr

50.0 2 20.0 5 99.9 94.0 65.0 10.0 10 90.5 71.0 40.0 5.0 20

86.0 63.0 40.0 18.0 2.0 50 63.0 39.0 22.0 9.6 1.0 100 39.0 22.0 12.0 5.0 0.5 200 18.0 9.5 5.0 2.0 0.2 500

9.5 4.8 2.5 1.0 0.1 1,000 5.0 2.3 1.2 0.5 0.05 2,000 2.0 1.0 0.5 0.2 0.02 5,000

Basic to flood frequency analysis is the construction of a “flood-frequency curve.” This is a plot of time (or some function of time such as recurrence interval or probability) on the horizontal axis and peak flow discharge on the vertical axis. Because this curve is put together using existing historical records, the accuracy of the prediction may be low in the low frequency (i.e., high recurrence interval) end of the graph. Indeed, it is common for many rivers in which monitoring has only begun this century not to have attained a “50-year” or “100-year” flood. In the case of incomplete data (which is the normal situation in much of North America), it is common practice to fit a curve to the limited data that is available and then project this curve to higher discharges/ recurrence intervals using the arithmetic relationship. The most commonly used equation to approximate the distribution of data on a flood-frequency curve and the discharge of a flood of a given magnitude is:

( )SKlogYlogX ∗+=

where X is the discharge of the flood of interest, Y’ is the discharge of the mean flood in the available data; S is the standard deviation of all the flow data, and K is a constant dictated by the flood magnitude (recurrence interval) and the anticipated shape of the curve.


In addition to this mathematical approximation and curve fitting, it is also common practice to simply extend the flood-frequency curve into areas of limited data “by eye.” This technique has the advantage of being rapid and not assuming a particular mathematical relationship. Thus, it is possible, given a time series of flood discharges, to easily approximate the flood flow of any magnitude simply by graphical means. First it is necessary to calculate the recurrence interval for each flow in the time series. This is done using the expression:

M1NRI +

=

where RI is the recurrence interval (in years), N is the number of years of flood data; and M is the order number of the particular year in the series with 1 being equal to the largest flood flow, 2 being equal to the second largest flow, etc. The calculated recurrence intervals (or probabilities) are then plotted versus the corresponding discharge, and a “best fit” line is drawn through the points.

Flood prevention and mitigation of losses There are few areas of environmental geoscience that are as controversial as flood control. The many methods of flood control and flood management can be subdivided into five main groups.

Hazard perception education How humans responds to flood hazard depends on how they perceive the hazard. As with many other geologic risks, the perception of a flood hazard is controlled by two main factors: • the frequency of floods; and • the cultural and technical state of the society.

Obviously, the more often individuals are adversely affected by floods, the more perceptive they will be of the hazard. In technologically advanced cultures, flood control is often seen as a very important function of society and necessary in order to allow the group to operate effectively. In contrast, technologically primitive groups are less likely to demand flood control and, instead, view floods as acts of god that are to be repeatedly endured.

Legal action Land use zoning is one of the most cost efficient means available to prevent large economic losses due to flooding. Zoning, as well as other legislative techniques such as building codes and tax incentives, depends on the accuracy of the flood frequency and magnitude analyses that, in turn, are directly a function of the length of time the river has been monitored.

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Upstream watershed management As discussed above, the physical conditions of the land in the watershed have a significant impact on the amount of runoff reaching the river during a storm event. The main point of upstream management schemes is to improve the ability of the land to hold water. There are many ways of doing this including: reforestation, reseeding, planting cover crops, reducing the slope and hillside gradients by terracing, construction of contour drainage ditches, contour plowing, and building small “check” dams on creeks in order to increase the storage capacity of the watershed. Upstream management methods are most effective for small to moderate sized storm floods but are not effective for mitigating the flood hazard due to major storms. This is because, in a major storm, the large amount of rainfall is usually able to completely saturate the soils.

Downstream structures The construction of engineering works or major landscape modifications is a very common method of alleviating the flood hazard in medium to large rivers. Each type of construction has its own specific goal and purpose. Floodwalls or artificial levees/dikes are constructed in an effort to confine the flood to the channel. The major problem with this type of construction is one of complacency, the presence of a dike or levee implies that there is little risk of flood in the floodplain areas, and development often proceeds without adequate perception of the true potential for levee failure. Channel improvements, such as straightening a curved channel or enlarging the channel cross section, attempt to allow the river to transport a larger volume of water with the idea that the sooner the flood waters pass the area, the less damage that will be sustained. River diversions are well known to local residents of southern Manitoba as the area boasts of two major diversions: one designed to route flood waters of the Assiniboine River into Lake Manitoba; the other to divert Red River flood flows around the city of Winnipeg. Although expensive to build and maintain, river diversion schemes, such as those in Manitoba, have generally proven to be cost effective. Finally, dams are one of the most straightforward downstream ways of controlling riverine floods. There are two basic types of flood control dams: those which block the drainage of the main river and those that control or impound water in small secondary reservoirs on tributaries to the main channel.

The purpose of a dam is, from a flood control viewpoint, very simple: to delay the peak flood flows in the channel by storing water in a reservoir behind the dam during the critical high water stage or season. The high cost of construction has, today, necessitated the building of dams for more than one purpose. Indeed, most dams constructed now in North America are designed not only for flood control but also for power generation, water resources/irrigation, and recreation. Because several of these purposes are in direct conflict with one another (e.g., to be used most effectively for flood control, the reservoir should be as empty as possible, whereas to be used for power generation, the reservoir should be full), the subject of dam construction and reservoir use has led to heated debates and controversies. At the center of many of these debates is the question of “one big dam or many small dams.” In general, a single large dam is most effective for


power generation, recreation, and water resources purposes. However, large dams (and large reservoirs) have much greater adverse effects on both the upstream and downstream areas of the river (upstream siltation, downstream erosion). Large dams also pose a much greater flood risk in the event of failure, as discussed earlier, and are frequently capable of initiating earthquakes.

Summary Floods are the most costly of all natural geologic hazards in terms of loss of life, property, and land. Floods occur in nearly all rivers and are part of the natural fluvial processes. Flooding can range from short duration, 1- or 2-hour flash flood events to massive inundations affecting thousands of square kilometres and lasting several weeks. Most floods are predictable. In most areas of North America, flood hazard management involves several distinct steps: • recognition and perception of the hazard; • forecasting and warning based on geologic and historical analyses of

frequency and magnitude; • flood fighting and installation of emergency systems; • construction of flood control and flood-proof structures; and • development of land use zoning and other regulations to discourage

urbanization in high risk areas.

For an environmental geoscientist involved with flood hazard prediction and mitigation, the following data and information are essential: • topographic maps, aerial photos; • river profiles and cross sections; • river hydrographs and flood frequency curves; and • historical records of geographic extent, elevation, and causes of past floods.

The “human element” of flood hazards is very noticeable and possibly more problematic than in any other geologic hazard. Before the risk can be reduced, the public must be made aware of the potential hazard. In the case of riverine flooding that has been controlled by structures, there is a tendency for the occupants of the “protected” floodplain to have complete faith in the control structures. In actual fact, the design of the control structure(s) was most likely a compromise between high initial costs (i.e., design for very severe but low probability floods) and high annual property losses and maintenance costs (i.e., design for less severe, high frequency floods). Another problem associated with the public’s perception of flood hazard is the terminology used by water resources managers and flood hydrologists; the public frequently is under the misconception that, for example; a “100-year flood” happens once every 100 years.

The single positive aspect of flood hazards is the fact that they are usually predictable. But even in this regard, the interaction between various branches of the government responsible for flood prediction and protection, the scientists, and the general public is often poor. As is the case with earthquakes, officials

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are reluctant to issue false alarms. When an official warning is issued, unless it is accompanied by exact information about what to do, interpretations of the message can be problematic or incomplete. It has been repeatedly shown in many flood events of the past in North America that the single most effective mechanism of flood awareness is the flood event itself. Persons who have personally experienced flooding in an area are more apt to be sensitive to the problem of floodplain use and occupation. Time greatly decreases this awareness.


base level channelization cfs crest discharge downstream flood drainage basin flood frequency curve flood floodplain hydrograph hydrologic cycle infiltration jokulhlaup

lag time levee Mississippi River Commission Natural flood frequency one hundred year flood permeability porosity rating curve river training recurrence interval stage TVA upstream flood


1. Give two examples of how flooding benefits society.

2. What is an El Niño event?

3. List two ways in which urbanization increases flood hazard risk.

4. Calculate the probability of a flood with a recurrence interval of 10 years occurring in the next three years on the Red River at Winnipeg.

5. The fifty-year flood on the Little Souris River near Brandon is one with a discharge of 400m3 per second or more. In May of 1990 this stream reached 410m3 per second discharge at Brandon. What are the chances (i.e., probability) of another 400m3 per second flow occurring this year on this river?


6. Why is the probability of flood hazard damage so much greater today than it was 50 or 100 years ago?

7. Compare upstream floods with downstream floods with respect to cause and extent.

8. List the major groups of flood control/management methods.

9. What is recurrence interval, and how is it calculated?

10. Compare and contrast stream hydrographs for a basin characterized by high relief, little vegetation, and low base flows versus a basin characterized by low relief, abundant vegetation, and high base flows.

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Notes


Unit 9 Exogenic Geologic Hazards: Landslides and Mass Movements

Topics Definitions and classifications of landslides/mass movement

Basic mechanics of landslides

Causes of landslides

Landslide assessment, prevention, and control

Introduction This unit deals with landslides and a wide variety of related phenomena that are termed mass movements, mass wasting, or ground failures. The topic of landslides is very broad; the processes involved in the various types of mass movements and the causes of these phenomena are extremely diverse. The single unifying theme that ties this unit together is gravity; all of the exogenic hazards described and discussed in this section are related because they are simply downslope movements of Earth materials under the influence of gravity. The differences arise in the physical and/or chemical conditions that trigger the downslope movement.

We will first categorize the various types of landslides and mass movements in terms of a number of important physical criteria. We will then inspect the forces acting on the slide material and try to resolve these using basic slope mechanics theory in order to better understand when, why, and how a slope fails. Once we have this understanding, it is an easy matter to re-examine various geologic settings and environments with respect to slope stability and to illustrate how specific natural and human-modified geologic conditions respond. Finally, we will touch on the main strategies and techniques for landslide prevention and mitigation of the hazard.

Learning objectives Ground failures, including landslides, avalanches, flows, and soil creep, are a significant geologic hazard in every Canadian province and throughout much of the rest of North America and the world. Although individual landslides are generally not as spectacular or as costly as some other geologic hazards, they are much more widespread. Our overall goals in this section are: to understand the main factors controlling the stability of slopes; to be aware of the major natural and human-induced causes of downslope mass movements of Earth materials; and to describe techniques and strategies by which landslide hazards can be corrected or the impact mitigated.

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• define mass movement and related terminology;

• categorize landslides and mass movements with respect to the speed of movement, the amount and type of material involved, and the nature of the movement of the material;

• summarize the mechanics of a typical landslide;

• calculate the safety factor of a slope;

• discriminate among the various physical and chemical factors that can increase the landslide hazard in an area;

• explain specifically how each of the factors of water, topography, climate, and time affects mass movements;

• describe, with the use of appropriate examples, how human activities can influence the magnitude and frequency of landslides;

• summarize and evaluate the various landslide prevention and correction techniques; and

• devise both long-term and short-term landslide hazard mitigation strategies for an area.

Learning activities 1. Read chapter 6 in your textbook and answer the study questions. Note: this

reading assignment will also be used for the following unit on Subsidence and Problem Soils.


Study notes Introduction Almost every year a major disaster in North America is caused by a landslide or another type of mass movement. On a worldwide scale, some of these landslides are rapid and dramatic events, destroying entire towns and villages in a matter of minutes; most, however, are much more subtle and slower but still capable of inflicting costly damage to property, roads, and foundations.

All slopes have a tendency to move, some more than others, some faster than others. Mass movements vary greatly in terms of origin and speed of movement, amount and type of material involved, and the distance of movement. Consequently, there is a very wide spectrum of geologic hazards that must be considered and understood by the environmental geoscientist under the general category of “landslides.” Rockfalls, rock slumps, debris avalanches, rock slides, rotational slumps, block slides, block glides, rock fragment flows, sand runs, loess flows, Earthflows, debris flows, soil falls, planar slides, debris slides,


translational slides, soil slips, soil heaves, soil creep, talus creep, solifluction, gelifluction, mudflows, and topples are all examples of landslide or mass movement hazards; all have varying characteristics and, hence, differing impacts on human activities. All of these mass movement hazards, however, have two common characteristics:

• the movement is induced and controlled by gravity; and

• the movement occurs because the strength of the slope is exceeded by the stresses within it.

Definitions and classifications of landslides/mass movement In the broadest possible terms, landslides and mass movements can be defined as any downslope movement of rock or surficial material mainly under the influence of gravity. This can occur in a wide variety of terrains, ranging from vertical cliffs to land having slopes of less than one degree. Indeed, in many large landslides there is often a component of “run-up,”—the movement of slide material up the slope.

Landslide “Landslide” itself is a somewhat confusing and unusual term because it can be applied to both the geologic process as well as to the resulting landform. Its use today certainly implies a catastrophic movement of material, but there is considerable variability even within this category depending on the angle of slide and the type and volume of material making up the slide.

Falls and slides A fall is generally defined as any type of free-fall movement. Obviously, this occurs mainly from cliffs and very steep (near vertical) mountainous slopes. In addition to simple free-fall, the material can also move by rolling or bouncing. In many respects, falls are the easiest type of mass movement to understand and quantitatively model. The velocity of the falling material is controlled by the vertical distance of the fall and the acceleration due to gravity according to:

( )0.5heightg2velocity ∗∗=

where g is equal to 9.8cm/sec. Rockfalls, debris falls, and soil falls are all commonly occurring examples of this type of mass movement. A topple is a kind of fall in which large slabs of rock fail by tilting, rotating forward, and overturning.

In contrast to a simple free-fall, a true mass movement slide involves movement and displacement along a definite shear surface. Generally slide movement does not involve major disruption or fragmentation of the moved material; the ideal slide simply moves along a single plane of failure. If this failure surface is curved the movement is termed a rotational slide; if the shear plane is not curved the movement is a planar slide. Depending on the amount of fragmentation that does occur in the slide material, the terms block glide

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(essentially no disruption of the material) or rock/soil/debris slide (much fragmentation) can be applied. An avalanche is generally considered to be an exceptionally large slide or glide. However, the term has also been applied to large flows.

Flows Flow is the movement of material as a viscous mass. This mechanism of movement implies that there is some fluid or moisture content available in the material and that it is, in part, due to an excess of this pore fluid that the mass movement actually occurs. Flows are generally differentiated from slides on the basis of the amount of distortion and mixing of the moved material. If the material acts as a homogeneous mass or becomes a homogeneous mass during the movement, it is a flow; otherwise, if the material maintains its overall integrity and particle-to-particle fabric and orientation, it is a slide. Flows often have a distinctive morphology consisting of a wide lobate toe at the base and a narrow flow track connected to the flow scarp or scar on the hillslope.

Although flows imply some type of fluidization, the fluid does not necessarily have to be water. The behavior of some of the large “slides” that have been observed and described during historical times suggests that air may act as the fluid required to account for the motion and mechanism of the particulate material. The movement of the material making up one of Canada’s most famous landslides/avalanches, the 1903 Turtle Mountain Slide at Frank, Alberta, is a classic example of this type of air lubrication/fluidization. Several possible hypotheses have been suggested to account for the unusually high speed, the absence of pore water, and the unique morphology and fabric characteristics of the Frank Slide (and other large slides such as the 1970 Huascaran avalanche in Peru):

• Movement by simple fluidization Somehow after initiation of the movement, probably originally as a block glide, the material fragmented into large grains 5 to 10 metres in diameter. Continued rapid downslope movement forced air in between the grains, and the slide was transformed into a flow. The main problem with this hypothesis is that to get the air into the interior of the slide and between the large grains requires a turbulent flow and complete homogenization of the material. The orientation and fabric of the slide debris making up the Frank Slide suggests that this type of homogenization did not occur.

• Movement on an air cushion Despite the large size of the individual grains in the slide material, the slide/flow had a great speed (estimated to be in excess of 170 km/hr) and was travelling over relatively hummocky, irregular ground. In addition, the slide climbed nearly 150 metres up the opposite side of the valley before stopping. These features suggest that the slide was moving on a cushion of air trapped beneath the material. This cushion of air acted as a lubricant, reducing the friction and allowing the high speeds and great run-up.

An interesting alternative explanation for the Frank Slide is that the mass acted like a fluid due to the dispersion of fine grained material (sand and silt) among


the larger metre-sized blocks. By decreasing the internal friction in the moving mass, the fines acted in much the same way as pore water does in a subaqueous turbidity flow. This type of mass movement has been termed a sturzstrom. Some investigators contend that the immense landslide features that have been observed on Mars and the moon can only be accounted for by this type of “dry” fluidization.

Creep Creep is caused by the disturbance and subsequent settlement of soil particles on a slope. The movement is slow and generally caused by heaving due to frequent repetition of freeze-thaw cycles or wetting-drying (hydration-dehydration). The particles are moved up perpendicular to the slope during the soil expansion phase and then let down in the contraction phase. Because the settling is in a vertical direction, the net result will be a downslope dislocation of the particle. Creep is generally a slow process with rates of movement ranging from less than 1mm/year to 5m/year. The speed and amount of movement is controlled by the degree of slope and the amount of expansion/ contraction. For example, in areas of permafrost, in which permanently frozen ground is found a short distance into the soil, vertical frost heave of as much as 20cm is common, thereby giving rise to relatively rapid rates of creep. Furthermore in a permafrost region, when the upper layer of soil thaws, the water cannot readily drain downward into the frozen Earth, thus increasing the fluid pore pressure of the upper sediment and reducing its strength. This often results in a type of flowage called gelifluction.

Because gravity-induced movement is the single feature tying all of these hazards together, a commonly used classification scheme of landslides and mass movements is based on the type of gravitational movement rather than on the geometry of the resulting landform.

This classification system recognizes six basic types: • fall; • topple; • slide; • lateral spread (creep); • flow; and • composite.

In practice, of course, it is very difficult to pigeon-hole the processes of mass movement in this manner because most occurrences involve a combination of movements rather than just one.

Another, somewhat more descriptive “classification” scheme is presented in Figure 9.1. This system emphasizes the continuum of processes in terms of speed (slow to fast) and identifies whether the material making up the mass is consolidated or loose.

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Basic mechanics of landslides Landslide processes involve the transport of solid material down a slope. These processes can be examined in terms of relatively simple, fundamental mechanical forces, with the variables being: the weight or volume of material being moved, the angle of the slope, and the type of movement that occurs. Basically, there are forces that tend to promote movement and forces that resist movement. Whether or not movement will take place depends simply on the balance between these two sets of forces. Nearly every kind of mass movement can be resolved in terms of these fundamental mechanical principles.

