PREDICTING LOCATIONS OF POSTFIRE DEBRIS-FLOW INCISION AND DEPOSITION IN THE SAN GABRIEL MOUNTAINS OF SOUTHERN CALIFORNIA

Debris flows following wildfire pose severe hazards to communities located adjacent to the San Gabriel Mountains in southern California. Following wildfire, timely hazard assessments are needed to identify locations where debris flows may erode channel material or deposit large volumes of mud, rock, and debris. Locations of debris-flow incision are needed to help identify where debris-flow volumes rapidly increase. Similarly, models for debris-flow deposition and locations for where deposition begins are needed to predict areas that may be inundated by debris flow.

In this study we develop a logistic regression model for predicting locations where debris-flow incision and deposition are expected. Air photos document numerous debris flows that occurred in the San Gabriel Mountains following the 2003 Grand Prix fire; these photos were used to map locations of debris-flow channel incision and deposition. A variety of terrain characteristics expected to influence debris-flow processes were calculated for each mapped point using 10 meter resolution digital elevation models. The model predicts the probability of debris-flow incision and deposition as a function of channel slope, planform curvature and the length of the longest upstream flow path. The model was validated using an independent database of mapped debris flow processes and found to predict locations of debris-flow incision and deposition with 83% accuracy and 89% precision. This model may be used to provide more specific estimates of the type of debris-flow hazards that exist within a recently burned area and provides insight into the terrain characteristics that influence debris-flow processes. In addition, information about where debris-flow initiation and deposition occur may help guide the implementation of existing postfire models for debris-flow probability, magnitude, and inundated area.