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Open-File Report 03–475
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Emergency Assessment of Debris-Flow Hazards from Basins Burned by the Grand Prix and Old Fires of 2003, Southern California

By Susan H. Cannon1, Joseph E. Gartner1, Michael G. Rupert1, John A. Michael1, Dean Djokic2 and Sreeresh Sreedhar2

1U.S. Geological Survey, Box 25046, DFC, MS 966, Denver CO 80225
2Environmental Systems Research Institute, Inc., 380 New York Street, Redlands, CA 92373-3112

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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 Old and Grand Prix Fires of October 2003 in southern California in response to the 25-year, 10-year, and 2-year 1-hour 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 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 range between 0 and 85% and estimates of debris flow peak discharges range between 460 and 5,900 ft3/s (13 to 167 m3/s). These maps are intended to identify those basins that are most prone to the largest debris-flow events and provide critical information for the preliminary design of mitigation measures and for the planning of evacuation timing and routes.

Version 1.1

Posted 2003

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