To illustrate these principles, consider the case of an individual block of rock sitting on an inclined slope. There is a failure plane located at the base of the block that is parallel to the underlying inclined surface. The block of rock has a mass, M; the angle of the slope is τ. The gravitational force, which is directed vertically downward is g (9.8cm/sec). The downslope force, termed shear stress, is the product of g, the sine of the angle of slope, and the mass of the block. Thus, the forces acting on the block in an attempt to drive it downslope are:

shear stress = Msinτg ∗∗

(Of course, if this were an actual real-world example, we would also have to consider other forces from various external agents, such as flowing water, wind, rainfall, freeze-thaw, etc.) In contrast, the forces holding the block in place (i.e., shear strength) are the frictional forces, which can be calculated as:

shear strength = ( )τcosMµ ∗

Environm

ental Earth S

ciences 007.136 133

Type of movement (increasing speed ==>)

Slide

Type of material

Rational Planar

Flow Fall

Bedrock Rock slump Rock-slide

Block slide

Rock avalanche Rock fall

(incr

easi

ng c

onso

lidat

ion

==>)

Regolith Earth slump Debris slide Debris flow Soil flow Debris avalanche

Sediments Sediment slump Slab slide

Earth flow

Liquefaction flow

Sand flow

Sediment fall

Figure 9.1

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where µ is a coefficient of friction. (Again, if this were a real-world exercise, other external resisting forces would have to be considered, such as the amount of induration of the material and the degree of cementation to the lower surface, the nature of the discontinuity at the base of the block, and so on.) The main point of this illustration is to demonstrate that as the angle of the slope increases, the sinτ increases, and cosτ decreases, thereby making movement more likely. The ratio between the resisting forces and the driving forces is defined as the safety factor or F. This safety factor can be calculated according to:

M sinτ gM) µ(cosτF =

If F is less than or equal to 1, the slope is considered unstable and very likely to fail. If F is between 1 and 1.25, the slope is conditionally stable but does required some remedial action to increase the safety factor to greater than 1.25.

Causes of landslides Landslides are an excellent example of the threshold concept we discussed in the first section. In the above example involving a block of rock sitting on a slope, if the factors that contribute to the stability of the block on the slope (shear resistance strength) are exceeded by factors contributing to the instability of the block (shear stresses), the slope will fail, most likely in a sudden and catastrophic manner. There is seldom only one cause of slope instability. Usually a number of factors all contribute to creating shear stresses that are in excess of the shear strength of the material. It is also important to realize that landslides can be generated by either increasing the shear stress or by decreasing the shear strength of the material.

Some of the most common ways in which the shear stresses can be increased are: • steepening the slope;

• removing part of the base of the slope which acts as underlying or lateral support;

• adding more weight to the top of the slope; and

• the application of transitory external stresses due to, for example, a seismic disturbance associated with an earthquake.

Factors that can lead to a decrease in the shearing resistance strength of the slope include: • increasing the internal pore pressure of the slope material (usually by the

addition of water);

• decreasing the cohesion of the material; and

• increasing the internal deformation of the material by fracturing, breaking, or weathering.


Because there is such a wide variety of possible situations in which the shear stresses can exceed the shear strength of the material, it is difficult to predict the likelihood of a landslide without detailed geological and geomorphological analyses. Hence, generalization is difficult. In broad terms, any situation that causes or leads to a loss of support at the base of a slope or a change in the physicochemical composition of the material within the slope will likely decrease F. The most common causes of landslides revolve around the following factors:

• Changes in the internal properties of the Earth materials. Such things as the degree of consolidation or cementation, the orientation of bedding surfaces, or the number and orientation of faults, fractures, and joints can all be gradually changed by normal weathering or other near-surface processes as well as tectonic events.

• Changes in the geomorphologic setting of the slope, including factors such as relief, steepness, shape, orientation, amount of vegetation cover, and moisture conditions.

• Application of independent external triggering mechanisms. These are phenomena that produce immediate but temporary stresses which exceed the threshold condition of an already marginally stable slope. The most common triggering mechanisms are: excess precipitation; human activities; and earthquakes.

Excess precipitation weakens the material by replacing air in the pores with water, thereby increasing the pore pressure along the shear surface. The conditions most conducive to failure due to excess precipitation are the presence of steep slopes and the occurrence of dipping strata of porous and permeable material that are underlain by lower permeability units.

The human activities that can increase landslide risk all involve either changes in the slope configuration (such as increasing hillslope gradients, undercutting the base, or adding weight to the top of the slope by construction) or conditions that permit more moisture to enter the substrate. Highway construction has long been recognized as one of the most common causes of human-induced landslides. Highway engineers can prevent, or at least reduce, much of the landslide potential by benching the slopes, installing drains, and applying various physical constraints such as rock bolts, pins, and retaining walls. In addition to highway and road construction issues, urbanization of hilly terrain also invariably increases the landslide potential of the area. Such things as excavation of hillsides for streets, utility lines, and building lots cause steepening of the slope and undercutting. Construction of houses on the slopes increases the weight on the slope. The creation of large reservoirs can also cause unstable slope conditions by modifying the water table conditions.

The complexity of human activities relative to landslides is well illustrated in the example of the landslide and flood at the Vaiont reservoir in northern Italy

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in 1963. This landslide/flood was one of the largest such disasters in history, destroying five towns and killing nearly 3,000 people. A large hydroelectric dam had been constructed in the Vaiont River gorge, raising water levels some 250m in the reservoir behind it. About three years before the disaster, geological investigations noted that a large mass of rock on the southern side of the reservoir was undergoing creep. Rates of movement for the creep were quite high, 5 to 10cm per day, suggesting that a more rapid flow or slide was imminent. Because a high groundwater table was likely responsible for the decreased shear strength of the material, they decided to drain the slope and lower the water levels in the reservoir. They found that by adjusting the level of the water in the reservoir the rate of movement of the creep could be controlled. High rates of movement were associated with relatively high levels in the reservoir; conversely, when the level of the water was dropped some 50m, the movement halted. Armed with this apparent ability to start and stop the creep motion, the reservoir was allowed to fill completely during 1962 and the first half of 1963 to a point some 100m above the level at which creep was previously halted. Of course, if creep movement started again the engineers intended to simply lower the water level in the reservoir to curtail it. Unfortunately, at this higher water level, the slope failed not by creep but by a massive slide. The slide measured about 2km long by 1.5km wide and 150m thick. Over 200 million m3 of rock filled the reservoir, displacing the water over the top of the dam causing it to surge down the Vaiont River valley as a wave over 70m high.

Landslide assessment, prevention, and control The environmental geoscientist is faced with two main problems in attempting to mitigate the hazards due to landslides and mass movements. First, the geoscientist must be able to recognize and predict which areas are susceptible to landsliding. The second task is to identify the controlling environmental characteristics that are most relevant to the particular landslide threat and suggest action to correct or control these features. Superimposed on this dual aspect of landslide assessment is the fact that the cost of preventing landslides and mass movements is far less than the costs involved in correcting the situation once the slide has occurred. Studies conducted in landslide prone areas of northern California suggest that costs may be as much as 100 to 150% more after the movement has taken place. Because many individual mass movements are relatively small, local features, the costs of prevention and mitigation of landslide hazards often falls on local municipalities.

The identification of potential landslide situations is reasonably straightforward and is certainly one of the easier tasks that an environmental geoscientist may be called upon to undertake. Areas in which landslides and mass movements are to be expected include: steep-sided valleys, steep hillslopes, scarps, and intensely dissected terrain. In general, flat, level terrain with little stream dissection has a lower potential for landslides, but care must be taken to properly assess the Earth materials and the water conditions in these types of geomophologic settings. As we learned earlier, fluidization and liquefaction of the nearly flat-lying, water saturated, unconsolidated sediments in south central


Alaska caused massive mudflows and landslides during the 1964 Good Friday earthquake. In any type of slope condition, cohesive, fine grained sediment is much more likely to become saturated in a heavy rain storm and, therefore, is very susceptible to slope instability. The presence of groundwater seeps on slopes can suggest high pore pressures in the subsurface. Obviously, any location in which undercutting is occurring, such as river bank areas or shorelines, is susceptible to failure.

Recognition of which of the causative factors is mainly responsible for a given slide potential can, in itself, suggest a means of correction. For example, excessive pore water pressure can be relieved by enhanced drainage of the site. Overloading at the head of the slide area can be remedied simply by removing the load and by installing retaining structures at the toe of the slide. In the case of small mass movements (less than about 2 x 106 m3), complete removal of the slide material is one of the most economical ways of mitigating the hazard. In cases of larger mass movements, avoidance methods are the only practical and the safest solutions. However, in areas where land values are high or usable land is scarce, avoidance is not always possible.

Hazard mitigation techniques generally fall into one of four main categories:

• Avoidance

• Water control methods When excess water is the main factor contributing to the low F, attempts must be made to decrease the stress within the slide material and increase the shear resistance of the material.

In the case of surface water entering the slide area, ditches and trenches can be constructed to divert the flow around the slide site, or sealants, such as concrete or clay, can be used to prevent the surface water from infiltrating the substrate. Grouting replaces the pore water in the sediment with cement and is very effective in preventing mass movements associated with highway construction. In relatively warm, moist climates, planting trees and shrub vegetation has been proven effective in removing water from the shallow subsurface of slopes. Deciduous trees such as poplars, alders, and elms remove surprisingly large quantities of water by transpiration, with amounts in excess of 150,000 litres/year/tree removed.

• Excavation methods Because of the high costs, excavation is used only when the slide is small enough, and there is an exceptionally high danger of failure. Excavation techniques range from complete removal of the slide material to unloading the head of the slide only. Other techniques such as slope reduction, slope modification by benching, and complete regrading are also possible on a local scale.

• Restraining structures There are many practical engineering procedures and methods used to control landslides that may occur during or due to construction activities. The purpose of installing restraining structures at the base of the slide is to

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increase the resistance to movement. While these structures generally have the advantage of relatively low initial costs compared to excavation, dewatering, or avoidance, they are not particularly cost effective in the long-term and can only be applied to relatively small failures.

Summary Landslide and mass movement are general terms for any downslope movement of rock or Earth material under the influence of gravity. There is a wide spectrum of mass movement types and various ways of classifying the processes and the resulting landform. Landslide potential and mass movement hazards are evaluated by calculating a safety factor, F, which is the ratio of the resisting forces to the driving forces. Factors that tend to lead to increased shear stress and a smaller F include:

• removal of lateral support mainly by erosion, highway construction, and mining;

• loading the slope by building houses and roads on or at the top of the slope;

• increasing the weight of the slope material by increasing the water content and saturation of the material;

• ground vibrations such as tremors caused by earthquakes;

• repeated cycles of freeze-thaw or hydration-dehydration; and

• tectonic uplift resulting in a change in slope angle.

Factors that contribute to a reduced shear strength of the material include:

• the greater potential of clay for water absorption and volume changes than other material;

• rock weaknesses such as faults, joints, and bedding surfaces;

• high pore pressures; and

• deterioration of intergranular cements due to weathering or acidic groundwater conditions.

Despite the high dollar value losses due to landslides and mass movements in North America, this is one of the easiest geologic hazards to identify and evaluate. The cost of preventing the mass movement is much less than the cost of correcting it after the movement has occurred. Unfortunately, until recently there has been little concentrated effort to educate nongeoscientists about this hazard identification and prediction.



angle of repose avalanche block glide creep debris flow driving forces fall g gabion gelifluction landslide mass movement mass wasting planar slide quick clay

resisting forces rock bolt rockfall rotational slide shearing stress shear strength shear surface slide slump sturzstrom topple Vaiont Reservoir/Dam τ µ F


1. Discuss the parameters that can be used in the classification of landslides.

2. a. Describe the Vaiont River reservoir disaster. b. What was the cause(s) of this event?

3. What two factors influence slope stability?

4. Describe several ways in which hillside development may aggravate landslide hazards.

5. Explain the movement of very large landslides.

6. Summarize the most practical mitigation techniques for large mass movements.

7. How can creep be identified?

8. How would you identify a landslide area on a topographic map?

9. How does water affect landslide potential?

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Notes


Unit 10 Exogenic Geologic Hazards: Subsidence and Problem Soils

Topics Subsidence

Mechanisms of human-induced subsidence

Subsidence management

Expansive soils

Permafrost

Introduction In many places throughout North America land levels are dropping. Sometimes this settling is so gradual that it goes largely unnoticed by the local population; Mexico City has been gradually sinking into the soft lake clays of the valley for hundreds of years so that it is now necessary to walk down a flight of stairs to get to the ground floor of many of the older buildings. At other times the collapse occurs suddenly and without warning; in May 1981 a 300m wide and 50m deep hole opened overnight in Winter Park, Florida, and gobbled up a house and garage, five automobiles, the municipal swimming pool, and parts of several commercial buildings. Whether fast or slow, land subsidence is one of the least known geologic hazards. Similarly, expansive soils and permafrost, the so-called “problem” soils, are often not even considered when discussing geologic hazards. Together, however, these problems cause about 40 billion dollars damage per year in North America. This unit will explore and examine the nature of the subsidence problem and related topics of expansive soils and permafrost.

Learning objectives You already understand the main factors controlling one very important type of ground failure: landslides. The overall goal of this unit is to identify and describe ground failure associated with land subsidence, expansive soils, and permafrost conditions. As is true with landslides and related mass wasting phenomena, there are a large number of causes of subsidence, both natural and human-induced. Thus, much of this section will concentrate on classifying these causes on the basis of the mechanism(s) involved in the failure.

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• list the most common causes of land subsidence;

• calculate the amount of time required for a sedimentary deposit to reach complete compaction;

• explain how fluid withdrawal affects the land surface;

• identify the various geologic conditions that increase the likelihood of subsidence due to fluid withdrawal;

• explain hydrocompaction;

• show where subsidence due to dissolution is most common;

• describe the nature of the problem with expansive soils;

• summarize the role of clay mineralogy in creating expansive soils;

• discuss various techniques for mitigating the damage due to expansive soils; and

• illustrate a typical cross section through a permafrost terrain.

Learning activities 1. Continue reading chapter 6 in your textbook and answer the study questions.


3. Complete assignment 4, and send it to the Distance Education Program Office.

Study notes Introduction Subsidence is a type of mass movement that results in sinking or lowering of the land surface. Often it takes place slowly over such a long period of time that it is almost imperceptible to an observer. This type of subsidence is usually not hazardous unless it is associated with a gradually transgressing shoreline along a coastal area. In other cases, subsidence is marked by a very sudden collapse of the Earth’s surface that, of course, can be catastrophic for any type of urban development or human activities in the area.

There are a great many near-surface environments and processes that give rise to subsidence. Subsidence is a natural part of delta sedimentation, for example. It commonly occurs in volcanic settings through the collapse of lava tubes and calderas. Dissolution of soluble bedrock such as salt, gypsum, or carbonates often leads to another type of large-scale, natural subsidence called karst. Humans are also capable of greatly accelerating these natural subsidence processes and, by mining and extraction of fluids from the subsurface, are able to create subsidence hazards that would not normally be present.


Soils and soft rocks which swell or shrink due to changes in moisture content are commonly known as expansive soils. Permafrost is perennially frozen ground, which, when allowed to thaw, loses strength and cohesion, undergoes compaction and liquefaction, and is susceptible to other types of mass movement such as solifluction. In much of North America and in parts of Europe and Asia, these two problems are common; it is estimated that expansive soils cause over ten billion dollars damage to homes, buildings, streets, and highways per year in North America alone, making this the “largest” geologic hazard.

Subsidence Nature of the subsidence problem Anyone who has visited or seen pictures of Mexico City or Venice is certainly aware of the magnitude of the subsidence problem. There are locations in Mexico City that have subsided over 7m in the last century and over 9m in the last two-hundred years. Similarly, landowners in Florida routinely suffer massive and catastrophic subsidence associated with the collapse of sinkholes. Unfortunately, the costs of most subsidence hazards are not easily determined. Man-induced subsidence in the Houston-Galvaston area of eastern Texas led to the inundation by water of about 100km2 of coastline, resulting not only in the loss of valuable beach property and structures but also in the necessity of constructing protective structures such as sea walls, levees, and artificial barriers. Furthermore, large areas of important waterfowl staging grounds have been lost. The costs are further amplified when one considers the increased susceptibility of the subsided area to flood hazards. In the Houston area, for example, Tropical Storm Delia in 1973 caused an estimated 53 million dollars extra damages compared to the situation if subsidence had not occurred. Large scale removal of oil and gas from subsurface reservoirs in the Long Beach area of California has resulted in about 100 million dollars damage to harbor structures, roads, and pipelines.

Classification of subsidence phenomena The causes of subsidence can be conveniently subdivided into natural causes and human-made causes. Subsidence is very much a part of many depositional environments and is a natural, ongoing sedimentary/diagenetic process that operates in relatively shallow subsurface settings. Subsidence can be brought about by ground vibrations associated with earthquakes, dissolution of soluble bedrock by groundwater, thawing of permafrost, oxidation of organic-rich sediments, and normal compaction/ consolidation of sedimentary deposits. Large-scale subsidence over vast regions is caused by tectonic forces such as downwarping of sedimentary basins and crustal plate interaction. However, human activities can play a major role in creating conditions of accelerated subsidence and in actually causing subsidence in areas where it would not normally occur. The two biggest causes of human-made subsidence are extraction of fluids (oil, gas, water) from the subsurface and mining. Other activities, such as loading of water sensitive clays and drainage of swamps and marshes can also lead to subsidence.

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Mechanism of natural subsidence Subsidence that is observed at the surface of the Earth is often the result of compaction and consolidation in the subsurface. Compaction is the natural compression of Earth materials causing a decrease in thickness of the deposit. Because of their high initial porosity, silty and clayey sediments are most susceptible to compaction. For example, the clay-rich lacustrine sediments on which Winnipeg sits contained about 80 to 90% water when they were initially deposited in glacial Lake Agassiz. Over the past 10,000 years these clays have dewatered by natural compaction and now contain less than 50% moisture. Associated with this dewatering was subsidence. However, because the compaction and subsidence took place gradually over a relatively long period and was largely completed by the time European settlement occurred in the basin, it is hardly noticeable.

Natural compaction of geologic material under a static overburden load takes place in two phases. The initial phase, primary consolidation, starts as soon as the material begins to be buried under younger sedimentary deposits. When freshly deposited sediment begins to be buried because of the very high initial moisture content, the overburden stresses are applied directly to the pore water. This stress increases the pore pressure which results in the fluids being expelled from the voids. As more sediment accumulates on top, more stresses are applied and more water is squeezed out. In special cases, such as when sedimentation rates are exceptionally high, or there is no easy escape route for the fluids, the pore waters are not expelled but rather the entire deposit loses shear strength and fluidization or liquefaction occurs. When this happens in the subsurface, the sediments are referred to as being geopressured or overpressured. In normal situations, however, there is usually adequate time and/or escape routes for the water and the end result of the primary phase of compaction is the transfer of the overburden load from the pore water to the solid material of the deposit. The entire compaction and, hence, the subsidence in this phase is due to water loss. This initial phase of compaction is usually complete under relatively shallow burial conditions, usually less than 100m.

The second phase, secondary compression, is the further reduction of the volume of the sediment by internal rearrangement of the solid granular material. Clay particles are often deposited with an “open” fabric or orientation analogous to a house of cards arrangement. Collapse of this open structure results in a more compact deposit and, therefore, additional subsidence at the surface. This second phase of compaction continues on into the deep subsurface at ever decreasing rates.


The amount of time required for a sedimentary deposit to reach complete compaction due to phase 1 can be calculated if several simplifying assumptions are made:

• The sediment is at all times 100% water saturated (i.e., there is no “empty” or air filled pore space).

• The permeability of the deposit does not become a limiting factor (i.e., even though the porosity of the sediment is being reduced substantially, the permeability remains high enough to allow the pore fluids to escape).

• There are adequate conduits for the water to escape (i.e., geopressuring is not a consideration in this simplified model).

