Landslide Hazard Zonation Mapping

BY Prof. A. Balasubramanian

Centre for Advanced Studies in Earth Science, University of Mysore,

Mysore-6

Landslide

• Is the down slope movement of loose soil, uncompact rock, and organic materials,

• under the effects of gravity along a sliding plane,

• Triggered by severe Rf, Sws, wt of overburden.

• Creates a new landform resulting from such movement.

• Hazardous/ disastrous.

Landslides

• associated with specific mechanics of slope failure and the properties and characteristics of failure types.

• Landslides are experienced in the hilly and mountainous areas, all over the world.

• May start from the top of a hill slope and move till the end of foot hill,

• or may start from the bottom of a hill slope, drawing all materials from the top to bottom.

Mass movement

• It is the movement of surface material caused by gravity. Landslides and rockfalls are examples of very sudden movements of this type.

• Of course geological agents such as water, wind and ice all work with gravity to cause a leveling of land.

Hill slopes

• Hills and mountains are typical geomorphic features.

• They are characterized by gentle or steep slopes.

• They are unique climatic zones.

• They are characterized by thick soil profiles, richness with soil moisture and plant growth.

Landslides as natural hazards

• Landslides normally occur on hill slopes.

• When the slopes are stable there will be no problem.

• When the slopes become unstable then they may lead to landslides.

• Landslides occur when the stability of the slope changes from a stable to an unstable condition.

• The change in the stability of a slope may happen due to several factors.

• These factors may be acting alone or acting together.

• The overburden may be mostly loose soil, may have a thick weathered zone, adequate natural or man-made vegetation, good root penetration, and all are under the direct influence of Climatic variations, especially rainfall.

Slope and bedrocks

• The Bedrock is related to the basement rocks of the hill.

• These may be massive or fresh, structurally weak or deformed.

• All of these may be under the influence of a tectonic force originating from the deep interior or a nearby huge structure like a dam.

• The next factor is the degree of slope of the area.

• Slope is a common factor for a landslide.

• The categories of slope is to be understood first.

• It varies depending upon the geomorphic conditions, bedrock, nature of overburden, vegetation and drainage systems.

Slope

• A slope is the rise or fall of the land surface.

• It is important for the farmer or irrigator to identify the slopes on the land.

TYPES OF SLOPES

• Gentle

• Steep

• Concave

• Convex

• The slope of line a is m=5/6

• The slope of line b is m= -1/4

• The slope of line c is m= 3/7

• The slope of line d is m= -6/3 or m=-2

• The slope of line e is m= 0 (but how do you use m= rise/run to get this number?)

Categories of slopes

• The following are the categories of slopes:

• Cliff > 80 degrees

• Precipitous 50-80 degrees

• very steep or steep 20-50 degrees

• moderate slope 6-20 degrees

• gentle slope 1-6 degrees

• flat terrain < 1 degree.

Slope (%) Approximate degrees

Terminology

0 - 0.5 0 Level

0.5 - 2 0.3 - 1.1 Nearly level

2 - 5 1.1 - 3 Very gentle slope

5 - 9 3 - 5 Gentle slope

9 - 15 5 - 8.5 Moderate slope

15 - 30 8.5 - 16.5 Strong slope

30 - 45 16.5 - 24 Very strong slope

45 - 70 24 - 35 Extreme slope

70 - 100 35 - 45 Steep slope

> 100 > 45 Very steep slope

Slope may be expressed in several ways

• but all depend upon the comparison of vertical distance (VD) to horizontal distance (HD).

• Before we can determine the percentage of a slope, we must know the VD of the slope. The VD is determined by subtracting the lowest point of the slope from the highest point.

Overburden soils and Bedrocks

• Rock type

• Structure

• Dip and strike

• Soil profile- properties of soils

• Rock material properties

• Rock mass properties

• & lastly Triggering mechanisms.

What do these pictures show?

Parts of a landslide zone

Landslide classification

• The landslide classification based on Varnes' (1978)system has two terms:

• -the first term describes the material type,-the second term describes the type of movement.

The material types- Rock, Earth, Soil, Mud and Debris

• Rock: is “a hard or firm mass that was intact and in its natural place before the initiation of movement”.

