Mass Movements/ Wasting
What are they? Mass movements include: Landslides Rock falls Avalanches Mud flows Debris flows Creep
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
Rotational landslide
Angle of Repose Varies for Different Materials
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 % 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…
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
Rock Falls
Slump occurs when one huge mass falls together. Describe: Mass of rock & soil suddenly slip down a slope in one huge mass What makes it happen? When water soaks the bottom of soil rich in clay
Slump Cartersville, Georgia
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
Creep geology/geo101/masswasting.html&usg=__NXT2vQd6NNlaYppssy5JkhmQtO8=&h=357&w=300&sz=33&hl=en&start=7&um=1&tbnid=T5WrN7Z S1oqFeM:&tbnh=121&tbnw=102&prev=/images%3Fq%3Dcreep%2Bmass%2Bmovement%26hl%3Den%26sa%3DN%26um%3D1 Bent tree trunk illustrating creep. National Zoo, Rock Creek Park, Washington, D.C.
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