In actual practice these assumptions are frequently violated, particularly with respect to maintaining an adequate permeability in a clay-rich deposit. Nonetheless, experience dictates that an “order of magnitude” estimate of the amount of time required to attain complete subsidence by phase 1 compaction can be calculated using:

K

V2h

t

2

τ∗∗⎟⎠⎞

⎜⎝⎛

=

where t is the amount of time required; h is the thickness of the layer of sediment undergoing compaction; V is a volume compressibility factor; τ is the specific weight of the pore water; and K is the permeability of the material. It is often convenient in field practice to determine, by laboratory analysis, a factor

called the coefficient of consolidation, Cv, which is equal to KτV ∗ . Thus,

v

2

C2h

t⎟⎠⎞

⎜⎝⎛

= . The total amount of subsidence is easily estimated using:

( )1

021

θ1TθθT

+∗−

=

where T is the total amount of subsidence due to compaction, 1θ is the porosity of the unit before compaction, 2θ is the porosity of the unit after compaction, and To is the total thickness before compaction.

Mechanisms of human-induced subsidence Withdrawal of fluids Subsidence induced by the withdrawal of subsurface fluids is one of the most common causes of land lowering. The withdrawal of fluids like groundwater and liquid hydrocarbons increases the grain to grain load, and, therefore, increases the likelihood of grain fabric rearrangement. The amount of

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subsidence that develops due to fluid removal is a function of a number of factors including:

• Mineral composition Clay minerals are most susceptible to reorientation and structural collapse.

• Sorting Better sorted clastic material will tend to undergo more compaction and subsidence than poorly sorted sediment.

• Degree of “natural” consolidation Obviously, in a sedimentary deposit that has already undergone nearly complete compaction and consolidation, fluid withdrawal will lead to a minimum of subsidence. In general, older sedimentary deposits are less susceptible to subsidence by fluid removal than younger deposits.

There are a large number of examples involving severe surface subsidence due to removal of subsurface fluids. The first documented occurrence was that of the Goose Creek oil field in northeastern Texas. This case is rather interesting because the subsidence of the oil field, which was originally on land, caused the Gulf of Mexico waters to transgress over the area and flood the field. Because the state holds the mineral and oil/gas rights to all “offshore” land, Texas filed for control of the oil production. Another well known example of subsidence associated with oil production is that of the Wilmington field, near Long Beach, California, which underwent nearly 9m of subsidence in 15 years. Both Venice and Mexico City have sustained considerable land subsidence due to excessive groundwater removal. In the case of Venice, total subsidence over the past 50 years has amounted to less than a metre. However, the low elevation and very low relief of the coastal plain on which the city is located, has resulted in a gradual transgression of the Adriatic Sea into Venice. The 9m of subsidence in Mexico City has significantly affected water supply conduits, waste disposal facilities, as well as buildings and other structures.

Hydrocompaction Some types of granular material have high strengths and can withstand surface loading and minor overburden pressures when dry. When wet, however, the sediment looses its strength, resulting in collapse and subsidence. Aeolian sediments and sediments deposited in relatively dry climatic regimes are most noted for this behavior. The conditions that favor the development of sediment susceptible to hydrocompaction are: • low density/high porosity; • moderate (10-15%) clay content; • smectitic clays; and • low moisture content.

Karst-related subsidence Large areas of North America, Europe, and Asia are underlain by closed depressions formed in soluble bedrock. Karst is a geomorphologic term for the


landform associated with dissolution of soluble rock such as limestone, dolomite, marble, gypsum, or salt. Although karst terrains are relatively easily identified, precise delineation of sinkholes is very difficult without closely spaced drilling and subsurface sampling. Development of karst terrain, and in particular formation of the dangerous, steep-sided caverns and sinkholes, takes place under the following optimum conditions:

• thick soluble bedrock;

• bedrock mantled with a thick layer of non soluble, unconsolidated sediments;

• well fractured bedrock that permits easy movement of groundwater through the system; and

• an original high groundwater table that subsequently was lowered quickly.

There are many classic examples of sinkhole subsidence creating a hazard for human occupation and habitat. Many of these examples demonstrate that not only is the formation of karst and the continuing dissolution of the bedrock a problem, but the hazard was actually brought about by some human-induced activity, most commonly lowering of the groundwater table. The Winter Park sinkhole near Orlando, Florida, received a certain amount of notoriety when, in a 10 hour period in early May 1981, it grew from a small shallow dish-shaped depression to a crater some 100m in diameter. Automobiles, pavement, and parts of houses and commercial buildings were lost in the 30m deep hole before it became plugged and stopped expanding.

Swamp/marsh drainage and oxidation The draining of wetlands is a major problem in many areas of North America. In addition to the obvious loss of wildlife habitat and waterfowl staging grounds, the drainage of these areas presents problems because the resulting dry ground is very susceptible to subsidence. When freshly deposited and water saturated, the sediment in a typical marsh or swamp has a density of about 1.1g/cm3. However, after the water is drained and the sediments are exposed to atmospheric conditions, drying and decomposition of the organics results in a decrease in density to as little as 0.05g/cm3. This decrease in density translates into a substantial subsidence of the land surface.

Subsidence related to mining One of the earliest human-made geologic hazards was that of mine subsidence. In North America, increasing urbanization and population growth has resulted in housing developments being constructed over old abandoned mines. Your textbook highlights one of the more unusual subsidence disasters that took place when an oil drilling rig accidentally drilled into the shaft of a salt mine beneath a lake in southern Louisiana.

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Subsidence management The key to hazard management of subsidence is prediction; the problem must be identified and realized before construction or urbanization is undertaken. Once the subsidence has occurred, there is little chance of effective mitigation. One of the best ways of curtailing human-induced subsidence is to regulate the activity responsible for its formation. Certain types of mining practices are more likely to lead to subsidence and, therefore, should be prohibited. Groundwater extraction must be carefully regulated and controlled in large urban areas sited on unconsolidated alluvial or lacustrine sediment or karst-susceptible bedrock. Similarly, the volume of oil and gas produced from an area should be monitored, with the same amount of fluid pumped back into the reservoir.

Expansive soils Nature of the problem Expansive soils, or surficial sediments that undergo significant volume changes depending on the moisture content, are common in large areas of western North America. As with all soil properties, the ability to swell and contract is inherited directly from the parent material in which the soil is developed. In North America, two main groups of parent materials lead to expansive soils: • volcanic ash and other deposits from volcanic eruptions; and • shales with high smectite content.

These two types of parent material are regionally abundant throughout most of the Great Plains, Great Basin, and Rocky Mountain areas; much of the Gulf of Mexico Coastal Plain; the Mississippi, Missouri, and Red River drainage basins; and the Pacific coast area. Estimates of annual damage to structures due to expansive soils run as high as ten billion dollars. Approximately 10% of all structures constructed on or in expandable soils undergo significant damage (i.e., beyond reasonable repair), and another 60% sustain moderate to minor damage. This includes houses, commercial buildings, roads, streets, and buried utilities and pipelines.

Clay mineralogy of expansive soils A clay mineral is a class of alumina-silicate minerals that has its chemical units arranged in a layered structure. The basic structural units or building blocks comprising the layers are: • silica tetrahedrons (SiO4); and • aluminum and magnesium hydroxide octahedrons.

The classification of the various groups of clay minerals is a function of how these structural units are arranged in the crystal. A 1:1 type of clay, the simplest structure, is a mineral made up of layers of one octahedral sheet alternating with one tetrahedral sheet. The mineral kaolinite is the most commonly occurring 1:1 layered silicate. These clays are characterized by their stability (i.e., they do not undergo extensive volume changes upon hydration or dehydration).


A 2:1 clay has a structure in which there is an octahedral sheet between two tetrahedral sheets. The most common and important 2:1 layered silicate mineral is smectite (sometimes called montmorillonite). These clays are extremely susceptible to significant volume changes due to substitution of ions and water molecules within the structure. The most common substitutions are: • aluminum for the silica in the tetrahedral sheet; • iron and magnesium for the aluminum in the octahedral sheet; and • various metallic ions and water in between the layers.

When this substitution occurs there is an expansion of the crystal structure and, therefore, an increase in the volume of the sediment in which the clay is present.

All clay-rich soils swell or shrink to some extent when subjected to moisture change. This is because of the large amount of pore space present between the clay-sized granular/particulate material of the soil. This type of expansion/contraction is known as interparticle volume change. Interparticle swelling and shrinking are controlled only by the physical properties of the soil, specifically the porosity and particle size of the material. Importantly, the swelling is not a function of mineralogy. A soil made up of quartz and one composed of smectite will undergo the same amount of interparticle volume change if the two deposits have the same particle size and porosity. In contrast, the physicochemical changes induced within the clay minerals themselves, which are termed intracrystalline volume changes, are controlled only by the mineralogy of the clays. Two soils, both composed entirely of the same clay minerals, will undergo the same amount of intracrystalline volume change regardless of their grain size characteristics. The potential for volume change of a soil composed of smectite is about 1,000 times greater than one composed of kaolinite.

In addition to clay mineralogy and particle size of the soil, other factors that help to determine the amount of expansion/contraction are:

• Density In general, the higher the density or consolidation, the more expansion that will occur per unit volume of sediment.

• Amount of moisture Obviously, the greater the amount of moisture available for intralayer substitution, the greater the amount of volume expansion.

• Amount and type of loading of the soil In general, soils that are lightly loaded by foundations and structures show the greatest amount of swelling and shrinking.

• Geometry of the soil mass A thick layer of expansive soil has the potential for much greater total volume change than a thin layer.

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Mitigation of the hazard The best method of preventing damage from expansive soils is to avoid them. Considering the large geographic area characterized by expansive soils, this is not possible in many cases. If it is necessary to place a structure on or in a potentially expansive soil the following remedial measures can be applied to reduce the damage:

• Replace the expansive soil with non-swelling material. Usually, however, the expansive material extends to such great depths that complete removal and backfill are not practical.

• Provide sufficient load to withstand and counteract the expansive forces of the soil. This is usually done in the construction of large commercial buildings, but it is not practical in house or road construction.

• Flood the area, either with water or some other type of chemical injection, before construction in order to cause maximum expansion of the soil. The potential for further expansion will be greatly reduced, assuming that the expansive soil is not allowed to desiccate (de-water).

Permafrost Permafrost, or perennially frozen ground, presents many special problems for construction and is a major environmental concern in large parts of North America and Europe-Asia. About 24% of the land surface of the world is characterized by permafrost. There has been a tremendous amount of research directed at understanding permafrost and the geotechnical aspects of constructing on it. A typical section through a permafrost soil shows three basic units or divisions: the uppermost layer, called the active zone or suprapermafrost zone, freezes and thaws on a seasonal basis. It is separated from the lower, perennially frozen ground by the permafrost table. The perennially frozen ground grades downward into the talik which is the unfrozen zone underlying the permafrost.

Climate is the single most important factor in dictating the presence of permafrost. Optimum conditions for the development and maintenance of permafrost are: • long, cold winters; • short, cool summers; and • little precipitation.

In addition, the water content, mineral composition, and texture of the sediment and soil play key roles in permafrost formation by controlling the thermal conductivity of the ground. A thick vegetation cover tends to produce a thinner active zone and a thinner perennially frozen zone. Complete removal of the vegetation, such as takes place during construction, results in a large increase in the thickness of the active zone. This, then, is the main problem with working with and constructing in permafrost terrain: thawing of the ice in the active zone releases water. This water cannot penetrate into the underlying perennially frozen ground and, therefore, stays near the surface in the active zone. This


increase in moisture content leads to a loss of cohesion of the soil and solifluction.

Mitigation techniques of the permafrost hazard fall into three main categories:

• Active measures involve replacing the entire active zone with material less susceptible to large moisture release upon thawing. Most commonly this is done by replacing the high water content clay-rich soils with sandy or silty sediments which hold less water.

• Passive measures attempt, such as the use of insulation, stilts, or pilings to eliminate the increase in active zone thickness that is associated with permafrost melting.

• Design measures try to produce a structure that withstands the freeze-thaw and solifluction cycles.


active zone compaction collapse expansive soil fabric geopressured zone hydrocompaction intracrystalline swelling interparticle swelling karst kaolinite layered silicate mineral Lake Peigneur montmorillonite permafrost permafrost table primary consolidation

secondary compression sinkhole smectite solifluction subsidence substitution suprapermafrost zone talik thermal conductivity Winter Park 1:1 clay structure 2:1 clay structure Cv h τ K θ

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1. What is the most common type of deposit involved in groundwater induced subsidence?

2. Figure 10.1 shows the distribution of subsidence in the Santa Clara Valley of California. a. Where is the area of maximum subsidence? b. What was the maximum annual rate of subsidence for the valley area? c. What problem do you think the city of Agnew is facing as a result of this

subsidence?

3. Figure 10.2 shows the record of subsidence in Long Beach, California. How do you account for the decline in the annual rate of subsidence after about 1953?

4. Sketch a profile through a typical permafrost zone.

5. Differentiate 1:1 clays from 2:1 clays.

6. Describe the conditions most favorable for karst development.

7. Where in Manitoba might there be a problem with subsidence related to karst?

8. Where in Manitoba might there be a problem with subsidence related to salt dissolution?

9. Differentiate interparticle swelling from intracrystalline volume changes.

10. Considering the nature of the Quaternary sediments in southern Manitoba, why do you think subsidence has not been a bigger problem than it has been in urban areas such as Winnipeg and Portage la Prairie?


Figure 10.1

Figure 10.2

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Notes


Unit 11 Water Resources and the Environmental Geoscientist

Topics Groundwater

Water use

Wetlands

Dams

Drought

Introduction This unit considers the use (and misuse) of water as a natural resource. We have already considered some aspects of surface water flow in connection with the exogenic geologic hazard of flooding. We now want to broaden our perspective. The importance of water availability for domestic use, agriculture, and industry is apparent, particularly in water stressed areas of the world. A satisfactory water supply is dependent upon a number of factors; among these, however, geologic considerations play a pivotal role. Thus, the environmental geoscientist is becoming more and more involved directly with the design of ecologically acceptable systems of collection, transmission, purification, and distribution of water for society.

Learning objectives Water is one of the fundamental substances necessary for life. The availability of water has not only permitted great civilizations to flourish but has also been a key factor in their destruction. The overall goals in this unit are, first, to familiarize ourselves with the different settings in which “usable,” water exists in near-surface environments of the Earth, and how we, as environmental geoscientists and hydrologists, recognize and evaluate this resource. We will then turn our attention to understanding how society uses this water and specifically how the scientists and engineers who are charged with the task of providing a supply of this resource for society cope with an ever increasing demand. Finally, we will study some classic efforts in water management schemes in order to emphasize the absolute necessity of sound management practices.

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• explain why water is essential on Earth, and list the most important properties of this unusual substance;

• illustrate and discuss the occurrence of water in the subsurface;

• describe, with the aid of appropriately labelled diagrams, the flow of water in the near-surface environment;

• construct a contour map of the water table on the basis of drill data;

• explain how Darcy’s law is applied to the flow of groundwater through porous media;

• calculate the discharge from an aquifer;

• describe and draw a diagram which shows a cone of depression resulting from drawdown around a pumping well;

• predict how salt water might intrude a freshwater aquifer in a coastal area;

• quantify the various uses of water on a global and regional basis;

• differentiate offstream uses from instream and consumptive uses;

• show how wetlands perform an invaluable service to society;

• classify the various types of dams;

• discuss the major controversies surrounding dam construction;

• critically evaluate the widespread use of dams in North America;

• distinguish upstream problems associated with dams/reservoirs from downstream problems;

• critically examine the impact of the Aswan High Dam on the Nile River; and

• using a sketch discuss the management scheme of the Colorado River.

Learning activities 1. Finish reading chapters 7, 8, 9, and 10 in your textbook (with emphasis on

chapters 7 and 8 for this unit) and finish answering the review questions, thought questions, and exercises at the end of each of these chapters.




Till taught by pain, men know not what good water’s worth. (Lord Byron)

When the well is dry, we learn the worth of water. (Benjamin Franklin)

Water is one of the most vital resources we have. It is essential for life: a person can live for perhaps a month without food but only for a few days without water. Water is also the primary controlling factor in the transfer of nutrients from the lithosphere to the biosphere; it plays a major role in dissolution and dilution of waste products; it serves as a raw material for photosynthesis. In many parts of the world it is the availability of usable water, rather than food, energy, or raw materials that acts as the limiting factor for population growth, urbanization, and most other human activities. As important as it is to have a constant source of usable water, many problems can arise if there is too much of it. Thus, water can take on a schizophrenic personality: lack of it causes drought and severely limits human occupation of an area; too much creates floods, triggers landslides, and destroys property; poor quality can lead to disease and sickness, poor crops, degraded landscapes, and even eventual destruction of human-made structures and habitats.

The study of water as a resource has historically been within the domain of engineers. However, the proper maintenance of this resource implies the detailed knowledge of several important geologic principles and concepts. Indeed, the Earth sciences are particularly well suited for this investigation because:

• much of the world’s supply of usable water is derived from geologic materials;

• many aspects of the hydrologic cycle are directly influenced by the geologic setting; and

• like flooding, many of the problems associated with the scarcity of this resource are related to basic geologic processes.

Water, a combination of the elements hydrogen and oxygen, is a compound that makes Earth unique compared with other planets of the solar system. It is so common that over 70% of the Earth’s surface is covered by water. Unfortunately, the vast majority of this water is either saline or locked up in ice sheets and, therefore, not usable to humans. Of the enormous amount of water present on Earth (about 1.5 x 109 km3), only about 0.3% of it is usable for most human purposes. Of this small amount, about 99% is either so remote that it has not yet been easily accessed, is too deep to effectively use, or is already polluted. This leaves approximately 0.003% of the Earth’s total water supply to draw from.

Water is a rather unusual substance. It is the only material that exists at the Earth’s surface temperatures in all three physical states. Unlike other geologic resources such as oil, gas, metals, or coal, water is a renewable resource,

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capable of recharge in a time period ranging anywhere from a few days (as is the case for atmospheric and fluvial H2O) to thousands of years (for groundwater and deep ocean water). It is a conducive medium for plant growth because it is clear and colorless. Likewise, it is a very attractive medium for transportation because it has a very low viscosity. Water is one of the very few compounds that expands when it solidifies, thereby making the solid material less dense than the liquid. This property is extremely important for aqueous plants and animals; if ice did not float on water, it would accumulate at the bottoms of lakes, rivers, and ponds and many lakes in temperate regions would never thaw completely. Water has a very high heat capacity, which means that it can absorb large quantities of heat energy without itself becoming hot. This property accounts for the moderating climatic effect that large bodies of water have on nearby cities. Finally, water is “wet.” This may seem obvious, but it also means that water is “reactive”; it is eager to adhere to other substances and to enter new chemical bonding, thereby making it the “universal” solvent.

Hydrologic cycle Our discussions in unit 7 have already provided some detail on the hydrologic cycle, so only a very brief summary will be given here. However, it must be emphasized that virtually any type of examination of water resources must begin with a thorough understanding of this important cycle. Indeed, much of the work of environmental geoscientists associated with water resources involves simply the measuring, documentation, and assessment of the flow of water from one part of the cycle to another.