• Soil: is “an aggregate of solid particles, generally of minerals and rocks, that either was transported or was formed by the weathering of rock in place. Gases or liquids filling the pores of the soil form part of the soil”.

• Earth: “describes material in which 80% or more of the particles are smaller than 2mm, the upper limit of sand sized particles”.

• Mud: “describes material in which 80% or more of the particles are smaller than 0.06mm, the upper limit of silt sized particles”.

• Debris: “contains a significant proportion of coarse material; 20% to 80% of the particles are larger than 2mm, and the remainder are less than 2mm”.

VARNES´ CLASSIFICATION OF SLOPE MOVEMENTS

• The five kinematically distinct types of movement are described in the sequence: - fall, - topple, - slide, - spread, - flow.

• Combining the two terms gives classifications such as:

• Rock fall,

• Rock topple,

• Debris slide,

• Debris flow,

• Earth slide,

• Earth spread, etc.

Landslides: Rock Fall

• Rock Falls are the detachment of rock from a steep slope along a surface on which little or no shear displacement takes place. The material then descends largely by falling, bouncing or rolling.

Rock Fall -Diagnostic Characteristics • A fall starts with the detachment

of soil or rock from a steep slope along a surface on which little or no shear displacement takes place. The material then descends largely by falling, bouncing or rolling. Free fall movement of material from a steep slope or cliff, EPOCH (1993).

• Toppling failures are distinguished by the forward rotation of a unit about a pivot point. Typically involving tall columns of rock Vertical or steeply dipping discontinuities behind the block allow the rock mass to topple out of the face and second set of orthogonal joints, which defines the column height.

Landslides: Rock Topple

• A topple is the forward rotation, out of the slope, of a mass of soil and rock about a point or axis below the centre of gravity of the displaced mass.

Landslides: Rock Topple Diagnostic Characteristics

• Toppling failures are distinguished by the forward rotation of a unit about a pivot point.

• Typically involving tall columns of rock. Vertical or steeply dipping discontinuities behind the block allow the rock mass to topple out of the face and second set of orthogonal joints, which defines the column height. Similar to a fall but involves a pivoting action rather than a complex separation at the base of the failure, EPOCH (1993).

Rock Topple Diagnostic Characteristics

Landslides: Rock & Block Slide

• A slide is the downslope movement of a soil or rock mass occurring dominantly on the surface of rupture or relatively thin zones of intense shear strain.

Types of landslides

Translational slides move along a flat slippery zone.

Rotational slides move along a concave (bowl shaped) slippery zone.

Common types of slope failure

What Causes Landslides?

• There are two primary categories of causes of landslides: natural and human-caused. Sometimes, landslides are caused, or made worse, by a combination of the two factors.

• Natural Occurrences

• This category has three major triggering mechanisms that can occur either singly or in combination —

• (1) water, (2) seismic activity, and (3) volcanic activity.

• Effects of all of these causes vary widely and depend on factors such as steepness of slope, morphology or shape of terrain, soil type, underlying geology, and whether there are people or structures on the affected areas.

Landslide Causes and Triggering Mechanisms

• Physical Causes—Triggers • Intense rainfall • Rapid snowmelt • Prolonged intense precipitation • Rapid drawdown (of floods and tides) or filling • Earthquake • Volcanic eruption • Thawing • Freeze-and-thaw weathering • Shrink-and-swell weathering • Flooding.

Natural Causes: Geological causes

• Weak materials, such as some volcanic slopes or unconsolidated marine sediments, for example

• Susceptible materials • Weathered materials • Sheared materials • Jointed or fissured materials • Adversely oriented mass disconti nuity (bedding, schistosity, and so

forth) • Adversely oriented structural discontinuity (fault, unconformity,

contact, and so forth) • Contrast in permeability • Contrast in stiffness (stiff, dense material over plastic materials)

Natural Causes: Morphological causes

• Tectonic or volcanic uplift • Glacial rebound • Glacial meltwater outburst • Fluvial erosion of slope toe • Wave erosion of slope toe • Glacial erosion of slope toe • Erosion of lateral margins • Subterranean erosion (solution, piping) • Deposition loading slope or its crest • Vegetation removal (by forest fire, drought)

Human Causes

• Excavation of slope or its toe • Use of unstable earth fills, for construction • Loading of slope or its crest, such as placing earth fill at the top of a

slope • Drawdown and filling (of reservoirs) • Deforestation—cutting down trees/logging and (or) clearing land

for crops; unstable logging roads • Irrigation and (or) lawn watering • Mining/mine waste containment • Artificial vibration such as pile driving, explosions, or other strong

ground vibrations • Water leakage from utilities, such as water or sewer lines • Diversion (planned or unplanned) of a river current or longshore

current by construction of piers, dikes, weirs, and so forth

Landslides and Water

• Slope saturation by water is a primary cause of landslides.

