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Mass Movements/ Wasting

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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Mass Movements/ Wasting

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  1. MassMovements/Wasting

  2. What are they? Mass movements include: • Landslides • Rock falls • Avalanches • Mud flows • Debris flows • Creep

  3. Anatomy of a rotational landslide

  4. >100 km/year <1 cm/year 0% ~40%

  5. 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)

  6. Factors in Slope Stability Gravity Water Earth Materials Triggering Events

  7. Gravity & steepening of a slope

  8. How to cause a landslide: add or subtract a mass …in the wrong place Common when building near slopes Common when building roads

  9. Rotational landslide

  10. Angle of Repose Varies for Different Materials

  11. Water decreases rock/soil cohesion

  12. Water decreases rock/soil cohesion

  13. Water decreases rock/soil cohesion Water circulating underground can dissolve cements that hold sedimentary rocks together

  14. Internal Causes for Slope Failure • Water (weight & interaction with clay minerals) • Decreasing rock cohesion • Incompetent/weak material • Adverse geologic structures

  15. 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

  16. La Conchita, CA March 1995

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

  18. La Conchita, CA

  19. 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

  20. Landslides in the Bay Area

  21. 1982 San Mateo County

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

  23. Devil’s Slide

  24. Rock Falls

  25. 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

  26. 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

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