Broad-scale climatic influences on rainfall thresholds for debris flows: Adapting thresholds for northern California to southern California

Abstract

A Landslide Warning System (LWS) operated in the San Francisco Bay region until late 1995. The LWS issued public advisories when rainfall conditions reached or approached critical levels for triggering debris flows ("mudslides"). Interest in an LWS for southern California was revived by the destructive landslides triggered by the storms of January and February 1993 and by the debris-flow problems created by the extensive areas burned in large wildfires the following autumn. Although a number of elements for an LWS already exist in southern California, a critical element must still be developed: the “threshold,” a defined set of values of rainfall intensity and duration that predicts debris-flow initiation within a specified region. Although reliable rainfall/debris-flow thresholds exist for the San Francisco Bay region, climatic dissimilarities between there and southern California produce differ-ences in the thickness, character, and behavior of the hillslope materials that necessi-tate adjustment of the thresholds. Of particular importance are the amount and distribution of precipitation, which, along the California coast, are controlled by ele-vation, distance from the coastline, and storm frequency. Storm frequency, in turn, is strongly correlated with geographic latitude. Although storms are less frequent in southern California, with a consequent decrease in mean annual precipitation, average rainfall amounts for individual storms generally equal those of storms farther north. A procedure is developed for modifying existing rainfall/debris-flow thresholds to account for these changes in precipitation patterns. Then, a set of interim rainfall/ debris-flow thresholds is derived for the greater Los Angeles region. As a demonstra-tion, these interim thresholds are compared with data on rainfall and debris-flow occur-rence during January and February 1993.