• Saturation can occur in the form of intense rainfall, snowmelt, changes in ground-water levels, and surface-water level changes along coastlines, earth dams, and in the banks of lakes, reservoirs, canals, and rivers.

Landslides and Seismic Activity

• Many mountainous areas that are vulnerable to landslides have also experienced at least moderate rates of earthquake activity in recorded times.

• Earthquakes in steep landslide-prone areas greatly increase the likelihood that landslides will occur, due to ground shaking alone, liquefaction of susceptible sediments, or shaking-caused dilation of soil materials, which allows rapid infiltration of water.

• Rockfalls and rock topples can also be caused by loosening of rocks or rocky formations as a result of earthquake ground shaking.

Landslides and Volcanic Activity

• Landslides due to volcanic activity represent some of the most devastating types of failures.

• Volcanic lava may melt snow rapidly, which can form a deluge of rock, soil, ash, and water that accelerates rapidly on the steep slopes of volcanoes, devastating anything in its path.

• Volcanic edifices are young, unconsolidated, and geologically weak structures that in many cases can collapse and cause rockslides, landslides, and debris avalanches.

Human Activities and landslides

• Populations expanding onto new land and creating neighborhoods, towns, and cities is the primary means by which humans contribute to the occurrence of landslides.

• Disturbing or changing drainage patterns, destabilizing slopes, and removing vegetation are common human-induced factors that may initiate landslides.

Human activities and landslides

• landslides may also occur in once-stable areas due to other human activities such as irrigation, lawn watering, draining of reservoirs (or creating them), leaking pipes, and improper excavating or grading on slopes.

• New construction on landslide-prone land can be improved through proper engineering (for example, grading, excavating) by first identifying the site’s susceptibility to slope failures and by creating appropriate landslide zoning.

Mining

Evaluating Landslide Hazards

• There are many different means of assessing landslide hazard for an area; it is always advisable to consult with an expert for the most accurate assessment, although this is not always possible.

• Two types of landslide hazard evaluation, direct observation and the use of technological tools.

Features that might indicate landslide movement:

• Springs, seeps, and wet or saturated ground in previously dry areas on or below slopes.

• Ground cracks—cracks in snow, ice, soil, or rock on or at the head of slopes.

• Sidewalks or slabs pulling away from structures if near a slope; soil pulling away from foundations.

• Offset fence lines, which were once straight or configured differently

• Unusual bulges or elevation changes in the ground, pavements, paths, or sidewalks.

• Tilting telephone poles, trees, retaining walls, fences. • Excessive tilting or cracking of concrete floors and foundations. • Broken water lines and other underground utilities.

Indications of sliding lands

• Rapid increase or decrease in stream-water levels, possibly accompanied by increased turbidity (soil content clouding the water).

• Sticking doors and windows and visible open spaces, indicating walls and frames are shifting and deforming.

• Creaking, snapping, or popping noises from a house, building, or grove of trees (for example, roots snapping or breaking).

• Sunken or down-dropped roads or paths.

Technological Tools for Evaluation of Landslides

• One of the guiding principles of geology is that the past is the key to the future.

• In evaluating landslide hazards this means that future slope failures could occur as a result of the same geologic, geomorphic, and hydrologic situations that led to past and present failures.

• Based on this assumption, it is possible to estimate the types, frequency of occurrence, extent, and consequences of slope failures that may occur in the future.

• However, the absence of past events in a specific area does not preclude future failures.

• Human-induced conditions, such as changes in the natural topography or hydrologic conditions, can create or increase an area’s susceptibility to slope failure.

Predicting landslide hazards

• In order to predict landslide hazards in an area, the conditions and processes that promote instability must be identified and their relative contributions to slope failure estimated, if possible.

