Mass Wasting

Introduction

While landslides area normal part oferosion and surfaceprocesses, they canbe very destructiveto life and property

Mass wasting is thedownward movementof material under thedirect influence ofgravity. The rate ofmovement variesgreatly.

What factors trigger mass wasting?

1) Gravity:Mass wasting occurs when the gravitational force acting on a slope exceeds the resisting force.

2) SlopeLarge slope angles have a greater chance for mass wasting.

The steepest angle thata slope can maintainwithout failure is theangle of repose

More Factors…3) Humans

Cutting through a hill for buildings and road construction.

Overloading - caused by human activities of dumping or piling

material onto a slope, increasing water pressure and slope strength

4) Fluvial (rivers/streams)Stream undercutting or wave erosion can steepening slopes

What Factors InfluenceMass Wasting?

4)Type of soil/rock• Mass wasting is more common in loose or poorly consolidated material than in bedrock

5) Weather and Climate• In areas of high temperature and precipitation weathering is deeper and produces tens of meters of unconsolidated material• In dry regions soils are thinner, but heavy localized rainfall

may result in mudflows

Factors…

6)Water content in soil• Water may increase the weight of a slope enough to

cause mass wasting.• Water reduces the amount of friction between particles,

causing the soil particles to easily slide past one another when wet.

7) Vegetation ( good for soil)• Decreases water content of slope materials• Root systems stabilize by binding soil and holding the soil to bedrock• Removal of vegetation can lead to mass wasting

What can trigger a mass wasting event?

Earthquakes– Rapid mass

movements can trigger vibrations, loosening up soil/slope.

Excessive water in short period of time.

Volcanic eruptions,explosions, and evenloud thunder maytrigger unstableslopes

What Are the DifferentTypes of Mass Wasting?

Classified on the basis of 3 things:1) Rate of movement - fast or slow– Rapid movements involve visible movements of material

– Slow movements are imperceptible except from their effects such as cracked walls and tilted trees or power poles

2) Type of movement - falling, sliding, or flowing

3) Type of material - rock, soil, and debris

What Are the DifferentTypes of Mass Wasting?

1) Falls•Rockfalls are a common type of rapid mass wasting.

•May occur along steep canyons, cliffs, and road cuts.

•Undercutting, earthquakes, frost wedging can weaken rock, leading to a fall.

•Sign of a fall:– Talus builds up at the base,

where fallen material collects– Loss of vegetation, clearing on

a hill (“scar”)

What Are the DifferentTypes of Mass Wasting?

2) Slides• Move along one or more

surfaces of failure

• May consist of soil, rock, or

both

• May move rapidly or slowly

What Are the DifferentTypes of Mass Wasting?

3) FlowsFlows move as a viscous fluid • Mudflows are fluid and move fastest, common in arid or mountainous regions*debris flows are more viscous*earthflows move as thick, viscous masses of wet regolith

http://www.youtube.com/watch?v=VMJiibdVYjE

Mudflows

• Consist of at least 50% silt and clay sized particles, at least 30% water• Usually follow pre-existing channels until the slope decreases, then fan out

Debris flows

4) Debris Flows• Composed of larger sized particles than mudflows• Don’t contain as much water as mudflows• Rarely confined to pre-existing channels

5) Soil Creep

Very slow movement of soil on a slope. Not easily visible…need to know the signs.