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Operation of a landslide warning system during the California storm sequence of January and February 1993

Raymond C. Wilson
Raymond C. Wilson
U.S. Geological Survey, 345 Middlefield Road, MS 998, Menlo Park, California 94025
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January 01, 1997


From 1986 to late December 1995, the U.S. Geological Survey and the National Weather Service operated a landslide warning system for debris flows triggered by intense rainstorms in the San Francisco Bay region. The Landslide Warning System tracked storm systems as they approached the region, determined actual rainfall with a network of radio-telemetered rain gauges, compared the rainfall to thresholds for initi-ation of debris flows, and issued the appropriate public advisories.

A series of intense rainstorms during January 1993 created hazards from landslid-ing and flooding over much of California. In the San Francisco Bay region, January rainfall was over 200% of normal, triggering debris flows on natural hillslopes and road cuts across Marin, San Mateo, Alameda, Santa Clara, and Santa Cruz Counties. The warning system issued Flash Flood/Debris Flow Watches during the most intense storms on January 13 and 15,1993. Most debris flows in this area were small and widely scattered, so damage was largely limited to several blocked roadways in mountainous areas. Storm damage was much heavier in southern California, where rainfall amounts were over 350% of normal for January, triggering flash floods and many landslides. This damage prompted inquiries about developing a landslide warning system for southern California.

A number of elements for a landslide warning system already exist in southern Cal-ifornia, including quantitative rainfall forecasting and a network of radio-telemetered rain gauges. Regional rainfall thresholds for debris flow initiation, consistent with the climate, topography, and geology of the region, remain to be developed. Such thresholds could probably be developed with a modest investment of research effort and resources.

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Figures & Tables


GSA Reviews in Engineering Geology

Storm-Induced Geologic Hazards

Robert A. Larson
Robert A. Larson
Los Angeles County Department of Public Works 900 South Fremont Avenue Alhambra, California 91803
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James E. Slosson
James E. Slosson
Slosson and Associates 15500 Erwin Street, Suite 1123 Van Nuys, California 91411
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Geological Society of America
ISBN electronic:
Publication date:
January 01, 1997



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