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Mass Wasting Sculpturing Earth’s Landscape What is Mass Wasting? • The downslope movement of material under the influence of gravity • May occur at rates of only a few inches per year – result in little damage and no loss of life • May occur at over 100 mph – result in great loss of life and property damage Mass Wasting – Factors Controlling Mass Wasting • Angle of Repose • Pore Water • Material • Orientation of Rock Layers • Vegetation Mass Wasting - Factors • Angle of Repose – The angle at which loose material sits at rest – Major determining factor in slope stability – Every material and slope has an inherent angle it becomes unstable. – Typically 25°-40° Angle of repose • In general, coarser grained, poorly sorted, and angular rocks have a higher angle of repose Mass Wasting - Factors • Pore Water – Water held within the void spaces – May provide surface tension When Saturated: – Adds weight – Provides lubrication Pore Water • No water – grains held together by frictional forces • Some water – Grains held together by surface tension • Too much water – Provides lubrication and grains move freely Mass Wasting – Factors • Material – Loose soil and sediment are more prone to slope failure – Sand material have greater pore spaces – Horizontal layers are more stable than loose sediment –Massive rocks tend to be the most stable Mass Wasting – Factors • Orientation – pertains to layered rocks – When layers dip in the same direction as slope, failure is more likely – Horizontal layers are more stable –Most stable is when rock layers dip into slope Mass Wasting - Factors • Vegetation – roots form an anchor for soil – reduces amount of water in pore spaces Mass Wasting - Factors Mass Wasting - Types • Slow Movement – Creep • Rapid Movement – Rock fall – Slump – Rock slide – Debris Flow Mass Wasting – Types (slow) • Creep – Gradual downhill movement of slope material – Result in tree trunks to curve at base Mass Wasting – Types (fast) • Rock fall – Free fall of detached pieces of material of any size – May occur as a result of freeze-thaw – May occur by the loosening action of plant roots Mass Wasting – Types • Slumps – A mass of material sliding along a curved, rotational surface (shaped like a spoon) – Scarp is crescent-shaped cliff at upslope end – Bulge is usually at toe where soil accumulates Mass Wasting – Types • Rockslides – Also called “landslides” – Blocks of rocks and soil move down a welldefined plane (sliding) – Among the most destructive – May be triggered by rain, melting of snow, or earthquakes Mass Wasting – Types • Debris Flows – Downslope movement of a viscous (thick) fluid – Commonly occur in volcanic areas – Typically follow drainage patterns (valleys) Mass Wasting – Classification Types Mass Wasting Potential Road Cut Brush Fires Volcanoes Heavy Rains Debris Flows Nevado del Ruiz Monterey Hills Mass Wasting Hazards Mitigation - Identification Mitigation – Identification • Studying aerial photographs to determine previous landslides • Field investigations of potentially unstable slopes Mitigation - Prevention • Steep slopes can be graded into gradual slopes • Retaining walls can be built • Drain pipes to reduce pore water