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 – Major determining factor in slope stability – Every material and slope has an inherent angle it becomes unstable. – The angle at which loose material sits at rest – 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 – Adds weight – Provides lubrication – Water held within the void spaces – May provide surface tension When Saturated:

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 well- defined plane (sliding) – Among the most destructive – May be triggered by rain, melting of snow, or earthquakes

Mass Wasting – Types Debris Flows – Commonly occur in volcanic areas – Downslope movement of a viscous (thick) fluid – 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