Mass Wasting Sculpturing Earth’s Landscape. What is Mass Wasting? The downslope movement of...

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