Emergency Assessment of Debris-Flow Hazards from Basins Burned by the Grand Prix and Old 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 de bris flows issuing from basins burned by the Old and Grand Prix Fires of October 2003 in southern California in response to the 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 function of burned extent, soil properties, basin gradient s and storm rainfall. The peak discharge maps are based on application of a multiple-regression m odel that can be used to estimate debris-flow peak discharge at a basin outlet as a function of basin gradient, burn extent, an d storm rainfall. Probabilities of debris-flow occurrence range between 0 and 85% and estimates of debris flow peak discha rges range between 460 and 5,900 ft 3 /s (13 to 167 m 3 /s). These maps are intended to identify those basins th at are most prone to the largest debris-flow events and provide critical information for the preliminary de sign of mitigation measures and for the planning of evacuation timing and routes.