MAPPING POST-WILDFIRE DEBRIS-FLOW IMPACT, SOUTHERN CALIFORNIA, USA

Wildfire is a frequent occurrence in southern California, and debris flows generated from recently burned basins can pose significant hazards to life and property. The potential impact of post-wildfire debris flows can be identified using a sequence of empirical models that first estimate potential debris-flow volumes, then identify the point where deposition becomes spatially continuous within each drainage basin, and last, by defining relations between volume and cross-sectional and planimetric deposit area. The model for calculating the volume of debris-flow material that can be expected at the basin mouth was developed using multiple linear regression analyses of a database measured from 42 basins burned by ten recent fires in southern California. Debris-flow volume is estimated using a combined function of basin relief, length of the longest flow path within the basin, the extent and severity of the fire, and peak 1-hour storm-rainfall intensity. The model to identify the upstream limit of continuous debris-flow deposition within a drainage basin was developed using field mapping of 16 post-fire, debris-flow producing channels. Field observations indicated that the location of the onset of debris-flow deposition could be consistently identified where the channel widens below the downstream-most bedrock step within each channel. The bedrock step can be identified before the passage of a debris flow on a 2-meter DEM as the downstream-most location where two or more consecutive cells (a length of four meters or more) have a gradient of 24 degrees or greater. Finally, relations between post-fire debris-flow volume and cross-sectional and planimetric area were defined from measurements of deposits of 22 post-fire debris flows in southern California. These relations can be implemented within a GIS to map the areas within the DEM that are likely to be inundated by debris flow. The resulting debris-flow inundation maps provide information necessary to identify and prioritize areas for post-fire erosion mitigation, warnings and evacuation planning in southern California.