Water is continually cycled through the lithosphere, hydrosphere, and atmosphere by an immense distillation and distribution system. Water is put into the atmosphere by evaporation from oceans, lakes, rivers, and soils and by transpiration from plants. This component of the cycle is termed evapotranspiration and is driven by solar energy. Cooling and condensing results in precipitation which falls back onto the land surface or ocean (meteoric water). Some of the precipitation moves overland as runoff to rivers and finally to the sea. Some of the precipitation also moves down into the ground through the zone of aeration (vadose groundwater) where the soil is not saturated with water. The infiltrating water may reach the level at which the sediment and/or rock is saturated with water. This is phreatic groundwater and its upper surface is termed the groundwater table. Phreatic groundwater moves slowly within porous and permeable units (aquifers). The flow rate and direction are controlled by elevation differences between where the aquifer is being recharged and where it is exiting the subsurface hydrologic system (i.e., a river, lake, or ultimately the sea).

All of the world’s water supply is eventually cycled through this sequence but at much different rates. Atmospheric water, river water, shallow lake water, and shallow groundwater are all recycled fairly rapidly (a few days to a few weeks to a few hundred years). In contrast, ocean water and deep groundwater require thousands and in some cases millions of years for renewal. For example, the groundwater in the Ordovician carbonate aquifers beneath Winnipeg, which


comprise a valuable source of water for both industrial and domestic purposes in southern Manitoba, is estimated to be some 15,000 years old.

Groundwater Nature of the resource The water table represents the top of the zone in which the pores in the soil, sediment, and rock are completely saturated with water. Unlike surface water, the movement of groundwater is very slow, usually measured in millimetres or centimetres per day rather than centimetres per second. An important point that must be kept in mind when working with groundwater is that however slow the movement is, the water in the pore system is moving, and its flow is controlled by gravity. Thus, much of the work of the environmental geoscientists and hydrologists involved with the use of groundwater is mapping the occurrence of the water in the various subsurface aquifers and identifying the rate and direction of flow of the water.

The use of groundwater as a water resource is increasing in North America. Presently, about 25% of the water demand in North America is met by groundwater sources. Nearly three-quarters of this groundwater is used for agricultural purposes. However, in areas of stressed surface water supplies, such as western Canada and western United States, the use of groundwater for municipal and industrial purposes is much higher.

Groundwater holds several distinct advantages over surface water sources:

• Groundwater is usually free of toxic organisms and other contaminants and, therefore, needs relatively little treatment or purification before use.

• Groundwater sources supply water of a constant temperature and composition.

• Groundwater is less significantly affected by short-term droughts relative to surface water impoundments.

• When it is available relatively near the surface, it is much less expensive to acquire than the construction of pipelines, diversions, dams, and reservoirs.

The problems with groundwater are that it is not always present relatively close to the surface and that the supply is not easily and quickly recharged. Because of this slow recharge, once the groundwater does become polluted, it cannot readily be flushed or cleaned up. Finally, in much of western Canada, the groundwater is (naturally) of poor quality, often too saline for use by humans.

Groundwater hydrology Whatever is known about the groundwater in a particular region must be derived by drilling. As a well is drilled, the cutting bit will pass through the unsaturated vadose zone and then encounter the sharp boundary of the water table marking the top of the phreatic zone. The elevation of the water table is an important reference point. As more wells are drilled the elevation of more points on the surface of the water table are known. Eventually, enough data points are available to plot on a map and contour. This elevation contour map

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portrays the highs and lows on the water table surface, just as a topographic map illustrates the highs and lows on the land surface. In fact, often the water table elevation contour map will simply be a subdued and smoothed version of the topographic map; where the land rises in a hill, the water table elevation will mimic this rise.

While topography affects the depth to the water table climate is a more important factor. In humid regions it is normally within about 10m of the land surface, whereas in arid areas it can be hundreds of metres below the surface. Because of the high costs of drilling deep wells, we rarely attempt to produce water from aquifers more than 300m deep.

Finally, unlike the land surface, the level of the water table varies with time. The groundwater is affected by atmospheric pressure changes and by input and withdrawal from the system. Input can come from seepage and infiltration from above through the vadose zone and from recharge by precipitation or streams and lakes. Withdrawal of water occurs by drilling a well and pumping water to the surface and by natural discharge into a lake or stream.

If the aquifer in which the groundwater is flowing is in direct contact with the Earth’s surface, then the pressure on the water at the water table is nearly equal to the atmospheric pressure. This is termed an unconfined aquifer. Many of the shallow aquifers in southern Manitoba are unconfined. If, however, the aquifer is overlain by strata of very low permeability (aquicludes), then the pressure of the water will likely be greater than that of the atmosphere. The aquifer in this case is confined. If our well is drilled into this confined aquifer, the groundwater will rise in the hole. The amount of rise is dependant on the pressure differential between the aquifer pressure and the atmospheric pressure; the elevation to which it will rise is termed the piezometric or potentiometric surface. An artesian system is one in which this piezometric surface is at a higher elevation than the ground surface.

The movement and flow of groundwater in the subsurface is controlled ultimately by two very simple factors: • the nature of the material through which it is flowing; and • the pressure exerted on the water to make it flow.

Quantitative studies of the flow of a fluid through a porous aquifer are based on an empirical relationship known as Darcy’s law or Darcy’s equation. Henri Darcy, a French engineer, was able to show experimentally over a century ago that the discharge of water through a tube of a certain cross sectional area that was packed with porous, granular material was proportional to the difference in elevation of the two ends of the tube and inversely proportional to the length of the tube. Specifically, the Darcy equation is:

∆l∆hKAQ =

where Q is the rate of flow (m3/day); K is the hydraulic conductivity (m/day), A is the cross sectional area through which the flow occurs (m2); and the quantity

Lh/∆∆ is the hydraulic gradient (i.e., the change in elevation, h∆ , per distance,


L∆ ). The hydraulic conductivity, like permeability, depends on both the nature of the material and on the characteristics of the fluid flowing through the material. Finally, the h∆ component of the hydraulic gradient is not merely an elevation difference but rather is the difference in head, or the level to which the water will rise in an observation well. Head is easily observed and measured in the well, or it is calculated by:

ZτPh +=

where P is the pressure; τ is the specific weight of the water; and Z is the elevation of the water above some arbitrary datum. By measuring the hydraulic gradient and experimentally evaluating the hydraulic conductivity, we can easily calculate the average velocity (V) of groundwater flow using:

LhKV

∆∆

∗=

This calculation is critical if, for example, a harmful substance is accidently introduced into the aquifer. It is important that the environmental geoscientist is able to evaluate how long it will take that substance to reach nearby groundwater wells or surface streams that perhaps are being used for municipal water supplies.

The Darcy equation allows us to examine the flow in one linear section or path of the water in the subsurface. However, by using the potentiometric surface map that we created by observing the elevation of the water table in a series of wells, we can easily evaluate the direction of flow or the three dimensional flow paths in a specific map area. The contour lines on this map join points of equal head and are termed equipotential lines. The groundwater in the aquifer will always move in a direction that is perpendicular to these lines, assuming, of course, that the material in the aquifer is homogeneous. Thus, a network of equipotential lines and orthogonal flow lines can be drawn and a flow net created.

This flow net is two-dimensional in the horizontal plane, but we can just as easily create a two-dimensional flow net in the vertical plane. To do this we use a cross section of the aquifer and plot the various equipotential lines that can be determined from the well data. The same rules are followed in drawing the flow lines perpendicular to these equipotential lines, except that now we are showing the movement of water in the vertical plane. The construction of these flow nets is important because not only does it help us to graphically visualize the complex three-dimensional flow of groundwater, but it also allows the geoscientist to independently evaluate the hydraulic conductivity. This latter calculation can be made using:

ndnfhK q =

where q is the discharge per unit width; K is the hydraulic conductivity; h is the total head drop over the region of interest; nf is the number of flow channels

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(i.e., the number of zones between the flow lines); and nd is the number of head drops (i.e., the number of equipotential lines crossed over the region of interest).

Water use Human use of water has increased dramatically during this century. Today we use approximately twice as much water as we did just two decades ago. The three major uses of water by people (other than transportation) are: • industrial use; • municipal and domestic use; and • agricultural use.

About half of this use consists of water lost to the atmosphere by evapotranspiration. The other half consists of degraded use; the water is contaminated by dissolved salts, other chemicals, heat, or biological components before being returned to the hydrologic cycle.

On a global scale, people use only about 8% of the total annual freshwater runoff, suggesting that there is an ample supply for human use. Furthermore, estimates indicate that it is economically feasible to tap as much as 20% of the world’s annual runoff. Of course, the major problem with this large supply of usable water is that it is unevenly distributed. Ultimately, this is due to the uneven distribution of precipitation and the variable rates of evaporation at the surface of the Earth. Consequently, many areas of the world are now withdrawing more water than can be replenished on an annual basis. Parts of western North and South America, Australia, the Mediterranean, northern Africa, and Middle East regions have all experienced severe water shortages in the past decade. A recently compiled United Nations report paints a rather pessimistic picture for the future. It is predicted that within the next 10 years, most of the Soviet Union, Europe, India, Australia, northern and western Africa, about two-thirds of North America, and substantial parts of Argentina and Brazil will experience severe water shortages. Superimposed on this are estimated increases of 100% in the demand for irrigation water, 2,000% increase for industrial water, and 500% increase for domestic and municipal use. Finally, the increased urbanization, particularly in developing nations, will lead to the pollution of approximately 40% of the world’s freshwater supply.

The use of various components of the hydrologic cycle vary greatly from one place to another. In North America, people now use about 2 x 109 m3 of water per day or roughly 12,000 litres per person per day. Most of this total is, of course, for industrial and agricultural purposes; the actual per capita use for purely domestic activities is only about 1,000 litres per day. Up to 1950, more water was used for agriculture than for industry. Today, however, industrial uses account for about twice as much withdrawal as agriculture. Most of this industrial use is associated with power plants and the water is mainly used for cooling purposes. The other large industrial use is manufacturing with the production of metals (245m3 of water per tonne of steel produced, 1,200m3 per tonne of Al), plastics (2,000m3 per tonne), and pulp/paper (450m3 per tonne) being the biggest consumers. However, a relatively small amount of this


industrial water is degraded. The largest concern is an increased temperature of the waste water, which presents problems for recycling and disposal.

Municipal use in North America amounts to about 1,000 litres per person per day. Of course, the biggest problem is maintaining an adequate supply for the large metropolitan areas. Some very elaborate water diversion schemes have been constructed to assure the major cities (primarily in California) of a supply of water. Even more grandiose diversions have been seriously proposed such as the infamous NAWAPA (North American Water and Power Alliance) designed to divert water from northern Canada to western United States at a cost of about 500 billion dollars, and to divert the Mississippi River into western Texas. Of the 100 largest cities in North America, most (70%) get their water from surface sources alone, while only 20% depend exclusively on groundwater sources. Of the 70 cities using surface water, 40 tap human-made reservoirs, 18 use river water, and 12 use lake water.

With increasing urbanization and industrialization, the relative proportion of water used in rural areas has decreased substantially. About half of the water used outside of cities and industry is used for irrigation. Irrigation began in western North America in the early 1800s and has continued to expand gradually since then. Today, the source of irrigation water is mainly surface reservoirs and rivers, with the Colorado River forming the world’s single largest irrigation management system.

There are several basic types of irrigation used today. The oldest but one of the least efficient techniques is simple diversion and channelization of surface water using a system of canals and ditches to funnel the flow into crop furrows. However, salinization, or the gradual build-up of salts in the soil, is a major problem because evaporation losses in this type of irrigation are relatively high. It is estimated that up to 50% of the water is wasted by leakage, infiltration into non-crop sites, and excessive water being applied to some areas of the cropland. The use of sprinkler systems to distribute the water has become very popular in the past several decades. In fact, the development of the large circular spinnakers, so commonly used and easily identified from the air by the unique geometry, has been called the most significant mechanization development in agriculture since the invention of the tractor! These circular sprinkler systems permit accurate and uniform application of water to the crops, can be used in rolling terrain (which obviously cannot be irrigated by conventional canals-ditches), decreases the evaporation losses, and is relatively inexpensive. Finally, drip irrigation uses a system of low pressure, underground pipelines to apply the water directly to the roots of the crops. This irrigation technique is expensive to install and maintain but is extremely efficient. It uses much less water and creates better crop yields because the water application is more constant. It is used in very arid climates.

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Wetlands Wetlands are transitional areas between terrestrial and aquatic systems where the water table is at or very near to the surface of the land. There are various types of wetlands including: • riverine wetlands associated with the floodplain and channel systems of

streams; • lacustrine wetlands; and • marine intertidal and estuarine wetlands.

Historically, wetlands have rarely been treated as a resource but rather were generally viewed as something to be drained, filled, or dredged. For example, in the United States, the “Swamp Reclamation Act” of 1849 gave jurisdiction for the sale and/or disposal of some 130 million acres of wetlands to the individual states. This resulted in the conversion of about half of all of the wetlands in the United States. About 60 million acres remain.

Today, it is generally recognized that wetlands perform many important physical and biological functions. They are economically important because they form the spawning grounds for about 65% of all the fish and shellfish harvested in North America. Their role in the ecological functioning of waterfowl is even more impressive; the many wetlands of the northcentral part of the continent are a duck factory, acting as staging grounds for over 90% of North America’s ducks. The wetlands are a hydrologic buffer in that they store water during floods and release water during dry periods, thereby reducing the severity of both floods and droughts. Recently, it has been shown that wetlands provide a way of mitigating water pollution by trapping, retarding and transforming many types of pollutants such as silt, toxic metals, pesticides, and excess organics. These pollutants are then broken down into less harmful constituents by microorganisms in the wetlands sediment. Ducks Unlimited has estimated that it would cost about $75,000 per acre to design and construct an “artificial” wetlands which would provide these same services.

Dams Introduction Ever since people started living in cities they have attempted to alter the normal drainage systems by the construction of dams. The dam-aquaduct systems of the ancient Romans, built between about 300 B.C. and 100 A.D., are well known and, indeed, remarkable even by today’s high-tech standards. The concept of irrigation also necessitated building dams to provide a satisfactory constant supply of water when it was needed. Reservoirs impounded behind dams have become the most common water storage system in the world. There are about 70,000 large (greater than 7m high) dams in North America with over 1,500 major reservoirs (greater than 1,000km2). The first dam constructed in North America was in 1634 in New England and was used for water power for a mill. In the last three decades, dam construction has progressed at a rate of about 5 dams per day. This is hardly surprising; if water is the single most essential


ingredient for civilization, society must have a means of obtaining, storing, and distributing it. Dams and reservoir systems provide this means. However, in altering the flow of a drainage system, inevitable changes are made in the hydrologic character of the stream on which the dam is located. These changes provide both positive and negative feedback mechanisms. A good dam is obviously one in which the negative feedback is minimized or one in which the positive features so overwhelm the negative aspects that the overall impact of the dam on society is positive.

Types of dams While it is realized that this is not an advanced course in river hydraulics or river management engineering, it is within the domain of an environmental geoscientist to understand the basic types of dams and to be able to discuss the attributes of the various types with the engineers. There are essentially three types of riverine dams: gravity, arch, and embankment.

Gravity dam A gravity dam resists the forces of the water in the reservoir simply by its weight or mass. In other words, the dam stays in place because the shear stresses of the combined water and sediment in the reservoir behind the dam are overcome by the shear resistance of the vertical component of the dam’s mass on the floor of the river. It is absolutely essential that the underlying bedrock at a gravity dam site be sufficiently strong to resist the added stress of the water, sediment, and mass of the dam.

Arch dam An arch dam, in contrast, gets its strength from the arcuate shape. The shape is designed such that the curvature transmits most of the water/sediment forces to the adjacent rock abutments. Consequently, arch dams can be much lighter, thinner, and easier to construct. Of course, it is essential that the rock abutments on either side of the river be strong enough to resist the added stress. There are also combinations of arch and gravity dams, such as the large Hoover Dam on the Colorado River, which use components of each of these types.

Embankment dam An embankment dam is made simply of excavated material (Earth fill, rock fill) without any additional binding substances. These dams “work” because the accumulated Earth/rock fill retards more water than can leak through the structure. Embankment dams are usually used on small streams, whereas arch and gravity dams are most often constructed on large rivers.

Impact of dams Positive aspects Dams and reservoirs have numerous positive aspects and are, in many ways, extremely beneficial to society. The reservoir can capture high spring flows from heavy rain or melting snow and release the water from these high flows gradually over time during periods when it is most required. Thereby dams also

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reduce the danger of flooding in downstream areas at the same time as providing a controllable and reliable flow of water for agricultural, industrial, and municipal uses. Some dams also supply “renewable” hydroelectric power, and many reservoirs serve an important recreational purpose.

Dam failures Despite these many positive aspects of dams and reservoirs, there is still a considerable amount of controversy over dam construction, as has been demonstrated by numerous water resources dam-reservoir projects in western Canada in recent years. One of the first problems that the public identifies is the safety aspect of dams. Considering the number of dams and reservoirs that are constructed in North America, this probably should not be a major consideration. The fact remains, however, that there have been “numerous” dam failures with, of course, catastrophic downstream repercussions. In short, dams are often perceived to significantly increase the downstream flood hazard risk in an area. Whether or not the other benefits (including flood protection, hydroelectricity, recreation, source of water) outweigh this negative aspect of increased risk must be carefully analyzed.

One of the most startling and unfortunate factors of past dam failures is that nearly 65% of them were due to faulty design, poor construction, improper operation and maintenance, or foundation failure which should have been anticipated. In North America today, it is estimated that there are about 10,000 dams that have a high probability of failure. One of the more classic examples of dam failure due to a lack of geological input into the construction is that of the St. Francis Dam in California in the late 1920s. This dam was sited at the contact of two lithological units; part of the foundation was a micaceous schist, the other part was conglomerate. The conglomerate contained abundant gypsum veins, partings, and cement. When this rock was dry it was quite strong, but wetting the rock led to dissolution of the soluble gypsum, disintegration of the conglomerate, and failure of the dam. In addition to property losses of about ten million dollars, some 500 lives were lost in the catastrophe. Unfortunately, this is not an isolated example; there are many other examples of dams siting on improper foundations. Some of the better known are:

• the Aswan Dam on the River Nile in Egypt, which is located on the porous and permeable Nubian Sandstone Formation;

• the Elwha River Dam in Washington, which is sited on gravels and sands that are so porous and permeable that the reservoir water undercut the dam; and

• a series of dams and reservoirs sited on karst-prone limestone terrain in which the reservoir water simply drains away.

Flood control A second major problem with many dams is that they are designed to prevent and control small- and medium-sized floods (i.e., high frequency floods) but cannot prevent very large floods. This point is well illustrated by the network of dams on the Colorado River in southwestern United States. People often


mistakenly believe that the presence of a dam on a river will protect them from all floods. Thus, urbanization of the floodplain below the dam proceeds without consideration for the true potential flood hazard risk. When a low frequency, large flood does occur, the damage is much greater than if the dam had not been built.

Water supply Although the construction of a dam and reservoir will usually help to maintain a constant supply of water, it will not increase the amount of water available. In fact, because large reservoirs in arid regions suffer considerable loss of water by evaporation, many times there is actually a net decrease in the water supply. Reservoirs also lose water by infiltration and seepage into the subsurface, thereby further decreasing the supply. Even the reservoir itself is sometimes controversial. While a dam offers some measure of downstream flood protection, it does this at the expense of permanently flooding large areas upstream from the dam. This dramatic change in ecosystems has been vigorously protested by many people because it displaces human and wildlife habitats and destroys scenic natural areas. Although the reservoir will create habitats for other organisms, these other species are often viewed as less desirable.

