USGS Releases Debris Flow Hazard Maps for Southern California

December 7, 2007 -- Maps showing the potential for destructive mudflows in the wake of recent Southern California wildfires were made available to the public and emergency responders today by the U.S. Geological Survey (USGS). The maps estimate the size of potential debris flows, commonly known as mudflows, and the areas that could be affected when rainfall begins on recently-burned areas.

USGS today unveiled 10 maps that show potential volumes of debris flows from basins burned by the Buckweed, Santiago, Canyon, Poomacha, Ranch, Harris, Witch, Rice, Ammo, Slide, and Grass Valley Fires in October and November. The maps will be used by the National Weather Service to provide debris-flow warnings to the public, by the Federal Emergency Management Agency (FEMA) to use in conjunction with their flood-inundation maps, and by local and state emergency managers.

The debris flow studies in the 2007 Southern California wildfire areas are 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.

More information about debris-flow and landslide hazards can be accessed at http://landslides.usgs.gov/research/wildfire/07sca/.