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© 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

Types of Landslides—Fall
 Falling – Free fall of earth
material
Figure 7.6a Figure 7.5

Slumping
Slumping – 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

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/enviro
nment/environment-natural-disasters/avalanches/
 http://videos.howstuffworks.com/adventure/avala
nche-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

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