AN ORDINARY STORM WITH AN EXTRAORDINARY RESPONSE: MAPPING THE DEBRIS-FLOW RESPONSE TO THE DECEMBER 25, 2003 STORM ON THE 2003 OLD AND GRAND PRIX FIRE AREAS IN SOUTHERN CALIFORNIA

On December 25, 2003 a rainstorm of 3 to 8 inches triggered deadly debris flows in an area burned two months prior by the Grand Prix and Old wildfires in southern California. Sixteen people were killed, hundreds of homes were damaged, many roadways became impassable, and numerous watersheds were eroded and heavily scoured as a result of the debris flows and flooding that occurred during the storm. Most rain gages within the burned area indicated that the return interval of the storm was less than two years.

The storm response within the Old and Grand Prix fire areas was documented using a combination of aerial photographic mapping and field checking. More than 97 percent of the 85 basins visited shortly after the storm displayed evidence of significant storm response, with more than 80 percent of those basins producing debris flows. Instrumented basins typically recorded a peak-flow equivalent to a 15-year event or more, which is in contrast to the storm return interval of less than two years.

Aerial photography consists of 2003 post-fire imagery and digital orthophotos taken immediately after the wildfires but before the storm, and 1:12,000-scale photography taken after the storm. A 2-meter resolution digital elevation model (DEM) was used as a base map. Comparison of the post-fire/pre-storm photos with the post-storm photos allowed for identification of the response specifically to the December 25, 2003 storm. Channel segments that showed alteration either by scour, a combination of scour and deposition, or deposition only were digitized using ESRI® ArcMap^TM 9.1. Each channel segment and depositional area was digitized as either a line feature or polygon within a geodatabase, and attributed accordingly. Altered channels and deposits were mapped in nearly 200 basins covering more than 150,000 acres. The resulting comprehensive database has been used to develop predictive models to locate the onset of debris-flow deposition and the area of inundation following future wildfires.