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Mass WastingToday:• Definition• Factors Influencing

Mass Wasting• Classification &

Examples

Slide La Conchita CA

Definition of Mass Wasting

• Down slopedisplacement of soiland rock under directinfluence of gravity– Rapid (e.g., avalanche)– Slow (e.g., creep)

• Result:– Landscape evolution– Hazard

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Landscape evolution:

• Mass wasting one of main processes of sediment transport• Results in change in shape of slopes (hills, cliffs, mountains, river

banks etc.)• Other processes that move sediment and shape landscape Rivers,

Glaciers, Wave, Wind

Mass Wasting: Examples

Slump and Earth flow Rock Falls

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More Examples:slide

Creep

Simple model for MW

• MW occurs when– force of gravity

exceeds stabilizingforces

– Friction, cohesion &strength

• Consider block oninclined plane...

Normal

Gravity

Down slope

Stabilizing

If Down slope force > Stabilizing force then...

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Factors influencing Mass Wasting: Slope

• Increase Slope– Increase down slope forces– Movement occurs when down-slope forces exceed

stabilizing forces• Rives and waves cut steep cliffs. (so does man)

Factors influencing mass wasting: Water• Loading:

– Add water = add mass– Increases down slope forces

• Lubrication:– Decrease friction

• Cohesion– Add water and increase

cohesion due to surfacetension (e.g. sand castles)

– Too much water decreasescohesion when sedimentgrains no longer touch (e.g.wet sand castle falls).

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Factors influencing mass wasting:• Deforestation

– Root systems bind soil– Remove it, and cohesion

decreases

• Earthquakes– Ground shaking can

trigger mass wasting

Types of Mass WastingFalls Slides Flows

Falls = movement through air

Slides = block maintains contact with surface

Flows = fluid movement in water or air flowing down slope

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Rock Falls (frost wedging)

Slides

• Block slides– down planar surface– Maintains contact– Surfaces can be bedding, fracture,

jointing etc.• Examples: Rockslides...

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• Turtle Mt. Slide(Alberta, Can)– 1903– 40 x 106 yrd– < 2 min

• EQ Lake - Madison R.– 1959– 80 x 106 ton Block

disaggregated– 28 killed

Vaiont Italy, 1963• Bedrock dips into canyon• Reservoir filled, water

wicked up bedding(lubrication)

• 240 x 106 m3• 90 m wave• 7 minutes 2600 dead

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Slump (another type of slide)

• Slide along curvedsurface

• Slump block rotates• Usually caused by

erosion at base ofslope

• Common hazard tohomes

Slump examples

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Flows: Fluid movement of rock in water or air

• Creep– Caused by frost heave (or expansion of

clay)– Volumetrically the most significant form

of mass waisting.

Examples of Creep

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Flow: Solifluction = liquefied soil over impermeable base

Mudflows• Cause: rapid mixing of

water withsediment/rock

• Snow melt onvolcanoes (Lahar)– Mt. Rainer

• Deforested hills S. CA– enhanced by forest

fires

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Mudflow in San Juan Mt. CO

Flow: Debris Flow & Avalanche• Most Rapid• Begins as slide• Block breaks into

small pieced• Cushioned by

air/ice/water• Flow great distances

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Mt. Huascaran, vilages of Yungay & Ranrahirca Peru

• Before 1970:• Village of Yungay, Peru• Off - shore EQ (benioff)

M~7

• After event:• Slide block of ice and rock

(>2.2 x 106 m3)• Debris flow > 9 km

distance• > 100 mph• >20,000 killed