PRODF: A REDUCED-COMPLEXITY MODEL FOR POST-WILDFIRE DEBRIS-FLOW INUNDATION

Recently burned slopes are particularly susceptible to debris flows. Increases in wildfire frequency, coupled with the growth of the wildland-urban interface, underscore the need to understand hazards posed by post-wildfire debris flows to downstream communities. In this study, we developed a reduced complexity debris-flow inundation model that we refer to as the Progressive Debris-Flow routing and inundation model (ProDF). While previous studies produced both empirical and hydrodynamic models that provide insight into the initiation and magnitude of debris flows, the routine use of debris-flow inundation modeling faces two primary challenges for rapid post-wildfire hazard assessments. One challenge is establishing that a model is appropriate for debris flows in a postfire setting, where sediment volumes and concentrations may differ from debris flows in unburned settings. A second challenge is the high computational burden of many existing hydrodynamic runout models. Here, we show that ProDF addresses both challenges with the simplicity of an empirical model while also performing well in unconfined areas, like more complex hydrodynamic models. We present results comparing simulations of debris-flow inundation to observed deposits for different burned areas in the western U.S. The input variables for ProDF are a debris flow volume and a digital elevation model. With these inputs, model parameters for yield strength and flow resistance can be calibrated based on observed debris-flow deposition maps. Specifically, we calibrated ProDF and assessed model performance in two contrasting landscapes: the Thomas Fire in southern California (underlain by sedimentary geology in an active tectonic area with pre-fire Chapparal vegetation) and the Schultz Fire in northern Arizona (underlain by Pleistocene-age volcanics with pre-fire Ponderosa Pine vegetation). ProDF reproduced over 70% of the mapped deposits at both sites. We also assessed model performance using mapped post-wildfire debris-flow deposits from across the southwestern US, including the Fish Fire in southern California and the Woodbury Fire in central Arizona. Results suggest that ProDF shows promise as a model that could be used to inform land managers of potential debris-flow runout zones.