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Mass WastingToday:• Definition• Factors Influencing

Mass Wasting• Classification &

Examples

Be certain to read Ch13 in your book - Iam curtailing it here

Slide at 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

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

Be certain to read about this inChapter 13 - I am skipping it here.

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Creep is a type of flow caused by frost heave orclay expansion

• Volumetrically the most significantform of mass wasting.

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Examples of Creep

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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Debris Flow at Mt. Huascaran, 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