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Physical Geography
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Landslides
Learning ObjectivesUnderstand basic slope processes and the causes of slope failure
Understand the role of driving and resisting forces on slopes and how these are related toslope stability
Understand how slope angle and topography, vegetation, water, and time affect both slope processes and the incidence of landslides
Understand how human use of the land has resulted in landslides
Know methods of identification, prevention, warning, and correction of landslides
Understand processes related to land subsidence
Mass WastingDefinition: mass wasting refers to a downslope movement of rock or soil as a more or less coherent mass. Comprehensive term all inclusive term for any downslope movement of earth materials
SlopesWeak and Soft RockForm slope with 3 segementsConvex upper partStraight central segmentConcave lower part
Hard RockForm free face with talus slope at base
Important types of mass wastingSlide downslope movement of coherent block of earth materialSlump is sliding along a curved slip plane producing slump blocksFall rocks fall from vertical faceFlow Downslope movement of unconsolidated material in which particles move about and mix within the massSubsidence is the sinking of of a mass of earth material below the level of surrounding material
Landslides are commonly complex combinations of slding and flowageUpper slump blockLower flow
Forces on SlopesThe stability of a slope expresses the relationship between resisting forces and driving forces
Driving forces forces which move earth materials downslopeDownslope component of weight of material including vegetation, fill material, or buildings
Resisting forces forces which oppose movementResisting forces include strength of material
Potential Slip PlanesGeologic surfaces of weakness in the slope material bedding, foliation, fractures
Safety FactorThe ratio of resisting forces to the driving forces RF/DFSF > 1 Slope is stableSF < 1 Slope is unstable
Factors Affecting Slope StabilityType of earth materialSlope Angle and TopographyClimateVegetationWaterTime
Rotational SlidesSliding occurs along a curved slip plane
Translational SlidesSliding occurs on a planar surface or on a slip plane
Slump (a type of slide)Indicators:ScarpHummocky terrain on and below (earthflow)
Slump
Debris FlowDebris flows are the downslope flow of relatively coarse material> 50% of particles in a debris flow are coarser than sandMovement may be very slow or very fast, depending on topographic conditions
Mudflows, debris avalanches, and debris flowsSmall to moderate magnitude events, occasional large magnitude events
Debris Flow
RockslideRock moves because theres nothing holding it back!Generally requires a pre-existing low-friction surface...
Rockslidelike a clay layer, once its wet...
Earthquake Lake, MT28 deaths in 1959, triggered by earthquake
Role of Earth MaterialsSlopes formed by weak rocks such as shale or have thick soil deposits typically fail by rotational slidesSlopes formed by hard rocks typically fail by translational slidesSoil slips occur above bedrock and fail by translational slides
Role of Slope and TopographyHillslope angle is a measure of the steepness of a slope = slope gradientSteeper slope = increased driving forcesSteep slopes associated with rockfallsSubarid to arid environments
Role of VegetationIn subhumid to humid environments, vegetation is thick and abundantLandslide activity includes deep complex landslides, earthflows, and soil creep.Vegetation influences slope stability by:Providing a cover that cushions the impact of rain falling on slopes and retards erosion on surfaceVegetation has root systems that tend to provide an apparent cohesion which increases resistance to landslidingVegetation adds weight to the slope increasing the driving forces
Role of WaterWater can affect slope stability by:Shallow soil slips can develop during rainstorms when slopes become saturatedSlumps or translational slides can develop months or years after slope is saturatedWater can erode the base or toe of a slope decreasing slope stability
Role of ClimateClimate influences the amount and timing of water in the form of water or snowInfluences type and amount of vegetation
Role of TimePhysical and chemical weathering can weaken slope materials decreasing resisting forces
Earthflowbasically a very viscous (thick) debris flowslow-movingfaster in wetter weather
Earthflow
Creepvery slowresult of freezing and thawing
Creep
shalesandstone
Creep
Triggers for rapid Mass WastingRainOversteepeningcutting at foot of slopepiling on head of slopeDeforesting / DevegetatingEarthquakes