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Mass WastingMass WastingSculpturing Earth’s Landscape
What is Mass Wasting?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 – TypesMass 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