• Useful conclusions concerning increased probability of landsliding can be drawn by combining geological analyses with knowledge of short- and long-term meteorological conditions.

Map Analysis

• Map analysis is usually one of the first steps in a landslide investigation. Necessary maps include bedrock and surficial geology, topography, soils, and if available, geomorphology maps.

• Using knowledge of geologic materials and processes, a trained person can obtain a general idea of landslide susceptibility from such maps.

Aerial Reconnaissance

• Analysis of aerial photography is a quick and valuable technique for identifying landslides, because it provides a three-dimensional overview of the terrain and indicates human activities as well as much geologic information to a trained person.

• In addition, the availability of many types of aerial imagery (satellite, infrared, radar, and so forth) makes aerial reconnaissance very versatile although cost-prohibitive in some cases.

Field Reconnaissance

• Many of the more subtle signs of slope movement cannot be identified on maps or photographs.

• Indeed, if an area is heavily forested or has been urbanized, even major features may not be evident.

• Furthermore, landslide features change over time on an active slide.

• Thus, field reconnaissance is always mandatory to verify or detect landslide features, and to critically evaluate the potential instability of vulnerable slopes. It identifies areas with past landslides (which could indicate future likelihood of landslides) by using field mapping and laboratory testing of terrain through the sampling of soil and rock.

• Mapping and laboratory testing for example, may identify vulnerable clays or other susceptible soils and show where they exist and their size and extent.

Drilling

• At most sites, drilling is necessary to determine the type of earth materials involved in the slide, the depth to the slip surface, and thus the thickness and geometry of the landslide mass, the water-table level, and the degree of disruption of the landslide materials.

• It also can provide suitable samples for age-dating and testing the engineering properties of landslide materials.

• Finally, drilling is needed for installation of some monitoring instruments and hydrologic observation wells.

• Note that drilling for information on stratigraphy, geology, water-table levels and for installation of instruments, for example, is also done for areas that have never had a landslide but for which the possibility exists.

Instrumentation

• Sophisticated methods such as electronic distance measurement (EDM), instruments such as inclinometers, extensometers, strain meters, and piezometers , and simple techniques, such as establishing control points by using stakes can all be used to determine the mechanics of landslide movement and to monitor and warn against impending slope failure.

Geophysical Studies

• Geophysical techniques (measurement of soil’s electrical conductivity/resistivity, or measurement of induced seismic behavior) can be used to determine some subsurface characteristics such as the depth to bedrock, stratigraphic layers, zones of saturation, and sometimes the ground-water table.

• It can also be used to determine texture, porosity, and degree of consolidation of subsurface materials and the geometry of the units involved.

• In most instances, such surface survey methods can best be used to supplement drilling information, spatially extending and interpolating data between boreholes.

• They can also offer an alternative if drilling is impossible.

• Downhole geophysical methods (nuclear, electrical, thermal, seismic) also can be applied to derive detailed measurements in a borehole.

• Monitoring of natural acoustic emissions from moving soil or rock has also been used in landslide studies.

Acoustic Imagery and Profiles

• Profiles of lakebeds, river bottoms, and the sea floor can be obtained using acoustic techniques such as side-scan sonar and sub-bottom seismic profiling.

• Surveying of controlled grids, with accurate navigation, can yield three-dimensional perspectives of subaqueous geologic phenomena.

• Modern, high-resolution techniques are used routinely in offshore shelf areas to map geologic hazards for offshore engineering.

Computerized Landslide Terrain Analysis

• In recent years, computer modeling of landslides has been used to determine the volume of landslide masses and changes in surface expression and cross section over time.

• This information is useful in calculating the potential for stream blockage, cost of landslide removal (based on volume), and type and mechanism of movement.

• Very promising methods are being developed that use digital elevation models (DEMs) to evaluate areas quickly for their susceptibility to landslide/debris-flow events.

• Computers also are being used to perform complex stability analyses.

• Landslide Evaluation Tools— Mapping, Remote Sensing, and Monitoring of Landslides

Mapping

• Maps are a useful and convenient tool for presenting information on landslide hazards.

• They can present many kinds and combinations of information at different levels of detail.

• Hazard maps used in conjunction with land-use maps are a valuable planning tool.

• Commonly, there is a three-stage approach to landslide hazard mapping.