An example of this type of ecological change that caused a considerable amount of scientific and political controversy is the Tellico Dam on the Little Tennessee River. The dam was begun in 1963 and construction proceeded as part of the Tennessee Valley Authority until conservationist groups successfully acquired an injunction to halt construction in 1973. The reason for this injunction was that the reservoir would have destroyed the habitat of a fish that was on the endangered species list. Legal battles continued until mid-1978 when the United States Supreme Court ruled that it was acceptable to finish the dam. In late 1978, however, Congress effectively overruled the Supreme Court decision by temporarily cutting funds for the project and turning the case over to a cabinet level inquiry board. This board unanimously agreed to recommend halting the dam construction even though it was over 90% complete. By the end of 1979, Congress reallocated sufficient funds to complete the dam.

Reservoir problems Another major problem with the construction of reservoirs is that they fill up. The first effect of a dam on a river is to reduce the flow velocity of the river upstream from the dam. As the river enters the newly created reservoir it drops its sediment load in the form of a delta. Delta progradation can significantly shorten the life of a reservoir and, hence, its usefulness. For example, the Anchicaya Dam on the Columbia River was built in 1955. By 1957 the reservoir was a quarter filled with sediment; eight years later it was completely filled. The reservoir of the Shihmen Dam in Taiwan was filled in ten years despite an anticipated 70 year life span when it was constructed. Lake Nasser behind the Aswan Dam receives an incredible 13 x 106 m3 of sediment per year. This was sediment that was originally carried down the Nile to be deposited in the large Nile Delta.

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Downstream problems In addition to these “upstream” problems, the areas downstream from the dam also suffer. One of the biggest downstream problems is increased erosion and downcutting by the rejuvenated river. The construction of a dam dramatically changes the hydrologic and geomorphologic balance of the stream system. Water that is released from the dam is usually devoid of sediment and is often at a much lower temperature than the original stream water. These factors combine to give the stream much more erosive power below the dam. For example, after the construction of Hoover Dam, the Colorado River channel below the dam was lowered over 3m. This resulted in substantially increased siltation farther downstream, which, in turn, increased the flood hazard downstream.

Economics Finally, many groups of society question the economic basis and benefit-cost ratios that are incurred by the building of some (many!) dams. Opponents cite many examples of “pork barrel” legislation and make-work projects in dam construction efforts. Some of the many examples include the following:

• A 48 million dollar dam on the Glover Creek in Oklahoma was constructed supposedly to supply water to the region. Nevertheless, only 6,000 people live in the vicinity, and their needs are already served by a reservoir designed to supply water for 90,000 persons.

• The Hillside dam in Kansas was constructed at a cost of 55 million dollars to protect 7,000 acres of downstream farmland from flooding, but the reservoir behind the dam flooded 14,000 acres of productive upstream land.

• The Bayou Bodcau project in southern Louisiana was to supply downstream flood protection for farmers; the cost was an incredible $300,000 per landowner.

• The Fruitland Mesa project in southern California cost about 130 million dollars and supplied water for 60 farmers.

Garrison dam/diversion Somewhat closer to home, residents of the northern Great Plains will remember the great amount of controversy that raged about 10 to 15 years ago over the Garrison dam and associated diversion schemes. Indeed, this project has had a long and controversial history. The project actually began with the first United States Congressional approval for funding in 1887. However, because of a lack of demand for the water, work on the project did not begin until forty years later. In 1944 additional money was allocated to the project, which was then known as the Pick-Sloan Missouri River irrigation project. By 1955 the Garrison dam on the Missouri was finally completed at a cost of 300 million dollars (1955 United States dollars). The reservoir behind this dam flooded over 300,000 acres of productive farm and ranchland, and, according to opponents of the scheme, land that was invaluable for wildlife habitat and historical/archaeological reasons. In an attempt to compensate for at least part


of this loss, the United States Congress authorized the diversion of some of the impounded reservoir water across North Dakota to irrigate 200,000 acres of land in eastern North Dakota. Unfortunately, this major diversion scheme called for the construction of some 7,500km of canals and artificial drainage systems and the loss of another 65,000 acres of farmland in the central part of the state. In addition, the canals, which are up to 35m deep in places, intersect bedrock aquifers and thereby lower the watertable of the surrounding prairie. This has resulted in the draining of many of the wetlands surrounding the diversion route. Superimposed on these geologic/ecological problems was the dramatic increase in costs of the project. The funding requests went from 250 million dollars in 1968 to 400 million dollars in 1973 to one billion dollars in 1979. The complete story and ultimate impact of the Garrison dam and diversions is still not known. A major environmental impact study was conducted in the 1970s that identified severe water quality degradation as well as significant negative biological repercussions that would be inflicted on the drainage systems.

Aswan dam The Aswan High dam and Lake Nasser in Egypt have become a classic example of problems and negative feedback mechanisms that can arise from construction of major dam-reservoir systems. This massive, 150m high dam on the Nile River, completed in 1967 at a cost of over one billion dollars, is truly a remarkable engineering feat. It was designed and constructed for several very worthwhile purposes: • to help control the annual flooding on the Nile; • to provide a constant supply of water for irrigation of the Nile valley; and • to generate electricity.

Unfortunately, like many other large river control systems, these benefits have only been partially met, and many of the positive features of the dam have been overshadowed by a plethora of unanticipated negative aspects caused by its construction.

One of the major problems with the construction of the Aswan dam is that it was undertaken without a proper knowledge base and without a sufficient understanding of the impact of such a major feature on the Nile drainage system. Many of the problems outlined below could have been anticipated if more study had been conducted before construction. On the positive side, the dam has partially accomplished several of its goals; it provides about 50% of Egypt’s electrical power and has eliminated downstream flooding along the Nile. Also, the water from Lake Nasser has been used to irrigate nearly a million acres of land, thereby substantially increasing the food production and cotton yield from the Nile valley. Finally, the impact of several major droughts in the region in the last twenty years has been significantly decreased because of the ability to release water from the 50,000km2 reservoir.

Notwithstanding these benefits, however, it is predicted that the dam will likely be an ecological and economic disaster for Egypt. For example, it was realized soon after construction started that the dual purposes of hydroelectric power

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generation and irrigation were not compatible in this area. The power generation has to be strongly seasonal because of the draw down of the reservoir during the summer months for irrigation. In contrast, while the climate of Egypt would permit year round cotton and food production if sufficient irrigation water was provided, crop planting has to be limited because of the necessity of keeping the reservoir as high as possible in order to provide hydroelectricity during at least part of the year. Similarly, one of the biggest problems to be faced in using the dam and reservoir is that, after twenty-five years of operation, Lake Nasser has never been more than half filled; there is simply insufficient water to fill the reservoir! This may seem incongruous considering the size and drainage basin area of the Nile, one of the largest and longest rivers in the world. However, the large expanse of open lake undergoes water losses of as much as 4m per year in the hot, dry climate of central Egypt. Substantial losses are also incurred by seepage into the very porous and permeable Nubian Sandstone, a major aquifer in the north Africa region.

The most costly problem of the Aswan dam is in the downstream end. Before the dam was built, the annual flooding of the Nile River, while problematic from a human occupation perspective, was important; it provided an annual supply of nutrient-rich sediment to the floodplain and washed accumulated salts out of the soil. Now, with the downstream floodwaters more or less eliminated, the nutrient-rich sediment is being deposited in the upper end of Lake Nasser. To compensate for the lack of natural nutrients, Egypt has had to spend about 300 million dollars per year on artificial fertilizers. Although these synthetic fertilizers are locally made, the fertilizer plants use much of the electrical power generated by the dam. The salinization in the downstream “floodplain” is significantly decreasing the yields of the previously very productive farmland. To date about 700,000 acres of land have been lost, offsetting by about 70% the amount of new land opened by irrigation from the reservoir. In order to correct for this build-up of salts in the soils, the construction of a complex artificial drainage system has been proposed that would annually flush the soils. The estimated cost for this project is about one billion dollars—as much as the dam itself!

The construction of irrigation canals and ditches in the downstream areas of the floodplain has provided ideal conditions for the rapid spread of schistosomiasis, a disease transmitted to humans by freshwater snails living in the slow moving waters of the canals. The infection rate in people living below the dam has risen from less than 10% before dam construction to about 90% today.


Because much of the sediment load of the Nile River now is being deposited in Lake Nasser rather than being carried downstream to the Nile Delta, numerous additional problems have been created:

• The reservoir is filling much faster than originally anticipated. Because the Aswan Dam was built without sluices, it will be virtually impossible to “flush” the sediment from the reservoir.

• The Nile Delta is foundering due to a lack of sediment. The Mediterranean Sea is very rapidly transgressing inland by as much as 2m per year in the Delta area, resulting in further loss of productive agricultural land.

• The lack of nutrients being delivered to the Delta and offshore area has resulting in the loss of Egypt’s sardine, mackerel, lobster, and shrimp industries. Not only were these fish an important source of food for the 40 million people in the metropolitan areas of the Delta and Nile River valley, but they also provide jobs for some 40,000 persons.

• The lack of sediment in the Nile River below the dam has caused considerable downstream downcutting and erosion. A possible solution to this is to build more dams below the Aswan structure. Ten addition dams are planned for the area between Aswan and Cairo at a cost of 500 million dollars each.

Colorado River The final example we will discuss is not so much an example of a single dam but rather the regulation and manipulation of a major river system. The Colorado River drains an area of over 600,000km2 in six states of western United States and Sonora and Baja of Mexico. The bulk of the drainage basin is within an arid region, with rainfall averaging only about 20cm over much of the area. Evaporation rates are high.

The river and its drainage basin have a relatively long and complex history of scientific investigation and engineering manipulation. River explorer, geologist, and director of the United States Geological Survey, J. W. Powell, was one of the first scientists to visit and study the basin. Direct and continuous monitoring of the flows at various places in this large basin were begun about one hundred years ago. Problems with conflicting views of the management of this river began almost immediately upon European settlement in the basin. The main problems have centered around diversions of water from the upper, more humid parts of the basin, thereby causing low supplies and flows in the downstream areas. The first diversions took place in Wyoming and Utah as early as 1840. Later in the 1800s more Colorado River water was tapped for use in agricultural areas of California and Arizona. A major large-scale diversion system supplied water to the Imperial Valley in California in 1901. In 1905 this diversion actually took all of the Colorado River and allowed it to flow into the Salton Sink (thereby creating the Salton Sea). Urbanization in the southern California and Arizona areas caused the further diversion of Colorado River water into Lake Havasu and, via various aqueducts, to Los Angeles, Tucson, and Phoenix.

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The management of the Colorado River and its watershed is a classic example of evolutionary concepts of water resources management coupled with legislative and jurisdictional controversies and agreements. The key principle that has evolved over the past 100 years is the doctrine of prior appropriation, which basically confirms “first rights” to the water. Specifically, the doctrine, which is the backbone of much of western United States and Canada water law, indicates that a user who has once established a pattern of beneficial use of previously unappropriated surface water has established a future right to continued water use on the scale initially established. For example, in the case of the Colorado River, the early demands for water by the “upstream” states (Wyoming, Utah, Colorado) and Mexico was much less than the demand by the “downstream” states (California, Arizona, New Mexico) and specifically the huge demand for irrigation water for the Imperial Valley/Salton Sea area. Because these use requirements had been established relatively early in the history of management of the basin, it would be very difficult to modify the allocations in light of new demands or a changing hydrologic budget, despite the fact that nearly all of the water originates in the highland areas of the upstream states.

By federal law and international agreement, the upstream states can “use” (i.e., extract for irrigation, municipal, or industrial purposes) 7.5 x 106 acre-feet (an acre-foot is a commonly used hydrologic volumetric unit equivalent to the amount of water one foot thick covering one acre of land), the downstream states can extract 8 x 106 acre feet, and Mexico can use 1.5 x 106 acre feet. This results in a total annual use of the Colorado River water of 17.0 x 106 acre feet. Unfortunately, it is now realized that the Colorado system cannot supply that quantity of flow in a normal year. The average annual discharge of the Colorado before any diversions or human uses was about 13.0 x 106 acre feet. Thus, the various states and regions of southwestern United States and northern Mexico have the legal right to consume more water than can be delivered by the Colorado system in an average year. This incredible situation came about by unintentionally using average flow records for the Colorado during a rather long period of high water discharge in the basin (1900-1930). In retrospect, it is now clear that these three decades unfortunately had the highest flows on record.

The Colorado River reservoirs have experienced a similar type of problem to that of Lake Nasser with respect to conflicts over multiple use. A major feature of the Colorado system is nine dams, which can hold over 60 x 106 acre feet at maximum capacity. However, because these reservoirs are also to be used for flood control as well as power generation and recreation, there is usually only about 50 x 106 acre feet of storage. Again the doctrine of prior appropriation dictates that the additional storage must be kept available in case of flood. The remarkable near-failure episode of the Glen Canyon dam in 1983 demonstrates the conflicts that arise in managing this major drainage basin and the scientific and technical constraints that must be placed on the legally assured rights of the water resource users.


It is evident that the Colorado River water supply system is being placed under considerable stress, in part from existing “legal” demands for its water and from the additional demands resulting from the expansion and urbanization that is going on in southwestern United States and northern Mexico. These demands and inevitable conflicts have forced geologists and hydrologists to closely examine the relatively short-term historical record of flows in the Colorado River basin (i.e., from 1880 to present), and, by using geologic “proxy” data, to identify the approximate level of flows over a much longer time frame. Geoscientists have now accumulated reasonably accurate flow information going back over 400 years. Any future water allocation schemes must use this data base and must attempt to arrive at a compromise with legal and social implications of water use within the constraints of lowered water availability.

The realization that the Colorado River cannot supply the existing demand has also generated some rather interesting alternatives and suggestions for ways of enhancing the flows. One of the most reasonable approaches sought is the termination of all future water resource development plans for the basin. Any further water consumption by either the upstream or downstream states will only deplete the balance even more than it already is. Land developers and those responsible for acquiring additional sources of water for existing urban areas obviously find this alternative difficult to accept. Proponents of the Central Arizona Project, which is to divert an additional 1.2 x 106 acre feet of water to the Phoenix-Tucson area, have been particularly vocal in their opposition.

There has been serious consideration given to a major diversion of water from the Columbia and Snake Rivers of northwestern United States into the headwater areas of the Colorado. As with the more grandiose NAWAPA scheme, this proposal has met firm opposition from hydrologists, geologists, conservationists, and ecologists.

Many scientists suggest that the only resolution to the Colorado River water supply problem is to change existing patterns of land use and water consumption. Agricultural use (i.e., irrigation) accounts for between about 70 and 90% of the total water withdrawal from the basin. At the present time, the fee structure for the Colorado River water is such that farmers can purchase water for irrigation purposes much cheaper than the water that is being used for municipalities. Thus, there has been little incentive among the agricultural groups to install more efficient irrigation systems or to improve the existing systems that are already there. Other ways of improving the efficiency of agricultural use have been suggested including augmenting the Colorado River water with groundwater, recycling and desalination the water after irrigation, and installing a distribution system consisting of covered pipelines rather than open ditches.

Drought Drought is commonly defined in general terms as a moisture deficiency that has serious, adverse effects on a community by reducing food production and water supplies. Drought is ubiquitous. It continually threatens mankind and

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historically has been responsible for massive human migrations, wars, and widespread human suffering. Sustained droughts over a period of years in developing countries such as Ethiopia, the Sudan, and the Sahel have caused long periods of famine, disease, debilitation, death, and widespread social and economic stress. Not only does drought affect humans, it also damages ecosystems, forests, wildlife, and soils. Nearly every country today is vulnerable to some degree to water shortages. Obviously, arid and semi-arid regions, which occupy nearly one half of the land surface of the Earth, are most susceptible to recurring droughts. The fact that droughts in western Canada and north-central United States over the last half century have not been more economically and socially threatening is due to the diversified economies of these regions. In all cases, however, drought has been stressful and costly.

It is a common misconception that drought affects only arid and semiarid regions; in fact, drought occurs in every type of climate from arid to humid, from tropical to arctic, and occurs over a wide spectrum of time scales and spatial scales. Thus, the impact and severity of the drought, or drought magnitude, is a function of the size of the area affected, the duration and timing of the event, its intensity, and the inherent vulnerability of the region.

The causes of drought can be divided into natural climatic causes and human-induced causes. From a climatic standpoint, drought in many regions is cyclical: a periodic variation in climate that produces precipitation short of “normal” expectations. In simplest terms, drought originates as a result of the stagnation or persistence of atmospheric high pressure systems over a region giving rise to subsidence of air masses, clear skies, and low humidities. This persistence blocks the usual sequence of wet and dry weather systems and the normal atmospheric circulation. It has been long recognized from historical records that weather fluctuations of this type are common in most climatic regimes. However, the periodicities of the cyclic events vary greatly from region to region and are usually difficult to establish and explain without detailed, long-term climatic records.

Human-induced activities, such as degradation of the land’s productive capacity and dramatically increasing the demand on groundwater and surface water, have been shown in the last century to be very effective in forcing drought occurrence. The loss of significant amounts of topsoil through overgrazing and improper farming practices has been the single most critical factor in the prolonged drought conditions in Africa. Some 650,000km2 of agricultural land south of the Sahara desert have been lost to salinization of the soils or completely lost due to wind and water erosion that, in turn, has led to the appalling consequences of recent years in the Sudan, the Sahel, and Ethiopia. This scenario, which has been repeated in numerous other areas, is caused by the cyclic nature of the drought-humid climatic conditions; during sustained humid periods the non-integrated expansion of populations, urbanization, and industrialization create demands for water, which far exceed the supplies when drought returns.


Summary No segment of environmental geoscience is fraught with as many problems and controversies as water resources management. A reliable and consistent source of usable water is an absolute essential commodity for society. Thus, elaborate water source projects have been proposed and created, some with disastrous environmental and ecological repercussions. Historically, water resource management has been built around engineering control structures such as dams, reservoirs, canals, and diversions. The future of water availability in many parts of the world is bleak. Thus, other management techniques have recently been proposed including large-scale desalination plants, towing icebergs, controlling weather patterns, and conservation.

Superimposed on this increasing demand is the conflict within societal groups over the use of the water. In general, domestic, industrial, and agricultural uses are not compatible, and compromises among all groups have been difficult to make in a water stressed environment.


acre-foot aquiclude aquifer aquitard arch dam artesian Aswan High Dam Central Arizona Project cone of depression confined aquifer Darcy’s law dipolar discharge drought embankment dam equipotential lines flow net flow velocity gravity dam hydrologic cycle hydraulic conductivity hydraulic gradient

meteoric water phreatic groundwater pressure differential potentiometric surface recharge residence time salinization schistosomiasis specific heat St. Francis Dam unconfined aquifer universal solvent vadose zone water table watershed wetlands ∆ h ∆ L Q K nf nd

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1. What physical property (or properties) of water:

a. allows lakes to freeze from the top down?

b. helps moderate the weather and protect us from the shock of sudden atmospheric temperature changes?

c. accounts for its widespread use as a solvent?

2. Critically evaluate the following statement: “Many areas of the world will run out of water within the next decade.”

3. If water is indeed a renewable resource, how can groundwater be depleted?

4. What are the main functions of dams?

5. Summarize the problems that dams create.

6. Explain why dams, despite, providing flood control, may lead to more flood damage than if they had not been built.

7. From where do most of the cities in North America get their water?

8. What is “unusual” about the use of water in the Colorado River basin?

9. What advantages are there to the drip irrigation system?

10. Summarize the advantages of the rotary or circular sprinkler irrigation system.


Unit 12 Coastal Zone Processes and Environmental Geoscience

Topics Special coastal area problems

Types of coastal zones

Waves and mechanisms of sediment movement

Beaches

Introduction This unit deals with the processes operating along shorelines. The processes controlling shoreline deposition and erosion have considerable impact upon human activities in all parts of the world. Likewise, man’s preferential occupation of coastal areas, both lake shores and ocean coastlines, has led to major disruptions of some of these natural processes and to severe land/water degradation.

