Natural Hazards, 2e

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Natural Hazards, 2e

• Mass Wasting
• Chapter 6

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Learning Objectives

• Understand slope processes and the different
  types of landslides
• Know the forces that act on slopes and how they
  affect the stability of a slope
• Know what geographic regions are at risk from
  landslides
• Know the effects of landslides and their linkages
  with other natural hazards

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Learning Objectives, cont.

• Understand how people can affect the landslide
  hazard
• Be familiar with adjustments we can make to avoid
  death and damage caused by landslides

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Introduction

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

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Types of Landslides Fall

• FallingFree fall of earth material

Figure 6.6a
Figure 6.5
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Types of Landslides Slides

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

Figure 6.6e
Figure 6.6b
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Types of Landslides Flows

• Flow Movement of unconsolidated material
• Creep Very slow flowage
• Earthflow, debris flow, and avalanche Rapid
  flowage

Figure 6.6d
Figure 6.7
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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
• Safety Factor (SF) is ratio of resisting forces
  to driving forces.
• Stable when gt1 unstable lt1

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Forces on Slopes Earth Material Type

• Mineral composition
• Shale or weak volcanic pyroclastic materials
  failure occurs as creep, earthflows, debris
  flows, or slumps.
• R0ck falls occur when very resistant rock
  overlies weak rock.
• Degree of consolidation
• Slumps are common in unconsolidated materials.
• Soil slip occurs when unconsolidated materials
  are over bedrock.

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Forces on Slopes Earth Material Type, cont.

• Presence of zones of weakness
• Slip planes, natural breaks in consistency of
  materials.
• Bedding planes in sedimentary rocks, weak clay
  layers, foliation planes in metamorphic rocks
• 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.

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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, and 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.

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Forces on Slopes Climate

• Arid regions prone to rock falls, debris flows,
  and soil slips
• Humid regions prone to complex landslides,
  earthflows, and creep

Figure 6.11b
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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.

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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.

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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.

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Snow Avalanches

• Rapid downslope movement of snow and ice.
• Depend on steepness of slope, stability of
  snowpack, weather.
• Angle of repose describes the steepest angle at
  which any lose material is stable.
• Slopes gt250 are unstable, depends on temperature,
  wetness, and shape of snow grains

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Geographic Regions at Risk from Landslides

• Anywhere that have significant slopes and
  mountains
• Factors expected to increase landslide activity
• Urbanization and development of landslide-prone
  areas
• Tree cutting in landslide-prone areas
• Changing global climate patterns

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Figure 6.18
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Effects of Landslides

• 25 people are killed each year, damages gt3
  billion.
• People are 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.
• Disease.

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Links to Other Natural Hazards

• Earthquakes, volcanoes, storms, and fires may
  cause landslides.
• Landslides may cause flooding or tsunamis.

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Natural Service Functions of Landslides

• Produce deposits that become mineral resources
• Creation of new habitats
• Increases plant and animal diversity

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Human Interaction with Landslides

• Expansion of urban areas, transportation
  networks, and natural resource use has increased
  landslide incidence.
• Grading of land surfaces can increase instability
  of surfaces.
• Building stabilizing structures and improving
  drainage can decrease landslide incidents.

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Human Interaction with Landslides, cont.

• Clearcutting and road construction
• Increases landslide-related erosion on unstable
  slopes
• Interrupts surface drainage, alters subsurface
  movement of water, and changes distribution of
  earth materials
• Urbanization increases landslide activity
• Removal of anchoring vegetation
• Construction of roads
• Building at higher elevations
• Cutting the base of slopes
• Placing fill materials on slopes

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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

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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

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Prevention of Landslides

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

Figure 6.22
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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 6.25
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Landslide Warning Systems

• Provide time for people to evacuate.
• Human monitoring for changes or small rockfalls.
• Electrical systems, tiltmeters, and geophones.
• Railroads have slide fences tied to signal
  systems.
• Wells can indicate dangerous amounts of water.

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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

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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
• Dont buy a home that has a landslide hazard

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End

• Mass Wasting
• Chapter 6