• The first stage is regional or reconnaissance mapping, which synthesizes available data and identifies general problem areas.

• This regional scale (sometimes called “small-scale”) mapping is usually performed by a Provincial, State, or Federal geological survey.

• The next stage is community-level mapping, a more detailed surface and subsurface mapping program in complex problem areas.

• Finally, detailed site-specific large-scale maps are prepared.

• If resources are limited, it may be more prudent to bypass regional mapping and concentrate on a few known areas of concern.

• We discuss three types of general mapping; (1) Regional,

• (2) Community level, and

• (3) Site specific.

Regional mapping

• Regional or reconnaissance mapping supplies basic data for regional planning by providing baseline information for conducting more detailed studies at the community and site-specific levels and for setting priorities for future mapping.

• Such maps are usually simple inventory or susceptibility maps and are directed primarily toward the identification and delineation of regional landslide problem areas and the conditions under which they occur.

• They concentrate on those geologic units or environments in which additional movements are most likely.

• The geographical extent of regional maps can vary from a map of a State or Province to a national map, which delineates an entire country.

• Such mapping relies heavily on photogeology (the geologic interpretation of aerial photography), reconnaissance field mapping, and the collection and synthesis of all available pertinent geologic data.

• Map scales at this level are typically at scales ranging from 1:10,000 down to 1:4,000,000 or even smaller.

Community-level mapping

• This type of mapping identifies both the three-dimensional potential of landsliding and considers its causes.

• Guidance concerning land use, zoning, and building, and recommendations for future site-specific investigations also are made at this stage.

• Investigations should include subsurface exploratory work in order to produce a map with cross sections.

• Map scales at this level typically vary from 1:1,000 to 1:10,000.

Site-specific mapping

• Site-specific mapping is concerned with the identification, analysis, and solution of actual site-specific problems, often presented in the size of a residential lot.

• It is usually undertaken by private consultants for landowners who propose site development and typically involves a detailed drilling program with downhole logging, sampling, and laboratory analysis in order to procure the necessary information for design and construction.

• Map scales vary but usually are about 1:600 or 25 mm (1 inch) equal to 16 m (50 feet).

Three Important Criteria for Landslide Maps

• The three types of landslide maps most useful to planners and the general public are

• (1) landslide inventories,

• (2) landslide susceptibility maps, and

• (3) landslide hazard maps.

Landslide inventory maps

• Inventories denote areas that are identified as having failed by landslide processes.

• inventories that depict and classify each landslide and show scarps, zones of depletion and accumulation, active and inactive slides, geological age, rate of movement, and (or) other pertinent data on depth and kind of materials involved in sliding.

• One way is to use aerial photography.

Landslide inventory details

• state of activity,

• certainty of identification,

• dominant types of slope movement, estimated thickness of landslide material,

• type of material, and

• dates or periods of activity.

Landslide susceptibility maps

• A landslide susceptibility map goes beyond an inventory map and depicts areas that have the potential for landsliding.

• These areas are determined by correlating some of the principal factors that contribute to landsliding (such as steep slopes, weak geologic units that lose strength when saturated or disturbed, and poorly drained rock or soil) with the past distribution of landslides.

• These maps indicate only the relative stability of slopes; they do not make absolute predictions.

• Landslide susceptibility maps can be considered derivatives of landslide inventory maps because an inventory is essential for preparing a susceptibility map.

• For example, overlaying a geologic map with an inventory map that shows existing landslides can identify specific landslide-prone geologic units.

• This information can then be extrapolated to predict other areas of potential landsliding.

• More complex maps may include additional information such as slope angle and drainage.

Landslide hazard maps

• Hazard maps show the areal extent of threatening processes:

• where landslide processes have occurred in the past, recent occurrences, and most important, the likelihood in various areas that a landslide will occur in the future.

• For a given area, hazard maps contain detailed information on the types of landslides, extent of slope subject to failure, and probable maximum extent of ground movement.

• These maps can be used to predict the relative degree of hazard in a landslide area.

• Areas may be ranked in a hierarchy such as low, moderate, and high hazard areas.

• Remote Sensing and Other Tools that Show the Features of Landslide Activity

Techniques to be adopted

• Topographic map: Indicates slope gradient, terrain configuration, drainage pattern.

