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Abstract

A storm that occurred January 3-5, 1982 in the San Francisco Bay area precipitated a host of mitigation schemes to provide protection against the now-perceived widespread debris flow/avalanche hazard. These procedures are now being designed and constructed in the Bay area. Removal of colluvium and loose bedrock in the source area mitigates the hazard of debris flows—a costly measure requiring access for equipment. Reinforcement of oversteepened slopes with rock riprap has proven to be an economical and expeditious means of supporting the oversteepened head scarp and flanks of a source area scar. In seismically active regions, the use of riprap for repair of source areas should be carefully scrutinized, especially when considered for critical structure sites.

Retaining walls are in common use to support unstable slopes in the source area. Their design is based on at-rest earth pressures that can range from an equivalent fluid pressure of 7,070 N/m3 to as much as 12,570 N/m3. Free-standing baffles of timber or steel, embedded in the main track of a confined debris flow/avalanche, reduce velocity but allow deposition to occur in an area accessible to maintenance equipment. Baffles can be spanned by heavy-gauge chain-link or gabion wire to provide a similar effect.

Impact walls capable of arresting and containing more than 150 m3 of debris, after sustaining an instantaneous impact load of at least 19,640 N/m3, are constructed on or at the hillside base. Deflection walls can be designed to direct flows to a specified depositional area. Sites positioned on fans or within drainages of limited area are protected by earthera basins designed for impact and containment of debris. Free drainage of a portion of the downstream face mitigates potentially adverse conditions that can develop due to captured runoff. The embankments are designed for an impact force of at least 19,640 N/m3, unless constructed with an upstream baffle system.

Lessons learned since 1982 in the San Francisco Bay area have changed the thinking of geotechnical consultants on new hillside development. Numerous sites have been protected by one or more of these engineered mitigation schemes. Thousands of hazardous sites remain, however, that are likely to be impacted by future debris flows/avalanches.

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