ESTIMATING POST-FIRE DEBRIS-FLOW HAZARDS IN THE WESTERN UNITED STATES PRIOR TO WILDFIRE

Following wildfire, the mountainous terrain of the western United States is susceptible to enhanced runoff and erosion, which results in an increased vulnerability to debris flows during intense rainfall. The convective storms that can generate debris flows in these recently burned areas often occur soon after the wildfire, making it difficult to develop and implement risk mitigation strategies prior to the first rainstorm. Here, we present a simple method for estimating post-fire debris-flow hazards prior to wildfire for the purposes of developing debris-flow risk mitigation strategies. We use historical data to simulate potential fire severity in the context of historical frequency, which we then use to estimate post-fire debris-flow hazards, including likelihood, magnitude, and rainfall intensity-duration thresholds. Our results demonstrate that hazard estimates using this potential fire severity simulation method are similar to predictions made using actual fire severity data collected after wildfire. We also demonstrate that the simulated fire severity produces hazard estimates that compare favorably to measured debris-flow occurrence and magnitude at a monitoring site in the San Gabriel Mountains of southern California. While we focus on application to post-fire debris-flow hazards, the methods presented here may be useful for improving our understanding of historical patterns and the temporal changes in fire severity associated with a changing climate. In addition, our methods are directly applicable to modeling other forms of post-fire hazards prior to wildfire, including flooding and erosion risk, as prediction of both of these hazards requires input data characterizing the severity of wildfire.