• Terrain Map

• Identifies material, depth, geological processes, terrain configuration, surface and subsurface drainage, slope gradient (also called surficial geology or Quaternary geology maps).

• Bedrock Map

• Identifies bedrock type, surface and subsurface structure, surficial cover (overburden), and age of rock over a topographic map base.

• Engineering Soil Map

• Identifies surficial material type, drainage, limited engineering characteristics, soils characteristics, vegetation cover.

• InSAR Imaging InSAR is an acronym for Interferometric Synthetic Aperture Radar.

• Both InSAR and LIDAR (description follows) use active sensors emitting a pulse of energy (from a satellite) and recording its return, from the ground, at the sensor.

• Most InSAR equipment is able to penetrate fog and rain and can be used in areas difficult to access by foot.

• LiDAR Imaging LiDAR is an acronym for Light Detection and Ranging, also known as ALSM or Airborne Laser Swath Mapping.

• Using a narrow laser beam to probe through dense ground cover, such as trees, LiDAR can produce accurate terrain maps even where forest cover gets in the way of traditional photography.

• The technique produces a very accurate Digital Elevation Model map (DEM)

Real-Time Monitoring of Landslides and Landslide Instrumentation

• The immediate detection of landslide activity that is provided by real-time systems can be crucial in saving human lives and protecting property.

• Traditional field observations, even if taken regularly, cannot detect changes at the moment they occur.

Earth Slope Stabilization/Mitigation

• Removal of soil from the head of a slide

• Reducing the height of the slope

• Backfilling with lightweight material

• Benches

• Flattening or reducing slope angle, or other slope modification

Strengthening Slopes

• Plastic mesh reinforcement

• Rock-fill buttresses

• Stream channel linings

• Check Dams

• Drainage Techniques-Ditches and drains

• Drainpipes

• Retaining Walls, Steel bin wall

• Reinforced earth wall

Slope Stabilization Using Vegetation

• Seeding with grasses and legumes reduces surface erosion, which can under certain conditions lead to landslides.

• Planting with shrubs adds vegetative cover and stronger root systems, which in turn will enhance slope stability.

• Biotechnical Slope Protection

Methods of approach

• Forest Cover Map • Identifies surface vegetation, topographic features,

surface drainage pattern, and in some cases, soil drainage character.

• Research Studies • May provide information on all of the above, plus

quantitative data on controlling factors and possibly local stability risk assessment.

• Aerial Photography Remote Sensing • Identification can be made of vegetation cover,

topography, drainage pattern, soil drainage character, bedrock geology, surficial geology, landslide type, and relationship to other factors.

• Landslides and flooding are closely associated because both are related to precipitation, runoff, and the saturation of ground by water.

• Flooding may cause landslides by undercutting banks of streams and rivers and by saturation of slopes by surface water (overland flow).

Rock Slope Stability Analysis: Limit Equilibrium Method

• • Plane failure analysis

• • Wedge failure analysis

• • Toppling failure analysis

• Stability analysis of slope : circular failure

• Stability Analysis in Presence of Water

•

Classification systems in slope stability analysis

• Slope Mass Rating (SMR)

• Chinese Slope Mass Rating System (CSMR)

• Rock slope rating (RSR)

• Slope stability rating (SSR) classification system, and

• Dump mass rating

Hill slopes

• Hill slopes have a good drainage of rainwater.

• The bed rock configurations are also very unique.

• The Subsurface Conditions of a hilly terrain may have two distinct zones as overburden and the basement rock.

• Forest Cover Map • Identifies surface vegetation, topographic features,

surface drainage pattern, and in some cases, soil drainage character.

• Research Studies • May provide information on all of the above, plus

quantitative data on controlling factors and possibly local stability risk assessment.

• Aerial Photography Remote Sensing • (Examples shown in figs. B4 through B7.) Identification

can be made of vegetation cover, topography, drainage pattern, soil drainage character, bedrock geology, surficial geology, landslide type, and relationship to other factors.

RISK MANAGEMENT PROCESS

• The Risk Management process comprises three components:

• · Risk Analysis

• · Risk Evaluation, and

• · Risk Treatment.

HAZARD IDENTIFICATION

• it is important to define:

• The site, being the primary area of interest

• Geographic limits that may be involved in the processes that affect the site.