Learning objectives Coastlines vary greatly in the kinds and intensity of geologic processes that occur along them. One of the main goals of this unit is to gain an appreciation of the most common of these processes and to apply the knowledge of the processes to land use studies and management schemes in coastal environments.


• describe wave motion;

• use the basic parameters of wave mechanics and fluid flow to calculate the erosion and transportation of granular material in the nearshore zone;

• identify the main problems associated with human occupation of barrier islands;

• sketch a profile through a typical nearshore-coastline environment;

• differentiate the processes and motion of deep water from the processes of shallow water waves;

• calculate the velocity of a wave;

• discuss the relationships among wave height, form, motion, and beach morphology;

• summarize the main kinds of shore defenses, and outline their impacts;

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• describe the factors influencing storm surge;

• show how longshore drift occurs;

• construct a sediment budget for a given length of shoreline; and

• illustrate how breakwaters and groins influence the movement of sediment in the nearshore zone.

Learning activities 1. Finish reading chapter 10 in your textbook and answer the review questions,

thought questions and exercises on pages 323-324.



The water rose at a steady rate from 3 p.m. until about 7:30 p.m. when there was a sudden rise of about four feet in as many seconds. I was standing at my front door, which was partly open, watching the water, which was flowing with great rapidity from east to west. The water at this time was about eight inches deep in my residence, and the sudden rise of 4 feet brought it above my waist before I could change my position. The water had now reached a stage 10 feet above the road at Rosenberg Avenue and Q Street, where my residence stood. The ground was 5.2 feet elevation, which made the tide 15.2 feet. The tide rose the next hour, between 7:30 and 8:30 p.m., nearly five feet additional, making a total tide in that locality about twenty feet—By 8 p.m. a number of houses had drifted up and lodged to the east and southeast of my residence, and these with the force of the waves acted as a battering ram against which it was impossible for any building to stand for any length of time, and at 8:30 p.m., my residence went down with about fifty persons who had sought it for safety, and all but eighteen were hurled into eternity—During the last hour that we were drifting, which was with southeast and south winds, the wreckage on which we were floating knocked several residence to pieces. When we landed about 11:30 p.m., by climbing over floating debris to a residence on Twenty-eighth Street and Avenue P, the water had fallen 4 feet. It continued falling, and on the following morning the Gulf was nearly normal. While we were drifting we had to protect ourselves from the flying timbers by holding planks between us and the wind, and with this protection we were frequently thrown great distances. Many persons were killed on top of the drifting debris by flying timbers after they had escaped from their wrecked homes. In order to keep on top of the floating masses of wrecked buildings one had to be constantly on the lookout and continually climbing from drift to drift. Hundreds of people had similar experiences.

Sunday, September 9, 1900, revealed one of the most horrible sights that ever a civilized people looked upon. About three thousand homes, nearly half of the residence portion of Galveston, had been completely swept out of existence, and probably more than six thousand people had passed from life to death during that dreadful night. The correct number of those who perished will probably never be known, for many entire families are missing. Where 20,000 people lived on the


8th, not a house remained on the 9th, and who occupied the houses may, in many instances, never be known.

(E.B. Garriott, “Forecasts and Warnings,” Monthly Weather Review 28, 1900, 371-377.)

This graphic description of the Galvaston storm surge of 1900, which was, in terms of loss of life, the greatest single natural disaster in North America, emphasizes the tremendous vulnerability of urban areas that are sited along the coasts and shorelines of the world. The coastal environments of the world have a great deal of historical significance. The zone where land and water meet has been the locus of settlement and commerce/trade routes and, therefore, is the site of intense human activity and construction.

The post-World War II episode of urbanization and industrial expansion in North America saw considerable conflict in how society would use the coastal areas of the continent. The conflicts centered around use of the coastal areas for: • urban expansion; • public recreation; and • power generation, refineries, and other industries.

Today it appears that urban expansion and public recreation have won out over the third competing use. Although the coastal zone amounts to only about 5% of the surface of the Earth, over 75% of the Earth’s population lives in the zone. Sixteen of North America’s twenty largest cities are sited in the coastal zone, with more than 90% of urban growth since 1950 being concentrated there.

Although coastlines appear impregnable and unchanging, our experiences over the last several decades show that many are extremely fragile and very susceptible to environmental degradation through improper land use and poor planning. The main culprits in this degradation of coastal areas are:

• construction of coastline “protection” structures such as breakwaters, jetties, seawalls, and groins, which are aimed at modifying the natural process-response of the coastal system in areas of intense urbanization or industrialization;

• dredging, draining, and filling backbeach, lagoon, and coastal swamp areas for the purpose of creating “usable” land; and

• massive municipal and industrial waste disposal.

Special coastal area problems Coastal zone areas have a unique set of natural geologic and geomorphologic features that give rise to a number of special environmental problems. The boundary between coastal land, coastal wetlands (i.e., swamps, lagoons, marshes), and water shifts constantly, making the coastal area a high-risk region from the standpoint of flood hazard. As discussed earlier, coastal wetlands and nearshore marshes, swamps, and lagoons are economically and biologically important. Draining and filling in of these areas for urban and industrial land results in loss of staging grounds and important fisheries. Similarly, the

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wetlands and nearshore coastal areas are very susceptible to chemical degradation. Finally, the coastal regions of many parts of the world are prolific oil and gas production areas.

These various special features of coastlines make a very good case for development that should be strongly regulated and rigidly controlled, with adequate planning and assessment preceding any construction or alteration of the natural setting. Unfortunately, shoreline erosion and degradation due to increasing urbanization, industrialization, and poor planning have not been significantly improved in most of the world’s coastal areas during the last several decades. Much of this lack of response is due to a combination of social and political factors.

In a shoreline environment, the key concept that must be kept in mind in the planning of any type of development is that the coastal area is usually very delicately balanced between erosion and sedimentation (i.e., marine transgression and regression) and is very poorly buffered. Thus, any modification, any structure, or any building will likely alter this balance. Man-made construction along the shoreline also reduces the flexibility of the coastal environment and often has significant feedback mechanisms. Furthermore, once shoreline construction and modification has begun, it cannot be stopped.

Types of coastal zones One of the first things that strikes a new investigator examining coastlines in North America is the basic differentiation between “east coast” versus “west coast” types of zones. Much of the east coast of North America is characterized by large, wide, and very long beaches, bordered seaward by relatively broad expanses of shallow continental shelf areas. Landward from the beaches are many types of wetlands, lagoons, bays, and estuaries depending on the local geomorphology and recent geologic history. This type of coastline is sometimes referred to as barrier island coasts. Nearly 3,000km of eastern North America are bordered by this type of coast.

In contrast, the west coasts of the continent are generally characterized by little or no shelf area, very short stretches of beach/bar areas, little backbeach development, a domination of sea cliffs, and a relatively rugged inland topography. Many consider these sea cliff shorelines to be very attractive for residential development. However, the apparent stability of the coastline is misleading: sea cliff coasts often experience relatively long periods of no retreat punctuated by bursts of very rapid retreat. These short episodes of rapid retreat have happened several times in the past half century on the sea cliffs of California and also on the cliff shorelines of several of the Great Lakes, resulting in large-scale losses of developed property, houses, and other structures.


On a world-wide basis, coastlines can be classified into eight basic types: • drowned river valley and estuary; • delta; • fiord and glaciated; • volcanic; • active fault area; • transgressing or wave eroded; • vegetated, mangrove swamp area; and • coral reef/atoll.

Waves and mechanisms of sediment movement What is a wave? Much of the above classification of shorelines is controlled directly by the amount, type, and duration of wave activity impinging on the coastline. Sea waves have attracted considerable attention and study for many years. Despite this long-term scientific interest, complete understanding of wave generation and propagation across the vast distances of the oceans is still incomplete. Nevertheless, some facts about waves are well established, and it is necessary for the environmental scientist working in coastal areas to understand them.

In our earlier discussion of EMR, we learned that it is not easy to define a wave. A wave simply transfers a disturbance from one part of a medium to another. It is important to realize that the disturbance is propagated through the medium without much substantial movement of the material itself. For example a floating cork merely bobs up and down on waves in a pool; it actually experiences very little overall movement in the direction of travel of the waves. Of course, we already realize from our investigation of EMR that it is energy that is being transported, not material.

Waves that travel through the material are called body waves; earlier we examined the impact of body waves generated by earthquakes on various human-made structures. The waves we are dealing with in this discussion are mainly surface waves. The most familiar surface waves are those we can see at the interface of a body of water and the atmosphere, but strictly speaking, any interface can show surface waves. Oceanographers and limnologists frequently identify internal waves that occur at the interface between two layers of ocean water or lake water of differing densities. Whatever the interface, surface waves are caused either by forces resulting from relative motion between the two fluids (i.e., the air blowing over the water) or by some external force that disturbs the fluid (e.g., boats, earthquakes, tidal attraction).

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Wave terminology A terminology system has emerged over the years to describe various components of an ideal water wave. Although much of this terminology has already been introduced in your introductory geoscience course, we will briefly review the basic definitions and concepts.

Wave height (H) refers to the overall vertical change in height between the wave crest or peak and the wave trough.

Wavelength (L) is the distance between two successive peaks (or two successive troughs).

Steepness is defined as wave height divided by wavelength (H/L).

In addition to having spatial dimensions at a fixed point in time, waves have temporal dimensions at a fixed point in space. The time interval between two successive peaks (or troughs) passing a fixed point is the period (T). The number of peaks passing the point per unit time is the frequency (f).

There are mathematical relationships linking the wavelength, wave period, and wave height to the speed at which the wave travels in deep water and the energy of the wave. The speed of a wave (c) is defined as the time taken for one wavelength to pass a fixed point. It is calculated according to:

TLc =

Wave number (k) is the number of waves per metre:

L2k π

=

Angular frequency (σ) is the number of waves per second:

T2πσ =

These relationships are valid for waves travelling over deep water. In shallow water, water depth has an effect on wave speed, and the equation for c becomes somewhat more complex:

5.0

Ld2 tanh

2Lgc ⎟

⎠⎞

⎜⎝⎛∗

∗=

ππ

where g is the acceleration constant due to gravity (9.8m/sec2), d is the water depth in metres, and tanh is the hyperbolic tangent. For our purposes, it is sufficient to approximate tanh (x); if the quantity is small (less than 0.05), then tanh approaches x; if the quantity is larger than π , then tanh (x) approaches 1.


Carrying the math one or two steps farther, it can be shown that in water deeper than half the wavelength, the wavelength is the only variable that affects wave speed. Thus,

5.0

2Lgc ⎟

⎠⎞

⎜⎝⎛ ∗

=π

In water very much shallower than the wavelength (i.e., when d is less than L/20), water depth is the only variable affecting wave speed and wave velocity becomes:

( ) 5.0dgc ∗=

Finally, when water depth is between L/20 and L/2, it is necessary to use the complex hyperbolic tangent equation above to precisely calculate c.

Wave energy The energy “possessed” by a wave is in two forms:

• kinetic energy, which is the energy inherent in the orbital motion of the water particles; and

• potential energy possessed by the particles when they are displaced from their position.

The total energy (E) of a wave per unit area can be calculated according to:

)Hg (ρ 0.125Ε 2∗∗=

where ρ is the density of the water in kg/m3.

As you watch a wave approach the shoreline it is apparent that the wave height and steepness increase until the wave breaks. Because the speed of the approaching wave (in the shallow water) is related to depth, the speed, c1, at a point farther from shore (depth = d1) is greater than the speed, c2, at a point closer to shore (depth = d2). Thus, both the wave energy and the square of the wave height are inversely proportional to the wave speed according to:

21

22

2

1

1

2

HH

cc

==ΕΕ

The breaking of a wave in the nearshore area is a highly complex phenomenon. Even some distance before the wave actually breaks, the sinusoidal shape is substantially distorted. Thus, in detail, the mathematical relationships expressed above are only approximations and the actual calculations are much more complicated. The point, however, is that the breaking of the wave dissipates much of the energy that the wave received from the original source (i.e., wind, earthquake, passing ship, etc.). Some of this energy is reflected back out to sea, some is used to break up, erode, and transport granular material up the beach slope, and finally some energy is dissipated as heat in the mixing of water and foam.

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Breaking wave morphology The type and morphology of the breaking wave is controlled mainly by a combination of factors, including beach and offshore bottom slope, wavelength, wave height, and period.

Spilling breakers are characterized by a great deal of foam and turbulence at the wave crest and are most usually identified on a gently sloping shoreline. The spilling is caused when the layer of water at the crest moves forward faster than the wave as a whole.

Plunging breakers are the classic surfers’ wave with a convex back and concave front. The crest curls over and plunges downward with considerable force, dissipating its energy over a very short distance. These types of breakers are most characteristic of intermediate to steeply sloping foreshores.

Collapsing breakers are similar to plunging breakers except, instead of the crest curling over, the front face collapses. These breakers also occur on beaches with intermediate to steep slopes but are usually associated with a strong wind.

A final type of breaker is the surging breaker, which is found only on very steep foreshores. Surging breakers are formed from long, low waves that remain intact until they meet the actual beach. They are also usually found in calm wind conditions.

These four breaker types form an important continuous series or spectrum with the spilling breaker (short period, large height) and surging breaker (long period, small height) making up the two end members.

Sediment movement It was mentioned above that a cork in a pool of water experiencing a passing wave seems to merely bob up and down and is not displaced in the direction of the wave’s movement. In actual fact, the cork (or a single particle of water) will trace a circular path. The diameter of this circle is greatest at the interface (i.e., top of the water) and decreases rapidly with depth in the water. The maximum depth at which the circular path of the motion of the water can still be identified is approximately equal to one-half the wavelength. This is an extremely important dimension for geologists because the erosion and transportation of sediment is, of course, controlled by the motion of the water above the sediment. A wave will not “touch” bottom if the depth is greater than 0.5L. As the wave approaches shallower water and its orbital motion impacts with the substrate, the path of the water movement will become distorted and will approach an elliptical outline. In order for us to be able to assess what size of sediment can be moved by the passing wave, it is necessary to calculate the maximum horizontal orbital velocity of the water at the bottom of the wave as it interacts with the sediment. In deep water (i.e., the wave is just touching bottom), the velocity (V) is calculated by:

d)(ksinh THV

∗∗∗

=π


where k is equal to L

2π .

In shallow water (i.e., d < 0.5L):

dc

2HV ∗=

From the standpoint of sediment movement, the transformation from “deep water” waves to “shallow water” waves is very important. This transition occurs over a range of depths from approximately 0.5L to 0.05L. As we noted above, the surface characteristics of the wave change, but also the orbital paths are distorted. The orbits first change from circular to elliptical, and then with shallower water, are eventually flattened completely. At this point the oscillatory motion of the wave’s particles is replaced by a net forward motion.

Refraction Another important phenomenon that occurs when waves approach a shoreline at an angle is refraction. Snell’s law, which describes the refraction of light rays through materials of different refractive indices, can be applied equally well to wave bending. If you know the bottom topography of a nearshore area, then the angles of the wave at different depths (Figure 12.1, d1 and d2) as it approaches the shore are related to wave speed according to:

2

1

2

1

cc

sinθsinθ

=

where θ is the measured angle between a line drawn perpendicular to the trace of the wave crest and a line drawn perpendicular to the shore. This ratio of angles, in turn, can be easily shown to be equal to the ratio of the water depths:

0.5

2

1

2

1

dd

sinθsinθ

⎟⎟⎠

⎞⎜⎜⎝

⎛=

186

Figure 12.1

Using these concepts, it becomes easy to understand why waves often appear to converge on headlands areas and diverge in bay areas. If we draw the wave rays, which are simply lines perpendicular to the wave fronts, for a series of waves approaching an irregular shoreline, diffraction of the waves caused by the irregular bottom topography will concentrate the energy of the waves at the headlands, thereby increasing erosion of the headlands. In contrast, the intervening bay areas will experience wave divergence, a lower energy level, and a net buildup of sediment.

Wave refraction is also responsible for the generation of longshore drift of sedimentary material on the shoreline. If the waves are approaching the shoreline consistently at any angle other than perpendicular, the wave energy will tend to move sediment laterally in the foreshore area. In many coastlines this results in a net flow of sand along and parallel to the beach rather than landward or seaward. The process of longshore drift has been recognized for hundreds of years; some of the earliest human-made structures on coastlines were constructed in an effort to stabilize the beach and keep sediment from moving laterally along the coast.

Seiche waves Small ocean basins, closed harbors, and many lakes experience a special kind of wave called a seiche. These can wreak considerable damage to shorelines and nearshore human-made structures. The long north-south basins of Lake Winnipeg and Lake Manitoba are particularly susceptible to seiches. The movement of a seiche wave can be best illustrated using a bathtub filled with water. We can start the motion of a seiche by forcing the water at one end of the tub down. This will result in a net movement of the water to the other end of the


tub and a rise in water level at that end. As we release the force at the “down” end, the water will rush back to that end of the tub in the form of a wave. This wave will continue to propagate back and forth in the tub until the energy is eventually dissipated.

In a real world situation such as the Lake Manitoba basin, the force initiating the seiche might be a major wind storm in the north end of the lake pushing water to the south. When the storm abates the seiche wave will form, and the two ends of the basin will experience alternating high and low water levels. The length of the basin (L) is equal to half of the wavelength of the seiche wave. If the average water depth (d) in the basin divided by (L) is less than 0.1, we can consider the wave to be behaving as a shallow water wave, with the period of oscillation (T) calculated by:

0.5d)(g2LT∗

=

The value T is also referred to as the resonant period. Since most lakes, bays, and estuaries where seiches might occur have relatively shallow water compared with the seiche wavelength, this is the normal equation used to calculate the wave’s period.

Tides The longest oceanic waves are those associated with tides and are characterized by the rhythmic rise and fall of sea level over a period of hours. The high water part of the wave, the rising tide, is referred to as the flow, whereas the falling tide is called the ebb. From earliest times it was recognized that these regular oscillations in sea level had something to do with the moon and the sun; tides are highest when the moon is full or new. We now realize, of course, that the influence of the relative motions of the Earth, sun, and moon on the oceanic water mass is quite complicated but can be precisely calculated. The moon-Earth gravitational interaction generates two high tides and two low tides per day. It takes 12 hours and 25 minutes to complete one tidal cycle. The sun-Earth interaction is superposed on this but at a different timing, such that at approximately two week intervals the Earth-moon-sun alignment is optimum to create higher than normal tides termed spring tides. Similarly, every two weeks the alignment is optimum for negative influence such that lower than normal tides, or neap tides, occur. Spring and neap tides are approximately 20% higher or lower than normal.

Thus, in addition to classifying coastal areas in terms of the morphology and geology of the setting as described above, it is common to refer to the tidal range of the coastline according to the following categories: • microtidal: less than 2m range; • mesotidal: 2 to 4m range; and • macrotidal: greater than 4m range.

The tidal range is not only controlled by the time of year (i.e., spring versus neap tides, etc.) but also by the geomorphic configuration of the land and the

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weather conditions at the time of the tide measurements. Because of a funnelling effect, bays, estuaries, and narrow inlets tend to have higher tidal ranges. Likewise, high tides are often enhanced by strong wind waves, storm waves, tsunamis, and seiches.

