Northern California Landslide Working Group
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What is a Landslide?The term landslide refers to the downslope movement
of rock, soil, or artificial fill under the influence of gravity.
Although many types and processes exist, landslides in Northern
California can be grouped into three basic types: rockfalls, deep-seated
landslides, and shallow landslides that often mobilize into debris
flows. Rockfalls usually occur on cliffs or steep slopes. They begin when slabs of bedrock break free and fall, roll, or bounce through the air to the ground below at speeds of up to 10 ft/sec. The resultant piles of angular rocks are typically referred to as talus. Joints and fractures within the rock weaken the cliff’s stability, allowing rockfalls to be triggered by such events as earthquakes or large rainstorms. Deep-seated landslides, as displayed in the diagram to the upper right, have one main sliding surface. These landslides, such as the one on Mission Peak in Fremont, CA, usually move in pulses during or following extended wet periods and are dormant during dry times. These massive failures express a wide range of velocities; ranging from ft. per year to ft. per day. A combination of gravity, rainfall, earthquake shaking, and undercutting by rivers or human modifications, can start or accelerate their movement. Debris flows (i.e. mudslides, mudflows, or debris avalanches) originate during intense periods of rainfall on already water saturated ground. They start as shallow landslides atop fairly steep hillsides that eventually liquefy and flow downslope at speeds over 10 ft/sec. Multiple debris flows can travel downhill into a stream channel, combine into a larger flow, and travel thousands of feet from their source. Because of their speed and far reach, debris flows are extremely hazardous to life and property. Large earthquakes or intense storms (such as the El Niño storms of 1982 and 1998) can trigger hundreds or thousands of debris flows. A man-made flume, depicted to the lower right, is being used by the USGS to study debris-flow initiation and run out characteristics.
Mitigating Landslide HazardsNumerous different Federal, State, and local agencies, academia, as well as private companies, address landslide mitigation in the United States. While federal and state agencies coordinate research priorities, generate regional maps and assessment standards, and improve techniques such as monitoring, local and private groups tend to focus more on mapping and assessing hazards at a site-specific scale. For additional information, please see the National Landslide Hazards Mitigation Strategy. The National Landslide Hazards Mitigation Strategy is developing new partnerships between government at all levels, academia, and the private sector, in addition to expanding landslide research, mapping, assessment, real-time monitoring, forecasting, information management and dissemination, development of mitigation tools, and emergency preparedness and response. Click here. For additional information regarding landslide types,
their hazard, and suggestions on what you can do if you are concerned
about landslide hazards, click here. |
Block diagram of idealized landslide model (Varnes, 1978)
Debris slide used for studying debris flow dynamics (Iverson, et al, 1992) |
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