• Hazard (landslide) identification requires an understanding of the slope processes and the relationship of those

• processes to geomorphology, geology, hydrogeology, climate and vegetation.

Basic documents required

• The basic documents for the hazard evaluation are: • - Topographical data: Official topographical maps and DEM, • - Geological data (lithological map), • - Joint sets • - Land use planning, existing buildings and other facilities, • - Thematic maps (rockfall sources, menacing blocks), • - Event catalogue (derived from chronicles, silent witnesses

in the field and disaster • documentation), • - Measurements of the distribution of block sizes at sample

plots of the talus slope.

Hazard zoning

• – The subdivision of the terrain in zones that are characterized by the temporal probability of occurrence of landslides of a particular size and volume, within a given period of time. Landslide hazard maps should indicate both the zones where landslides may occur as well as the runout zones. A complete quantitative landslide hazard assessment includes:

• • spatial probability: the probability that a given area is hit by a landslide

• • temporal probability: the probability that a given triggering event will cause landslides

• • size/volume probability: probability that the slide has a given size/volume

• • runout probability: probability that the slide will reach a certain distance downslope

•

• Landslide Hazard Zonation: A Case Study in Garhwal Himalaya, India

• Author(s): S. Sarkar, D. P. Kanungo and G. S. Mehrotra

• Mountain Research and Development, Vol. 15, No. 4 (Nov., 1995), pp. 301-309

Study area

Geology and structure

Slope

Distribution of landslides

Statistical Analysis

Slope & Landuse

Landslide Susceptibility Index

LH Zonation map

GIS - Links Data Sets

• GIS software links the location data and the attribute data:

Grid based approach

ArcGIS Spatial Analyst Tools

Types of DEMs

Triangulated Irregular Network (TIN)

A Mesh of Triangles

Triangle is the only polygon that is always

planar in 3-D

Points Lines Surfaces

Tin Triangles in 3-D

(x3, y3, z3)

(x1, y1, z1) (x2, y2, z2)

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y

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Projection in (x,y) plane

Flow On a Triangle

2.0

13.0

9.0

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Flow On a TIN

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Conceptual Model

Triangulate

Enforce Breaklines

Interpolate Z

5: Create TIN

Defining Basins

Computing Basin Data

• Area

• Slope

• Flow Distances – Slopes

• Aspect

• Stream Lengths – Slopes

• Others

A=3.18 acr

BS=0.0124 ft/ft

AOFD=140.06 ft

A=5.39 acr

BS=0.0243 ft/ft

AOFD=158.33 ft

A=7.21 acr

BS=0.0200 ft/ft

AOFD=93.47 ft

• Landslide susceptibility map – A map showing the subdivision of the terrain in zones that have a different likelihood that landslide of a type may occur. The likelihood may be indicated either qualitatively (as high, moderate low, and not susceptible) or quantitatively (e.g. as the density in number per square kilometres, or area affected per square kilometre). Landslide susceptibility maps should indicate both the zones where landslides may occur as well as the runout zones.

Various approaches used in GIS modeling

Analyze Landscape Structure

Use Statistics to Compare Map Surfaces

Identifying Convex and Concave features by deviation from the trend of the terrain

Example of a micro-terrain deviation surface

Calculation of slope considers the arrangement and magnitude of elevation differences within a

roving window

Longitudinal slope calculates the steepness along a linear feature, such as a pipeline

Modified transverse slope is calculated as the average of off-route slopes surrounding a location

Analyzing the slope differences along a proposed route

2-D, 3-D and draped displays of terrain roughness

Map of surface flow confluence

2-D, 3-D and draped displays of surface flow confluence

Identifying Upland Ridges

A simple erosion potential model combines information on terrain steepness and water

flow confluence

Extended erosion model considering sediment loading potential considering intervening

terrain conditions

Gentle slopes surrounding streams are visually apparent on a 3D plot of elevation

Slope values for the terrain surface are calculated and gently sloped areas identified

Valley bottoms are identified as flat areas connected to streams

Use Surface Area for Realistic Calculations

Visual comparison of different slope maps

Comparison of Fitted versus Generalized slope maps

GIS Modeling

PURPOSE OF LANDSLIDE ZONING MAPS

• Landslide zoning may be developed by preparing different maps that, according to the type of zoning, can be distinguished among (see also the definitions given Chapter 2):

• • Landslide inventory map; • • Landslide susceptibility zoning map; • • Landslide hazard zoning map; • • Elements at risk map; • • Consequence scenario map; • • Landslide risk zoning map.

