A METHOD FOR RAPID ASSESSMENT OF POST-WILDFIRE DEBRIS-FLOW HAZARDS

The increased incidence of catastrophic wildfires in the western United States and the encroachment of development into fire-prone ecosystems have created a critical need for methods to quantify potential hazards posed by debris flows produced from burned watersheds. Debris flows are one of the most hazardous consequences of rainfall on recently burned hillslopes, and land-management agencies need tools to determine both the probability and magnitude of such destructive events from individual drainage basins. Such tools can be used to focus pre-fire treatments and post-fire hazard-management efforts. A model for estimating the basin-scale probability of debris-flow occurrence was developed through analysis of a database consisting of measures of basin response; basin-specific rainfall triggers; and basin morphology, physical properties, and burn characteristics from 398 basins in 15 recent fires throughout the western U.S. A multiple-regression model to estimate post-fire debris-flow peak discharge at a basin outlet was developed using data from 62 recently-burned basins including estimates of debris-flow peak discharge, basin gradient, burned extent, and storm rainfall. These models were used to generate debris-flow hazard maps following the extensive wildfires in October 2003 in southern California. In response to 1-hour rainstorms with 25-year, 10-year, and 2-year recurrence intervals, conditional probabilities of debris-flow occurrence ranged between 0 and 99%, and estimated peak discharges ranged from between 10 and 190 m3/s. These maps identify basins that are most prone to large debris-flow events and provide information for mitigation prioritization and evacuation planning. Evaluation of the response of basins recently burned by the Grand Prix and Old Fires to a rainstorm on December 25, 2003, indicated that 67% of the basins were correctly classified as having a moderate to high probability of debris-flow production.