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Mass Movements/ Wasting. What are they?. Mass movements include: • Landslides • Rock falls • Avalanches • Mud flows • Debris flows • Creep. Anatomy of a rotational landslide. >100 km/year. <1 cm/year. 0%. ~40%. Mass Movements. Material moves downslope due to the pull of gravity

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Presentation Transcript

What are they?

Mass movements include:

• Landslides

• Rock falls

• Avalanches

• Mud flows

• Debris flows

• Creep


Anatomy of a

rotational landslide


>100 km/year

<1 cm/year



mass movements
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)

Factors in Slope Stability



Earth Materials

Triggering Events

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

Common when

building near


Common when

building roads


Water decreases rock/soil cohesion

Water circulating underground can dissolve cements

that hold sedimentary rocks together

internal causes for slope failure
Internal Causes for Slope Failure
  • Water (weight & interaction with clay minerals)
  • Decreasing rock cohesion
  • Incompetent/weak material
  • Adverse geologic structures
the weight of water
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, CA

March 1995


It happened again in 2004…

in exactly the same place…

debris flows or mud flows
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
  • 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

Risk factors to increase likelihood of mass movement

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