Chapter 7 Mass Wasting

© 2012 Pearson Education, Inc. Introduction videos   nment/environment-natural-disasters/landslides- and-more/landslides/ nment/environment-natural-disasters/landslides- and-more/landslides/  -ultimate-compilation-of-the-best-landslide- videos/ -ultimate-compilation-of-the-best-landslide- videos/

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Types of Landslides—Fall  Falling – Free fall of earth material Figure 7.6aFigure 7.5

© 2012 Pearson Education, Inc. Slumping Slumping – Sliding along a curved plane

© 2012 Pearson Education, Inc. Types of Landslides—Slides  Sliding – Movement of material as a coherent block  Slumping – Sliding along a curved plane Figure 7.6bFigure 7.6e

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 7.10

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 7.18

© 2012 Pearson Education, Inc. Avalanche Videos  Y Y  nment/environment-natural-disasters/avalanches/ nment/environment-natural-disasters/avalanches/  nche-videos-playlist.htm#video nche-videos-playlist.htm#video-30965

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 7.19

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 7.22

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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