Storm surge Storm surges, such as the one that destroyed Galvaston, Texas, in 1900, are unusually high water conditions brought about by a combination of factors. As the name implies, storm surges are normally associated with an approaching hurricane or typhoon. The most devastating surges are those that are superimposed on high astronomical tides. In addition to the Galvaston disaster, storm surges have been responsible for some of the most destructive and life-taking catastrophes in human history. For example, in a single two day storm surge, associated with an offshore cyclone in the Bay of Bengal in 1970, over 300,000 people perished in Bangladesh and northern India. The surge put nearly 9m of water onto the land in the coastal areas, wrecking nearly 400,000 homes, killing about 300,000 cattle, and destroying over half of the fishing fleet of the region (some 99,000 boats). These losses, in turn, led to the starvation of another 50,000 people. The widespread coastal flooding of eastern United States in 1972, which caused three billion dollars damage, was, in part, the result of surges associated with Hurricane Agnes. A storm surge/seiche combination created much destruction in Darwin, Australia, in advance of Cyclone Tracy in 1976.

Storm surges and swells are generated by the intense atmospheric conditions connected with those large low pressure disturbances and move out from the center of the storm. The surges can travel thousands of kilometres ahead of the actual storm. As graphically described in the Introduction section of this unit, the surge is characterized by a rapid increase in water level above high tide. As with other waves, the factors that affect the surge height are:

• Angle of approach of the surge Maximum surge is usually associated with a perpendicular approach.

• Coastline shape Maximum surge is associated with the funnelling of water into a concave coastline or up a narrow bay or estuary.

• Shape and width of the continental shelf Maximum amplification of surge and tide effect occur in wide, gently sloping continental shelf areas.

Beaches and shoreline erosion Coastal erosion is a major problem for developed shorelines everywhere in the world. In some countries, such as Bangladesh, erosion threatens the peoples’ way of life. In the atoll nation of the Marshall Islands, the actual physical existence of the country is at stake. In the United States, Canada, and Western Europe, the problem, while not as fundamental as in Bangladesh or the Marshall Islands, still represents a major economic and environmental threat.


Like many geologic hazards, the erosion crisis is to a certain extent human-induced. If no one lived near the shore, there would be no erosion hazard. As increasing numbers of people migrate to the coastal zone of the ocean or lakes, and as sea-level rise causes an increase in rates of landward shoreline movement, coastal erosion becomes an ever more important geologic hazard.

During periods of static or rising sea level, shoreline erosion will inevitably be a widespread process. Judging from the recent history of barrier island evolution, however, there is some evidence that erosion has become more extensive in recent years. Many barrier islands in eastern North America are regressive, indicating that a seaward accretion or widening of the islands occurred over the last 3,000-4,000 years. Within the last several decades, however, a profound change has taken place, and shoreline retreat has replaced shoreline progradation. Most regressive barrier islands, such as Galveston Island of eastern Texas, are now eroding on both the ocean and lagoon sides. Presumably, this island narrowing is in response to a recent increase in the rate of sea-level rise, but other factors such as sand supply reduction may also be involved.

The role of humans in causing coastal retreat ranges from global to local impact. Global production of excess CO2 is widely assumed to be leading to greenhouse effect-related sea-level rise, which in turn causes accelerated erosion. Over the past decade, many scientific studies have predicted increases in the rate of sea-level rise due to the greenhouse effect. Indeed, there is now general agreement that the current global trend of sea-level rise is likely an important factor behind the serious erosion and land loss that is taking place in many coastal areas of the world. Locally, the damming of rivers is cutting off a major source of sediment for many beaches, which also accelerates erosion. This problem is particularly acute in beaches on the west coast of North America. Increasingly, there is widespread concern that efforts to save buildings along the shorefront leads to the destruction of the recreational beaches that were often the reason the buildings were built there in the first place.

The consequences of sea-level rise can be classified into three categories: flooding, salt water intrusion, and shoreline retreat. The most important consequence of a rise in sea level for low-lying areas, such as Bangladesh and the Mississippi Delta region, is land loss due to a combination of flooding and shoreline retreat. For coastal areas where there is sufficient elevation and coastal zone steepness to avoid inundation, such as the Pacific coast of Canada and United States, shoreline retreat will be the most important process, but much less property loss will occur.

Responses to this erosion problem vary considerably. Where sufficient capital exists, such as in western Europe, North America, and Japan, massive structures are built and maintained to hold back the forces of the sea. However, where poverty is the norm, as in most developing countries, buildings routinely tumble into the sea, or they are moved back step-by-step, keeping pace with shoreline retreat.

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What is coastal erosion? A variety of terms are used to describe the erosion process, including drowning, coastal erosion, shoreline erosion, beach erosion, shoreline retreat, beach retreat, and shoreline recession. Most commonly, retreat and progradation refer to a change in shoreline position; erosion and accretion refer to volumetric changes in the exposed part of the beach. The term beach erosion is deeply ingrained in the public language and usually refers to any form of shoreline retreat or erosion.

The widespread use of the term erosion to describe landward-moving beaches is a poor choice and unfortunate because it implies a loss of land and material. On sandy coasts, such as those of eastern United States and Canada and the Gulf of Mexico barrier island coasts, what is usually called beach erosion is simply the changing of the beach’s location. Much of the sand that was on the beach in its former position remains within the beach system. In other words, to the owner of threatened property, the landward movement of the shoreline may well seem like erosion because land in front of the property is being lost. But to a swimmer or surfer on an undeveloped beach, the same landward movement of the shoreline, if noticed at all, is simply the movement of the beach to a new location. It is critical that you, as an environmental geoscientist, are able to make the public community understand that erosion does not threaten beaches; it only threatens buildings. It is also crucial to distinguish between erosion and an erosion problem. Many kilometres of undeveloped Canadian shorelines are retreating landward, and as a rule such locations are not considered to have an erosion problem. It is only when humans interfere or get in the way of shoreline erosion that it becomes a problem. The point is that there is no erosion problem until we build something next to an eroding shoreline.

Erosion at the shoreline can be measured in a number of ways. It is frequently expressed by the change in some measure of mean shoreline position. Commonly, a reference feature such as the wet-dry (high water or high tide) line on the beach, the vegetation line, or bluff edge is measured relative to a stable landmark. As we discussed earlier, aerial photographs, charts, and maps are often used by the environmental geoscientist to calculate erosion rates. Changes in subaerial and subaqueous sand volumes are also used to measure beach loss or gain. The latter approach is a particularly meaningful way to measure erosion in front of seawalls or to gauge the erosive loss of replenished beaches.

The problem of determining future erosion rates with a degree of precision useful to society, however, is complicated by the available choices of time spans of measurement. Is the rate of change over the last year the most useful? The last decade? The last 50 years? The last 1,000 years? There are numerous other complications as well, including the effects of storms, the lack of precision in measuring small changes using aerial and satellite photographs, and the quality of older map data.


Causes of shoreline erosion Causes of shoreline erosion are many and varied. Different types of shorelines (e.g., the rocky British Columbia coast, the unconsolidated glaciated coasts of Lake Winnipeg and Lake Manitoba, the barrier islands of the United States east and Gulf of Mexico coasts) erode by widely differing mechanisms. Factors involved include climate, local sediment supply, sediment or rock type, wave energy, sea-level change, and the effects of humans. A complete discussion of erosion mechanics is beyond the scope our course. We shall instead discuss in a very generalized way the mechanics of erosion on sandy beaches. Viewed on a broad scale, these processes are similar everywhere and amenable to a general treatment.

As we discussed earlier and as you recall from your introductory course in Earth science, beaches are systems in dynamic equilibrium. This equilibrium involves four factors: • sediment supply; • relative sea-level change; • wave energy; and • shape and location of the beach.

Each of these factors is to some degree dependent on the others; a change in one factor results in adjustments by the others.

Sediment supply Each shoreline segment has a unique combination of sediment sources. The major sources include rivers, eroding bluffs and cliffs, and, if we are dealing with oceans, the continental shelf. Locally, longshore transport of material from an adjacent shoreline segment may be important. The continental shelf contribution, via onshore transport by fairweather waves, is the most difficult to determine. In most sediment budget analyses, the continental shelf contribution is counted as the amount of sediment not accounted for from other sources.

Different sources produce or provide sediment of different textural quality. Eroding till banks along the southern margin of Lake Winnipeg, for example, commonly produces poorly-sorted material in a wide size range, which after sorting by wave action may produce a “protective” cobble or boulder lag on the beach. Similarly, bioclastic debris from a reef off the Florida Keys is typically coarse and very poorly sorted. Eroding dune bluffs on a west coast barrier island, on the other hand, produce well-sorted, fine-grained material.

Human interference with beach sediment supplies has been increasing dramatically in recent decades. The damming of rivers, for example, is cutting off a major source of sand for many beach systems. In North America, this problem is acute for west coast beaches of United States and Canada. Damming of rivers, however, does not affect most eastern North American barrier island erosion rates because rivers there are presently depositing their sand loads at the heads of estuaries; direct fluvial sediment input to the beach system is usually negligible. Seawalls, which are particularly extensive on European shores, have

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cut off the supply of sand normally contributed to beaches by eroding bluffs. The myriad seawalls, breakwaters, groins, and jetties that line the developed shorelines divert offshore, slow down, trap, and otherwise reduce the regional beach sediment supply furnished by longshore currents and, hence, cause increased erosion rates.

Sea-level rise Sea-level is rising at a rate of about 3.0mm/yr along much of the North American eastern coastal plain coasts and at varying rates along other coasts. Relative sea-level rise is made up of both tectonic and eustatic components. Part of the present sea-level rise can be accounted for by the recent melting of the world’s alpine glaciers. Thermal expansion of upper surface layer waters in the open ocean due to global warming is also a major factor. The highest rate of relative sea-level rise in North America is found in parts of the subsiding Mississippi Delta, where the rate of rise may exceed 1.0cm/yr. Locally rapid relative sea-level rise may also be due to human-induced subsidence as we have already discussed. Most sea-level rise scenarios assume melting and possibly the breakup of the portion of the West Antarctic Ice Sheet that is grounded on the continental shelf. The nature and complexity of the models used to predict greenhouse effect sea-level changes are beyond the scope of this course, but it is of value to remember that most models predict a eustatic rise of 0.5-2.0m above present sea-level by the year 2100!

Wave energy Remember that waves and wave- or wind-induced currents are the primary movers of nearshore sediment. As we discussed earlier, beach shape is determined by a combination of sand supply and the energy and type of incident waves. Different shorelines are adjusted to different wave climates. Eastern Canada shorelines, for example, frequently retreat on an annual basis in response to “northeaster” storms. Erosion of Florida’s Gulf of Mexico beaches, on the other hand, is most affected by hurricanes, often spaced decades apart. Northeasters or extratropical storms in general tend to cause more shoreline erosion than hurricanes because they move much more slowly than hurricanes. This allows them to pound on the beach for time spans up to three days.

Steep storm waves combined with storm-driven currents tend to move sand offshore. Fairweather waves between storms tend to slowly move sand landward, providing that the sand has not been moved below the fairweather wave base. It is usually assumed that sand removed by storms with a water depth deeper than 10-15m is lost permanently to the beach system. Often storm-caused erosion is substantially “repaired” by post-storm onshore and longshore transportation of sediment.

The most intriguing questions about the impact of waves and wave climate on future shoreline retreat involve an anticipated change in storm climate. Both the frequency and intensity of Atlantic hurricanes are predicted to increase in the coming decades. This will likely impact Gulf of Mexico shoreline retreat rates


more than the Canadian or United States East Coast, where Northeasters, rather than hurricanes, tend to be the more important shoreline erosion event.

Beach shape and location The fourth factor in the dynamic equilibrium is the shape and location of the beach. The spectrum of beach morphologies and their relationship to wave energy and erosion potential has already been discussed. Beaches typically respond to storms by retreating landward and either flattening their profile or forming offshore bars. In either case, the effect is to dissipate wave energy over a broad zone and thus reduce shoreline retreat.

The beach will move landward in response to a rising sea-level unless the sediment supply is sufficient to hold the shoreline in place. A reduction in sediment supply will likewise lead to beach erosion if other factors remain constant. The beach will also likely move landward if wave energy increases, as in a single storm event or in response to a longer-term increase in storminess (e.g., El Niño effects on the west coast of South America). Many variations in the dynamic equilibrium factors exist, leading to a wide range of possible shoreline responses.

Solutions to the erosion problem The basic problem in responding to the shoreline erosion crisis on most shorelines in Canada and United States is that two conflicting society priorities come into play that cannot be met simultaneously. One priority is the preservation of shoreline property. In Canada such property is often high-priced and as a result tends to be owned by influential individuals who are very active in defense of their property. The second priority is preservation of the beach. The beach is utilized and valued by many more people than just property owners. However, swimmers, surfers, fishermen, scientists, and beach walkers tend to be more dispersed geographically and less vocal as a group in defense of beach preservation. If one beach is damaged in the process of saving shoreline property and houses, the view of many is that there is always another beach to “use” down the road.

Broadly speaking, society has three main routes available to mitigate the beach erosion problem. These are: • hard stabilization; • soft stabilization; and • relocation or retreat.

Hard stabilization refers to any method of holding the shoreline in place using fixed objects such as seawalls and bulkheads. Soft stabilization, or beach replenishment, involves the emplacement of additional beach sediment as a means of holding the shoreline in place. The relocation route is not actually mitigation but rather simply involves moving threatened structures back or allowing them to fall into the sea as the shoreline retreats.

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Hard stabilization The main advantage to hard stabilization is simply that it is the most dependable way to save shorefront property. The disadvantages, however, are many including: • degradation of the recreational beach; • reduction of beach access; • relatively high expense; and • often unsightly.

Hard shoreline stabilization structures can be divided into two types: those that block wave energy and those that trap sediment. These shoreline structures are discussed in detail in the following section on Coastal Defenses and Coastal Geoengineering. Wave-energy-blocking seawalls are shore-parallel structures built on the exposed beach to protect the land behind them from wave attack. Seawalls are the most common type of hard structure used worldwide. Shore-perpendicular groins and shore-parallel offshore breakwaters are designed to increase beach width by interrupting the longshore transport system and trapping sand.

Seawalls and related structures often degrade the beach through a variety of mechanisms:

• Placement loss This term refers to the physical placement of the seawall on the active beach at the time of construction. If the seawall is built out on the recreational beach, then that part of the beach is lost to use. In other words, part or all of the recreational beach is missing because the seawall is sitting on top of it. Seawall placement was responsible for much of the beach loss in Miami Beach, Florida.

• Passive beach loss If a wall is placed on a shoreline that is retreating, the beach will narrow as it retreats toward the wall. This is referred to as passive beach loss. The same principle is obviously true of any fixed object on a retreating shoreline, not just seawalls. Since most shorelines are retreating, seawalls will degrade most beaches through passive means. This is usually a long-term (decades) process.

• Active beach loss This term refers to beach degradation caused by the direct impact of the wall on nearshore oceanographic/lacustrine processes. The mechanisms of beach degradation include intensification of longshore currents during storms and the seaward or lakeward reflection of storm waves, which cause offbeach sand transport.


Soft stabilization There are two distinct advantages to soft stabilization:

• It widens the beach.

• It protects buildings while the beach is still in place.

The disadvantages are that this technique is temporary and, therefore, costly.

Soft stabilization or beach replenishment involves the emplacement of sand to rebuild beaches that have retreated close to seawalls or to buildings. Sand is usually pumped by a dredge from inlets, tidal delta shoals, or the even the offshore area to the beach. In some cases, such as along the beaches of some of the Great Lakes and Lake Winnipeg, sand has even been trucked in from inland quarries.

The lifespan of replenished beaches is quite variable. Miami Beach, Florida, a 16km-long, 60 million dollar artificial beach is still largely in place after 10 years. On the other hand, a 5 million dollar beach in Ocean City, New Jersey, disappeared in a little more than two months in 1982.

There is little agreement among sedimentologists and environmental geoscientists as to how to predict the lifespan of a replenished beach once it is emplaced. This is because a replenished beach erodes at varying rates along its length, making it possible to choose different locations from which to interpret lifespan, and, frequently, it is assumed that sand removed from the subaerial beach resides just offshore, where it continues to have a dampening impact on storm waves. From the standpoint of the beach community, an underwater beach has little credibility or use. Mathematical models used to predict replenished beach behavior have routinely failed. This is because of our present lack of understanding of nearshore processes, especially where storm processes are concerned. We cannot model what is not yet understood. There is also a tendency on the part of both the public and the government to accept storms that “prematurely” remove replenished beaches as unavoidable accidents rather than design failures.

Clearly, the beach replenishment alternative to erosion mitigation is not viable for developing countries for the cost is simply too great. In addition, if sea-level rise continues or even accelerates, the price of holding the shoreline in place and preventing loss of property by beach replenishment along sandy, low-lying shorelines will likely become unacceptable even to wealthy nations.

Relocation and/or retreat The advantages of this final alternative are many including it: • offers a better long term response to sea-level rise; • preserves the beach; • reduces or even eliminates shoreline stabilization costs; and • preserves buildings.

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The disadvantages are: • It is politically a very difficult decision to make. • The actual loss of property and land may be costly.

Relocation is the general term for any shoreline erosion response that does not involve shoreline stabilization. This could mean relocating buildings, demolishing them, or simply letting them fall into the sea. A number of temporary defensive responses to shoreline erosion such as setback regulations, building restrictions, and land-use planning also fall into this category.

Prior to World War II, moving or abandoning buildings was a common procedure on rapidly eroding Canadian and United States shorelines, especially on barrier islands. Beachfront buildings along the east coast of North America have been moved back as long ago as the mid-1800s. In 1888, the famous Brighton Beach Hotel on Coney Island, New York was moved back over 600m with the aid of six steam locomotives. The hotel was moved because it was threatened by shoreline erosion. Many of the early homes in New Jersey were built on wooden runners so that they could be pulled back by horse teams or a tractor as the shoreline retreated. There is even an example of a 120-year old home in North Carolina that has been moved back five times (200m in total) and is now once again close to the shoreline.

The future Coastal erosion promises to be an increasingly visible geologic hazard as sea-level rises and the rush of development to the shore continues worldwide. Responses to the problem will vary dramatically. Public policy in Japan and Holland now maintains that these two countries intend to suffer no further land loss. In fact, Holland continues to increase its land area at the expense of the area of the sea floor. In contrast, the United States National Park Service in 1972 instituted a policy of noninterference with natural coastal processes on its lands, and many states have now opted for the retreat alternative. Considering the change in public perception of the value of beaches and the “buildings versus beaches” controversy, it is likely that future erosion response strategies will favor responses that preserve beaches.

What society needs now from environmental geoscientists is accurate bases for developing long-term strategies to deal with coastal erosion. Perhaps the most important applied research problem in coastal geoscience today is predicting the effect of sea-level rise on shoreline retreat rates. Were sea level to rise catastrophically 10m in the next year, there is no question that the shoreline would be inland at what is now the 10m contour. Predicting shoreline behavior at a much slower rate of sea-level rise is much more difficult. The problem is complex, and standard geoengineering models are often too rigid and are based on too many narrow assumptions to have wide application. In terms of the next century, there is a strong possibility that, if the scenarios of predicted greenhouse warming (and sea-level rise) come to pass, North America, Europe, and Australia will not be worrying about shorefront recreation versus property preservation, but rather concern and funding will focus on the fate of major cities.


