Chapter 7 Mass Wasting. © 2012 Pearson Education, Inc. Introduction videos

Chapter 7

Mass Wasting

© 2012 Pearson Education, Inc.

Introduction videos

http://www.youtube.com/watch?v=FZSTM3knaao http://video.nationalgeographic.com/video/enviro

nment/environment-natural-disasters/landslides-and-more/landslides/

http://blogs.agu.org/landslideblog/2012/01/21/the-ultimate-compilation-of-the-best-landslide-videos/


Introduction

Mass wasting Rapid downslope movement of rock or soil as a coherent

mass Include earth flows, rock falls, and avalanches Described collectively as landslides

Slope processes All slopes are constantly evolving and materials are

always in motion downslope Slopes are composed of different segments:

High cliff or free face Talus slope Convex slope Straight slope Concave slope

© 2012 Pearson Education, Inc.Figure 7.4


Types of Landslides—Fall

Falling – Free fall of earth material

Figure 7.6a Figure 7.5


SlumpingSlumping – Sliding along a curved plane


Types of Landslides—Slides

Sliding – Movement of material as a coherent block Slumping – Sliding along a curved plane

Figure 7.6b Figure 7.6e


Types of Landslides—Flows

Flow – movement of unconsolidated material Creep – very slow flowage Earth flow, debris flow, and avalanche – rapid flowage

Figure 7.6d

© 2012 Pearson Education, Inc.Table 7.1


Forces on Slopes

Driving forces move materials downslope Weight of slope material

due to things placed on the slope such as Vegetation, fill material, or buildings

Resisting forces oppose downslope movement Shear strength of the material

Resistance of material to sliding or flowing along slip planes


Forces on Slopes: Earth Material Type

Mineral composition Shale or weak volcanic pyroclastic materials

failure occurs as creep, earth flows, debris flows, or slumps

Rock falls occur when very resistant rock overlies weak rock



Forces on Slopes: Earth Material Type, cont.

Presence of zones of weakness Slip planes, natural breaks in consistency of

materials Rotational slides have curved slip surfaces Translational slides have planar slip surfaces

Permeability – Ability to transmit water Soil slips occur when layers have contrasts in

permeability


Forces on Slopes: Slope and Topography

Steepness of slope or incline Steeper the slope, the greater the driving force Steep slopes are associated with rock falls,

avalanches, soil slips Moderate slopes are associated with earth flows Gentle slopes are associated with creep

Topographic relief or height of hill above land Mass wasting occurs more in high relief areas


Forces on Slopes: Climate

Amount and timing of water that infiltrates or erodes slope

Type and abundance of hillside vegetation Arid regions prone to rock

falls, debris flows and soil slips

Humid regions prone to complex landslides, earth flows, and creep

Figure 7.14b


Forces on Slopes: Vegetation

Vegetation provides protective cover that slows surface erosion

Roots add strength and cohesion to slope materials

Vegetation adds weight to slopes


Forces on Slopes: Water

Water saturates soil causing soil slips and debris flows

Slumps develop after deep infiltration of water

Water erodes base of slope to decrease stability

Water can cause spontaneous liquefaction or quick clay Fine grained material that loses strength when disturbed

and flows like a liquid


Forces on Slopes: Time

Forces change with time

Driving and resisting forces change with season due to changes in moisture content or water table

Chemical erosion occurs slowly over time Carbonic acid from plants dissolves limestones


Snow Avalanches

Rapid downslope movement of snow and ice Sometimes with rock and vegetation Thousands occur each year in United States and Canada

Depend on steepness of slope Angle of repose describes steepest stable slope

Snow slopes >25° are unstable, depending on temperature, wetness and shape of snow grains

Stability of snowpack

Weather


Snow Avalanches, cont.

