Methods for the Emergency Assessment of Debris-Flow Hazards from Basins Burned by the Fires of 2007, Southern California

This report describes the approach used to assess potential debris-flow hazards from basins burned by the Buckweed, Santiago, Canyon, Poomacha, Ranch, Harris, Witch, Rice, Ammo, Slide, Grass Valley and Cajon Fires of 2007 in southern California. The assessments will be presented as a series of maps showing a relative ranking of the predicted volume of debris flows that can issue from basin outlets in response to a 3-hour duration rainstorm with a 10-year return period. Potential volumes of debris flows are calculated using a multiple-regression model that describes debris-flow volume at a basin outlet as a function of measures of basin gradient, burn extent, and storm rainfall. This assessment provides critical information for issuing basin-specific warnings, locating and designing mitigation measures, and planning of evacuation timing and routes.