Emergency assessment of debris-flow hazards from basins burned by the Cedar and Paradise Fires of 2003, southern California

Abstract

These maps present preliminary assessments of the probability of debris-flow activity and estimates of peak discharges that can potentially be generated by debris flows issuing from basins burned by the Cedar and Paradise Fires of October 2003 in southern California in response to 25-year, 10-year, and 2-year recurrence, 1-hour duration rain storms. The probability maps are based on the application of a logistic multiple regression model that describes the percent chance of debris-flow production from an individual basin as a function of burned extent, soil properties, basin gradients, and storm rainfall. The peak-discharge maps are based on application of a multiple-regression model that can be used to estimate debris-flow peak discharge at a basin outlet as a function of basin gradient, burn extent, and storm rainfall. Probabilities of debris-flow occurrence for the Cedar Fire range between 0 and 98% and estimates of debris-flow peak discharges range between 893 and 5,987 ft3/s (25 to 170 m3/s). Basins burned by the Paradise Fire show probabilities for debris-flow occurrence between 2 and 98%, and peak discharge estimates between 1,814 and 5,980 ft3/s (51 and 169 m3/s). These maps are intended to identify those basins that are most prone to the largest debris-flow events and provide information for the preliminary design of mitigation measures and for the planning of evacuation timing and routes.