MassMovements/

Wasting

What are they?

Mass movements include:

• Landslides• Rock falls• Avalanches• Mud flows• Debris flows• Creep

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Anatomy of a rotational landslide

<1 cm/year

>100 km/year

0% ~40%

Mass Movements• Material moves downslope due to the pull of gravity

• Can happen almost anywhere

• Commonly associated with other events (heavy rainfall or earthquakes, for example) and are therefore under-reported

• Movements can either be catastrophic (slope failure) or slow and steady (creep)

• The rate of the mass movement can be increased by various erosive agents (especially water)

Gravity

Water

Earth Materials

Triggering Events

Factors in Slope Stability

Gravity & steepening of a slope

How to cause a landslide: add or subtract a mass …in the wrong place

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Common whenbuilding roads

Common whenbuilding nearslopes

Rotational landslide

Angle of Repose Varies for Different Materials

Water decreases rock/soil cohesion

Water decreases rock/soil cohesion

Water decreases rock/soil cohesion

Water circulating underground can dissolve cements that hold sedimentary rocks together

Internal Causes for Slope Failure

• Water (weight & interaction with clay minerals)

• Decreasing rock cohesion

• Incompetent/weak material

• Adverse geologic structures

The Weight of Water

• Sedimentary rocks commonly have porosities of 10 - 30%

• If pore spaces fill with water, the weight of the material is increased substantially, creating instability

La Conchita, CAMarch 1995

It happened again in 2004…in exactly the same place…

La Conchita, CA

Debris flows or mud flows

• Mass movements that behave like fluids

• Unlike slides, flows are not controlled by a failure surface, but instead are dominated by internal movements

Landslides in the Bay Area

1982 San Mateo County

Devil’s slide area on Highway 1 north of Half Moon Bay

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Devil’s Slide

Rock Falls

Creep

• Downslope movement of soil and uppermost bedrock

• Creep happens at too slow of a rate to observe directly

• Instead, creep can be identified by it’s effect on objects

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Gravity - hill slopes more vulnerable (on top of a hill, on the slope, or at the bottom of a hill), modified slopes (road cut, cut flat area to build on, coastal erosion, etc.)

Water - risk is higher when ground is saturated and/or during heavy rains, El Niño events

Earth Materials - loose soils (particularly clay-rich) or fractured rock, and old landslides pose greater risk

Triggering Events - heavy rain during storm, rain after big storms or fires, earthquakes (when ground is saturated?)…are all triggers

Risk factors to increase likelihood of mass movement