Coastal defenses and coastal geoengineering Structures built along the coast to help protect or in some other way modify the natural shoreline processes vary greatly in terms of costs, design, function, and complexity. A simple groin or pier may cost as little as $100 per metre, whereas a seawall can be as much as $20,000 per metre. As a generalization, as soon as a structure like a jetty or groin is installed, it is necessary to continually add more protective devices to maintain the new beach/shoreline configuration.

A jettie or breakwater is often used to protect inlets and harbor areas from waves. It also serves to stabilize the channel within the harbor area by preventing shoaling due to longshore drift. Unfortunately, the blocked sand often accumulates on the updrift side of the harbor area, whereas shoreline recession/erosion takes place on the sediment-starved downdrift side.

A groin, usually a solid, narrow structure which projects seaward perpendicular to the shore, is also very effective at reducing the rate of longshore drift. Groins are often installed in order to stabilize and widen a beach, but the problem is that they do this at just the one spot; downdrift areas of the unprotected shoreline will suffer accelerated erosion rates. A classic example of this type of problem occurs today along the Toronto lakeshore. Five groins were installed to prevent a beach approximately 7m wide from eroding. After nine months, however, a beach several kilometres downdrift had eroded about 6m due to the structures blocking movement of sand along the shore.

Seawalls are wall/pavement-like structures designed to protect the shoreline against wave damage and also to keep unusually high tides from flooding the nearshore land areas. The first major seawall in North America was constructed in response to the Galveston storm surge in 1900. In places this seawall is more than 7m high and runs for several kilometres along the barrier island shoreline. The most serious problem with seawalls is that they are doomed to failure. With the construction of a seawall, the beach and exposed shoreline will strive to create a new equilibrium profile, probably one much steeper than the original. Then, when another large surge or severe high tide-storm wave combination hits the coast, the steepened seabed in front of the wall can be easily eroded, thereby undercutting the wall and collapsing the structure.

For many people living in a city immediately adjacent to a large body of water like one of the Great Lakes or the ocean, the presence of breakwaters, jetties, and groins is reassuring because it signifies that there is some degree of protection afforded to the urban area. In actual fact, we now realize that many of these structures cause severe beach deterioration and ultimately increase the hazard due to ocean or lake flooding.

There is much controversy even among Earth scientists about the rationale and ultimate feasibility of shoreline protection structures. One school of thought maintains that basically it is possible to build anywhere along virtually any coastline providing the proper geological and geotechnical studies have been done and these results are known and understood by the developer. The developer’s task is to assess these risks in suitable economic terms and to apply the appropriate legal, institutional, and economic constraints. Essentially, the

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higher the risk for coastal zone development, the higher the cost for protective measures.

A fundamentally different viewpoint of coastal zone management advocates a much more comprehensive environmental assessment of the zone. This school of thought maintains that it is insufficient to merely call upon society to establish “appropriate” structural and environmental standards for the coastline in economic terms. In some coastal areas it may not be in society’s best interest to place expensive and populated centers in a zone of potential hazard even if there is some degree of protection. Finally, there is also some question as to whether the real magnitude and frequency of the hazard is known.

Of course, as we already know from our study of other geologic hazards, the answer to these questions and a decision as to which type of coastal zone management is best utilized cannot be made by geoscientists alone. Unfortunately, many smaller communities facing the problems of flooding, beach erosion, rising sea level, or simply the question of whether to allow development along the coast can seldom be convinced to look ahead and provide a comprehensive plan of coastal zone management. As is shown by this quote from a conference on coastal engineering published over forty years ago, the problem is not new:

Along the coastlines of the world, numerous engineering works in various states of disintegration testify to the futility and wastefulness of disregarding the tremendous destructive forces of the sea. Far worse than the destruction of insubstantial coastal works has been the damage to adjacent shorelines caused by structures planned in ignorance of, and occasionally in disregard of, the shoreline processes operative in the area.

(D.L. Inman and J.D. Frautschy, “Littoral Processes and the Development of Shorelines” in The Proceedings,

Coastal Engineering Conference [Santa Barbara: CA, 1966]).


barrier island beach beach face berm breaker zone breakwater collapsing breaker ebb tide estuary eustatic sea level flow tide groin

regression revetment seawall seiche spilling breaker spring tide surge surging breaker surf zone swash zone transgression tsunami



jetties lagoon littoral transport littoral cell longshore current macrotidal mesotidal microtidal neap tide orbital path plunging breaker

wave climate wave energy wave frequency wave number wave refraction wave steepness c g H L a


1. Lake Winnipeg waves in the North Basin frequently have the following characteristics: H = 0.62m, T = 10 seconds, d = 21 m, L = 40m. What is the bottom orbital velocity of such a wave?

2. Discuss this statement: “Once constructed, shoreline engineering structures produce a trend in coastal development that is difficult, if not impossible, to reverse.”

3. Sketch a shoreline on which a groin has been placed to restrict littoral drift. Indicate where sand erosion and deposition will subsequently occur and how this will reshape the shoreline.

4. Summarize how the relative elevation of the land and sea can be altered.

5. What is the present trend in global sea level?

6. Differentiate east coast from west coast shoreline environments.

7. Sketch a typical shoreline in a microtidal environment; in a macrotidal setting.

8. Summarize the main differences between waves and currents.

9. Sketch a profile through a typical beach labelling the various zones and features.

10. Explain why waves preferentially attack headlands and deposit in bays.

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Notes


Answers Appendix

The following answers are brief suggestions only. Complete answers would require more detail.

Unit 1 What is Environmental Geoscience? 1. Any of:

• Not all geoscience is environmentally related.

• Reinforces the environmental interaction of the science.

• Emphasis is on both social impact of hazards and on society’s impact on environment.

• Numerous other “problem” terms.

2. If stress is applied to a system such that it exceeds a critical point, the system changes rapidly. There are many examples; see textbook and notes.

3. a. 70/2 = 35 years b. The amount of time required for a quantity to double in size.

4. There are many possible examples; see text and notes. Overpumping a groundwater aquifer leading to decreased carrying capacity of the land; use of off-road vehicles leading to increased siltation and high erosion rates.

5. Man is superior attitude, nature is self-healing attitude, nature is cyclic attitude, the now generation philosophy, the infinity complex

6. First to assess the carrying capacity of southwestern United States

7. Battle of Gettysburg in the United States Civil War; Allied landings in France during WWII.

8. It has the ability to be predictive as well as historical.

9. Resources: for the most part, the resources of the Earth are finite. Thus, by maintaining an ever increasing demand for a resource, even very large quantities can be quickly used up.

Pollution: the exponential growth of population means that the facilities that society has in place for pollution control and waste disposal must be constantly upgraded and enlarged.

10. Uniformitarianism most simply stated is “the present is the key to the past.” It is the basic guiding principle in all aspects of Earth science. In environmental geoscience studies, it is very important not to confuse rates of processes with uniformitarianism.

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Unit 2 Introduction to Earth System Science 1. Various ‘correct’ answers but probably most likely energy, hydrologic,

geobiochemical and rock.

2. Solar, geothermal, gravity (the text also uses the term tidal)

3. A very abbreviated answer would be it initiated the present Northern Hemisphere Ice Age; a more complete answer would discuss the northward diversion of the equatorial currents, positive feedback mechanisms, etc.

4. 5,000 x 1012 moles/yr x 110 kcal/mol = 5.5 x 1017 kilocalories

5. The greatest production of methane on land is from waterlogged soils, swamps, lakes, etc. In such environments there is little oxygen, and the methane is produced by bacterial action in anaerobic settings. The flux to the atmosphere is significant simply because these methane forming environments are so close to the boundary (i.e., air-soil or air-water interface). In oceans, the wind and waves keep the upper few hundred metres of water well mixed and oxygenated, so any initial methane formed is oxidized to CO or CO2 by the time it gets to the air-water interface.

Unit 3 Techniques, data, and investigative procedures 1. There are many examples given in the manual and textbook: topographic

maps can provide preliminary estimates of the properties of the bedrock and surficial deposits (e.g., steep slopes or high, massive uplands tend to suggest relatively hard, resistant materials); linear features on topographic maps may indicate faulting or geological contacts; springs and marshes, particularly if they are aligned, suggest a contact between a permeable and nonpermeable rock unit; morphological features such as fanshaped slopes indicate rapid and perhaps episodic sedimentation.

2. Lowest cost: Topographic map Hydrological data Remote sensing Geophysical survey

Highest cost: Geological drilling

3. EMR Velocity = Frequency x wavelength.

4. Temperature, composition, physical state of material.

5. Atmospheric attenuation and scattering of the UV EMR.

6. Vegetation studies.

7. Can penetrate atmosphere, haze, clouds, and even unconsolidated surficial material.

8. Multi-date, multi-band, and multi-scale.


Unit 4 The Earth’s cycles I: The Lithosphere 1. The outmost layer of the Earth; or also (more precisely, the crust and the

outermost portion of the mantle

2. A branch of tectonics that deals with the processes by which the lithosphere is moved laterally over the asthenosphere; or also the slow lateral movement of segments of the Earth’s outmost shell as a result of convection cells deep inside the Earth

3. A collisional or convergent plate boundary along which one plate sinks beneath the other along a steep shear zone.

4. Igneous

5. Igneous, metamorphic, sedimentary

6. Continental-oceanic; continental-continental; oceanic-oceanic

7. A type of plate boundary along which the plates are moving apart from one another.

8. A type of plate margin along which two plates slide past one another.

9. Oceanic crust is more dense and thinner than continental crust; there are also significant compositional differences: continental crust is richer in light elements than oceanic crust.

Unit 5 Endogenic geologic hazards: Earthquakes 1. T = 0.05 ∗ 100 ∗ 10 = 50 seconds; magnitude has nothing to do with the

period.

2. Ground shaking, ground cracking (least hazardous), ground failure (Tsunamis).

3. Primary wave: high velocity, can move through both liquid and solid, compression-dilation movement.

Secondary wave: low velocity, can move through only solid material, up and down motion.

4. Moderate because much more common than large and still powerful enough to cause much damage in urban areas.

5. The injection of fluids into the subsurface results in a lowered shearing strength of the rock: τ = τo + µ (σn - P).

6. Any of: retrofit, redesign, reduce use, removal.

7. Any of: West Coast, Yukon, Gulf of St. Lawrence, Offshore Newfoundland.

8. The transformation of granular material from a solid to a liquid by an increase in pore pressure.

9. logN = a - (b ∗ M).

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10. About 11 am Winnipeg time (but answers will vary depending upon which equations you use).

Unit 6 Endogenic geologic hazards: Volcanoes 1. First (and probably only) example of diverting and stopping a lava flow.

2. The nuees ardente.

Type of activity Composition Example

Shield nonexplosive low silica Hawaii

Composite mixed explosive and non explosive

intermediate silica Mt. St. Helens

Dome Explosive high silica Mt. Lassen

4. Ground tilting, earthquake activity, gas emissions, topographic changes on crater floor.

5. Create new land, soils formed on lava are often very rich.

6. A = energy line (nuees ardentes will not overtop topographic barriers outside of this line); B = convective-thrust portion of pyroclastic eruption cloud; C = gas-thrust portion of cloud; D = nuees ardente.

7. a. Caldera.

b. Very large.

c. Increased geothermal and geophysical activity.

d. 700,000 years ago for the Long Valley caldera; 600,000 years ago for the Yellowstone area.

8. Gases from the volcano collected and built up in the bottom water of Lake Nios. Something triggered an overturn of the lake such that the waters were allowed to mix and the gases escaped. The most toxic gases, being heavier than air, flowed down the side of the volcano and killed many people and animals.

9. The major events are as follows:

• Early March: increased seismic activity and small explosions.

• March 18: earthquake triggered a landslide which caused mudflows down the Toutle River.

• The landslide on the mountain released enough confining pressure to cause a lateral blast eruption.

3.


• Once initiated, the eruption continued for about 9 hours; other mudflows, probably generated by the pyroclastic flows, were channelled down the Toutle River valley.

Unit 7 The Earth’s Cycles II: The Hydrosphere and the Atmosphere 1. The troposphere is inherently unstable because of the unstable density

situation created by heating the air from below. In contrast, the stratosphere is heated from above.

2. Much of the incoming solar radiation is not absorbed by the atmosphere. It reaches the Earth’s surface as visible light where it is absorbed and re-radiated as longwave radiation. This infrared radiation is absorbed by water vapor and trace gases in the atmophsere and heat is released.

3. The upward increase is due to the absorption of solar energy by oxygen to make ozone.

4. 0.026 years (i.e., 9.6 days)

5. Dust would not be evenly distributed because, it is washed out of the atmosphere rapidly in precipitation.

6. nitrogen, oxygen, argon, and carbon dioxide; methane, nitrous oxide and ammonia

7. Maximum heating is at the equator where the sun is overhead. Warm air rises and moves toward the poles. To replace the rising warm air, cool air moves in from the north and south as tradewinds. The NE and SE orientation is due to the Coriolis effect.

8. Long wave infrared radiation is absorbed by greenhouse gases, and heat is released in the process. The amount of these gases helps to determine the temperature and other climatic factors of the Earth’s surface.

9. Cl, Na, S, Mg, Ca, K, C (note: sulfates, carbonates, phosphates are not elements!)

10. 54%

11. The material would probably stay entrapped in the large clockwise circulation gyre and perhaps come ashore at Bermuda.

12. The conveyor belt circulation pattern describes the major thermohaline circulation of the deep ocean currents; this conveyor system is driven by exchange of heat and moisture (evaporation) and density differences between cold water and warm salty water. Dense cold water forms at a number of sites in the North Atlantic and spreads south along the ocean floor to the Indian and Pacific Oceans where it meets the cold, salty deep current that is flowing adjacent to Antarctica. This flow shifts northward into the Pacific and gradually rises and warms to form a somewhat less

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salty, warmer return flow current past northern Australia, southern Africa and on through the Atlantic as the Gulf Stream.

Unit 8 Exogenic geologic hazards: Floods 1. Adds nutrients to floodplain soils, flushes away accumulated salts

2. A climatic situation in which the eastern equatorial waters of the Pacific Ocean become anomalously warm. This causes increased heat content of the atmosphere and disruption of the normal climatic patterns in large areas of the world.

3. By putting more people on the floodplain; by channelization and decreased infiltration.

4. 0.27

5. 2%

6. Urbanization and population increase.

7. Upstream: caused by intense rainfall, short duration, severe, small area; downstream: caused by storms of long duration, saturate the soil, affects a wide area.

8. Perception/education, legal action, upstream management, downstream structures, (do nothing).

9. Recurrence interval is the reciprocal of the probability:

P1

M1NRI =

+= .

10. High relief basin: short lag time, high peak flow, short duration; low relief basin: long lag time, low peak flow, long duration.

Unit 9 Exogenic geologic hazards: Landslides and mass movements 1. Speed of movement: fast to slow; type of material involved: unconsolidated

to bedrock; geometry of movement: homogeneous and fragmented to nonfragmented; type of movement: slide to fall.

2. a. Dam construction created a large and deep reservoir; the higher water levels initiated creep; the engineers believed the creep could be controlled by the reservoir water level; the combination of high reservoir levels and water saturated slopes from rainfall changed the creep to slide; the slide displaced water in the reservoir which overtopped the dam.

b. Combination of reservoir level and water saturated soils due to rain.

3. The forces that promote movement and the forces that resist movement.


4. Changing slope configuration by undercutting and/or adding weight to the slope and changing the internal moisture conditions of the slope material.

5. Movement by fluidization; movement on a cushion of air; movement by dry fluidization (sturzstrom).

6. Identification before development; control the amount of water entering the slide area (excavation and retaining structures are generally not practical in large slides).

7. Many possible answers: tilted trees, tilted utility poles, taut and sagging wires, hummocky topography, leaks in pools, breaking pipes, poor alignment of fences, doors, walls.

8. Look for lobate geomorphology, scarp head, toe, lines of springs.

9. It adds weight, increases the pore pressure, and decreases the cohesion.

Unit 10 Exogenic geologic hazards: Subsidence and problem soils 1. Poorly consolidated sands/sandstones.

2. a. Near Agnew.

b. About 6cm/yr.

c. Many possible problems: Loss of land due to transgression of the Bay; loss of sewage disposal system; increased flood hazard.

3. Most likely due to a decrease in the amount of oil and gas extracted.

4. Profile should show the active layer overlying the perennially frozen ground which grades downward into the talik. The permafrost table should be shown.

5. 1:1 clay is made up of layers of an octahedral sheet alternating with a tetrahedral sheet; a 2:1 structure has an octahedral sheet between two tetrahedral sheets. The 2:1 structure allows for more volume change due to substitution of ions or water.

6. Thick, well-fractured, soluble bedrock that is mantled by thick non soluble unconsolidated sediments. A rapid drop in the water table also enhances karst development.

7. Interlake area.

8. Western Manitoba.

9. Interparticle swelling: controlled by the porosity and particle size of the granular material; intracrystalline volume change: controlled by the mineralogy of the material.

10. Several possible explanations: most of the subsidence due to compaction had already occurred before Europeans arrived; subsidence is occurring at a very slow rate and has not been “noticed” yet.

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Unit 11 Water resources and the environmental geoscientist 1. a. Decrease in density upon freezing.

b. High heat capacity.

c. Wettability.

2. On a global scale, the supply of fresh, potable water many times exceeds the existing and projected demands. However, this usable water is very poorly distributed giving rise to local and regional shortages as the demand in “marginal” areas increases.

3. The renewal times are long thus permitting an aquifer to be temporarily depleted.

4. Flood control, recreation, power generation, water supply.

5. Upstream: ecological/habitat changes, loss of land, increased seismic activity; downstream: increased erosion, downcutting, loss of sediment, loss of nutrients, increased flood hazard.

6. Unrestricted downstream development on the assumption that the dam will protect the development against all flood hazards; dam failure.

7. Surface water sources.

8. It is “legal” to use more water than normally flows in the river.

9. Very efficient, creates better crop because the water is applied directly to roots, less salinization.

10. Can be used in rolling terrain, less evaporation, inexpensive.

Unit 12 Coastal zone processes and environmental geoscience 1. About 0.06m/sec.

2. By constructing any structure which modifies the natural equilibrium of the nearshore environment, additional structures are necessary to maintain the new equilibrium.

3. The sketch should show the groin placed perpendicular to the beach and the sand piling up on one side (upstream side) of the groin. The downstream side of the groin will suffer accelerated erosion because the source of sediment being supplied by longshore drift has been interrupted.

4. Water locked up on the land in the form of glaciers, land subsidence, major tectonic movements.

5. Rising.

6. East coast: large, wide, long beaches, broad expanses of relatively shallow water, abundant wetlands; West coast: little or no shelf, sea cliffs, rugged topography, short narrow beaches.


7. A microtidal shoreline should show a very long uninterrupted beach (barrier island); a macrotidal shoreline consists of very short, stubby barrier islands interrupted frequently by tidal channel-ebb tide-flood tide delta complexes.

8. Wave: very little actual horizontal displacement of water or material in the water, obeys specific empirical relationships among height, length, velocity, frequency; current: major displacement of water/material, independent of the wave relationships, often caused by waves.

9. Sketch should show, in order, offshore bar, wave breaker zone, near shore slope, wave run up zone, beach berm, dune, washover flats, marsh/lagoon.

10. The wave energy is being concentrated on the headlands and dispersed in the bay area.

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Notes

Documents

Course Manual - University of Manitobahome.cc.umanitoba.ca/~mlast/rpublications/assets/coursemanual.pdf · beyond just the bare facts to the environmental implications and interpretations