• Landslide inventory map may be used for susceptibility zoning and/or as information for policy makers and the general public;

• • Landslide susceptibility zoning map may be used to prepare the hazard map and/or, in combination with elements at landslide risk within the susceptible area, as information for policy makers and the general public. It may be also used as advisory where the available records of incident data allows the assessment of the societal risk (e.g., in terms of F-N curves) within the susceptible areas threatened by rapid to extremely rapid landslides (Cruden and Varnes, 1996);

• • Landslide hazard zoning map can be used as information, advisory or statutory to control the development of threatened areas, representing the most efficient and economic way to reduce future damage and loss of life. Such maps also provide the appropriate element of decision for considering the feasibility of the development with or without any stabilisation or protective countermeasures (Cascini et al., 2005);

• • Elements at risk map is used to prepare the consequence scenarios map and, in combination with the landslide susceptibility zoning map, may be used as information and advisory for policy makers and general public;

• • Consequence scenario map may be used as information and advisory showing the areas that require QRA. Using quantitative procedures, this map provides for each element the consequence scenario related to its vulnerability and a given landslide hazard; in such a case, it may be used as information, advisory and statutory.

• • Landslide risk zoning map may be used as statutory and allow the implementation of alert system aimed at protecting the human life. In addition, QRA provides a global view of the expected annual damage for the elements at risk due to the landslide hazard. It can be used as statutory and design and, on the basis of cost-benefit analysis, either control or stabilization works can be identified and designed for landslide risk mitigation.

ANALYSIS FOR LANDSLIDE HAZARD ZONATION:-

• Digital Elevation Model (DEM)

• Digitized contour map

• Elevation sliced map

• Slope map in degree

• Slope aspect map

• Flow direction map

• Flow accumulation map

• Village Map/ Road Map

• Google Earth Data

• Land Cover Map

• Lithology Map

A landslide hazard zonation map

• is prepared to assist mitigation planners in wake of landslide trigger. In the present study pre and post earthquake remote sensing data has been used to prepare landslide inventory.

• Remote sensing data is further used to delineate drainage pattern, photo lineaments, structural features, lithologial features, and land/use land/cover type of the area by applying digital image processing techniques.

• Geological features are analyzed using criteria such as colour, tones, topography and stream drainage pattern from the imageries.

• Digital elevation model data is used to generate primary topographic attributes namely, slope, aspect, and relative relief.

• For landslide hazard zonation (LHZ) different thematic maps such as land-cover map, slope map, relative relief map, structural map, lithology map, lineament buffer map, drainage buffer map, soil map, are assigned relative weight on ordinal scale to obtain landslide hazard index (LHI).

• Threshold values are selected according to breaks in LHI frequency and a LHZ map is prepared which contains very low hazard, low hazard, moderate hazard, high hazard and very high hazard zones.

• Study suggests that landslides in present area are influenced by the proximity to drainage, lineament and topographic attributes

• Landslides are triggered by many extrinsic causative factors such as rainfall, earthquake, blasting and drilling, cloudburst and flashfloods. Himalayan region has highly undulating terrain which is a witness of ongoing orogeny process.

WEIGHT ANALYSIS

• Weighted rating system is based on the relative importance of various causative factors derived from field knowledge.

• Input data layers such as soil map, lineament buffer map, slope map etc. are assigned weightage (out of total 100%) factor according to their corresponding impact on the landslide triggers.

• Different classes of input layers are given rating on the scale of 1 to 10 where 1 stands for the class which has minimum impact on landslide.

Landslide Hazard Index

• is prepared by assigning influence/weight to factor and rating to different classes. Weight factor of the input layer is multiplied by the corresponding rating given to that particular class on pixel basis.

• Finally summation of each layer is done.

• LHI = Σ Weight/influence of factors× ratings of classes/attributes

SUMMERY OF ASSIGNED WEIGHT AND RATING

GIS Lanslide Susceptibility Model in ArcGIS Model Builder

Ground Penetrating Radar

GPR

Lidar can show the

Lidar + GIS: Revolutionizing Landslide Mapping and Field Evaluation