Loose-snow avalanches Widen as move downslope

Slab avalanches Move as cohesive block More dangerous and damaging Triggered by overloading slope or zones of

weakness in the snowpack

Avalanches move down chutes Avoiding these areas can minimize hazard



Avalanche Videos

http://www.youtube.com/watch?v=B0RWLxOFGLY

http://video.nationalgeographic.com/video/environment/environment-natural-disasters/avalanches/

http://videos.howstuffworks.com/adventure/avalanche-videos-playlist.htm#video-30965


Geographic Regions at Risk from Landslides

Anywhere that have significant slopes and mountains are at risk

Factors expected to increase landslide activity: Urbanization and development of landslide-

prone areas Tree cutting in landslide-prone areas Changing global climate patterns



Effects of Landslides

Can do significant damage In United States 25 people are killed each year, damages

>$1 billion

People hit with or buried in falling debris

Slides may damage homes, roads, and utilities

Slides may block roads, impeding travel or block streams causing flooding


Links to Other Natural Hazards

Linked to almost all other hazards

Earthquakes, volcanoes, storms, and fires may cause landslides

Landslides may cause flooding or tsunamis


Natural Service Functions of Landslides

Produce deposits that become mineral resources Such as gold or diamonds

Creation of new habitats in forests and aquatic ecosystems Increases plant and animal diversity


Human Interaction with Landslides

Landslides are a natural phenomenon

Humans increase through: Expansion of urban areas, transportation networks,

and natural resource use

Humans can decrease incidence through: Grading of land surfaces can increase instability of

surfaces Building stabilizing structures and improving

drainage can decrease landslide incidents


Human Interaction with Landslides, cont. 1

Timber harvesting Clearcutting

Removing all of the trees from a defined area

Road building Building of network of logging roads

Increases erosion (soil slips) on geologically unstable land

Interrupts surface drainage, alters subsurface movement of water, and changes distribution of earth materials


Human Interaction with Landslides, cont. 2 Urbanization increases landslide activity

Removal of anchoring vegetation Construction of roads Building at higher elevations Grading

Benches are cut into slopes for home sites Placing fill materials on slopes


Identifying Potential Landslides

Crescent-shaped crack or terraces on hillside

Tongue-shaped area of bare soil or rock on hillside

Large boulders or talus piles at base of cliff

Linear path of cleared vegetation extending down a hill

Exposed bedrock with layering parallel to slope


Identifying Potential Landslides, cont.

Tongue-shaped masses of sediment at base of slope

An irregular land surface at the base of a slope

Information can be used to create slope stability map or landslide hazards map



Prevention of Landslides

Drainage control Keeps water from infiltrating a slope Drains can divert water Impermeable layers can keep water from infiltrating

Figure 7.23


Prevention of Landslides, cont.

Grading can increase slope stability Material from upper slope

can be moved to base

Slope Supports Retaining walls – concrete

or filled wire baskets

Figure 7.25


Adjustment to Hazard, Community Preparations

Movement of critical facilities Hospitals, schools and police stations should be

placed in safe locations

Landslide correction Landslide may be stopped after it starts using

drains


Adjustments to the Hazard, Personal Preparations

Get a geologic evaluation of property

Avoid homes at mouth of valley or canyon

Consult local agencies

Monitor small landslides on property

Look for cracks in house walls, leaning retaining walls, doors or windows that stick


Adjustments to the Hazard, Personal Preparations, cont.

Be wary of leaks in swimming pools or septic tanks, trees or fences that tilt, or sagging or taut utility wires

Be wary of small springs

Look for surface features

Don’t buy a home that has a landslide hazard


Things to be Aware of in Avalanches

Most avalanches are triggered by victims or others in their party

Obtain forecasts from nearest avalanche center before you go out

Most people who survive avalanches are rescued by the other members of their party

Learn avalanche safety procedures and how to evaluate snow conditions before traveling to avalanche-prone area

Documents

Chapter 7 Mass Wasting. © 2012 Pearson Education, Inc. Introduction videos