Some landslides move slowly and cause damage gradually, but others move so rapidly that they can destroy property and take lives suddenly and unexpectedly. Debris flows (also referred to as mudslides, mudflows or debris avalanches) are a common type of fast-moving landslide that generally occurs during intense rainfall on previously saturated soil. They commonly start on steep hillslopes as soil slumps or slides that liquefy, accelerate to speeds of 35 miles per hour or more and flow down hillslopes and channels onto gently sloping ground. Their consistency ranges from watery mud to thick rocky mud and they can carry boulders, trees and cars. Debris flows from different sources can combine in canyons and channels, where their destructive power can be greatly increased. Fast moving flows of mud and rock, called debris flows or mudslides, are among the most numerous and dangerous types of landslides in the San Francisco Bay region. When prolonged, intense rain falls on steep hillslopes, the saturated soils can become unstable and move rapidly downhill as debris flows. These catastrophic flows are capable of destroying homes, washing out roads and bridges, sweeping away cars, knocking down trees, and obstructing streams and roadways with thick deposits of mud and rocks. |
Hazardous Areas
A. Canyon bottoms, stream channels and areas near the outlets of canyons or channels are especially hazardous. Multiple debris flows that start high in canyons can be funneled into channels, where they can travel long distances to canyon mouths and beyond. |
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C. Roadcuts and other areas where hillslopes have been excavated or altered are especially susceptible to fast-moving landslides. Rock falls and debris flows onto roadways are common during rainstorms and often occur during milder rainfall conditions than needed for debris flows on natural slopes.
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B. Debris flows commonly begin in swales (depressions) on steep hillslopes, making areas downslope from swales particularly hazardous.
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D. Areas where water from surface runoff is channeled, such as along roadways and below outfalls of culverts, are common sites of debris flows and other landslides.
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What You Can Do If You Live Near Steep Hills
Prior to Intense Storms
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During Intense Storms
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For more information, please contact:
National Landslide Information Center
U.S. Geological Survey
Federal Center, Box 25046, MS 966
Denver, Colorado 80225
1-800-654-4966
http://landslides.usgs.gov
NLIC@usgs.gov
(This information was reprinted, with permission, from U.S. Geological Survey Fact Sheet 112-95, 1995 (revised and abridged November 1997)