OPERATION OF THE USGS/NWS LANDSLIDE WARNING SYSTEM (1986-1995)

Raymond C. Wilson

Debris flows are among the most numerous and dangerous of several types of landslides triggered by severe rainstorms in California. Debris flows begin when heavy rainfall saturates the soil on a steep hillside, which suddenly fails and transforms into a flowing mass of loose soil and rocks. Debris flows move rapidly (up to 60 km/hr) and can flow down streams or other channels for distances of up to several kilometers. They can smash homes and other structures, wash out roads and bridges, sweep away cars, knock down trees, and when they finally come to rest, bury large areas under several feet of mud, rock, and other debris. In early January, 1982, a disastrous rainstorm struck the San Francisco Bay region, triggering thousands of debris flows (mudslides) and other shallow landslides across the San Francisco Bay region, causing 25 deaths and many million dollars in property damage.

Following the 1982 disaster, the USGS Landslide Working Group in Menlo Park began a research project to study the triggering of debris flows by severe rainstorms. A major goal was to develop a system for warning the public when rainfall conditions reach or approach critical (threshold) levels where debris flows might be triggered. In developing and operating this warning system, the landslide group worked closely with forecasters at the National Weather Service office in Redwood City who could detect and track major storm systems using satellite imagery, radar, and surface data. Real-time rainfall data were monitored with the ALERT network of automatic, radio-telemetering rain gages spread across the Bay Area. The first public warnings were issued during a severe storm sequence in February 1986. Public advisories were also issued during or just before severe storms in 1991, 1993, and 1995.

The debris-flow warning system operated for nearly a decade (February, 1986 until December, 1995), before organizational changes in both the NWS and the USGS, followed by decreases in funding and staffing, forced the termination of the warning system in December 1995. During its brief lifespan, the warning system produced both significant technical advances and valuable operational experience that will be useful for the development of future debris-flow warning systems elsewhere in the country and the world.

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