Estimation of inundation areas of post-wildfire debris flows in Southern California USA

Prediction of the area of possible inundation is critical for debris-flow hazard management. This is typically done with inundation modeling programs, but many of these models are not specific to predict post-wildfire debris flows, a particularly dangerous subset. For this research, deposition patterns from flows triggered by the 2003 Christmas Day storm in areas burned by the Old and Grand Prix Fires in southern California were used to develop runout parameters specific to post-wildfire debris flows that can be incorporated into the LAHARZ inundation modeling program. Detailed mapping using aerial photographs, confirmed by field mapping, was used to document the cross-sectional area of the debris flows in drainages (A), the planimetric area of the flows (B), the volume of the flows (V), and the location of onset of deposition. These data were used to modify the power law equations required by the model, where the equations A = 0.26 V-0.40 and B = 7.4 V-0.81 are the best estimates for the post-wildfire data. These are statistically the 50th percentile values. More conservative values are also calculated as: A = 0.54 V-0.40 (80% prediction interval), A = 0.82 V-0.40 (95% prediction interval), B = 32 V-0.81 (80% prediction interval), and B = 73 V-0.81 (95% prediction interval). These values are expected to apply post-wildfire debris flows in the volume range of 10(2) to 10(5) m(3) in the mountain ranges in Southern California.