UC Irvine

Human development situated at the foot of mountains faces sediment-laden flood hazards characterized by high-velocity, erosive flows carrying mud and debris. In the western United States, sediment-laden flood hazards are increasing due to more frequent and severe wildfires, more intense precipitation, and the expansion of development towards mountain wildlands. Considerable work has focused on developing models of post-fire peak flows and sediment yields at the outlet of mountain canyons, but the risk to communities downstream of protective flood infrastructure common throughout the southwestern U.S. is largely unknown. We present an original modeling framework that captures the interactions between wildfires, storms, and flood infrastructure to estimate sediment-laden flood hazards. Stochastic modeling with a continuous simulation approach is used to quantify uncertainty and explicitly consider antecedent conditions. This work shows that compound post-fire flood hazards may be up to 6 times larger than suggested by the marginal hazard posed by extreme precipitation, and that future increases in wildfire severity and intensity could increase flood hazards by up to a factor of 11. Furthermore, modeling of compound post-fire flood hazards across the Santa Ana Mountains in Riverside County, California reveals hot spots of risk below catchments that last burned over 45 years ago. As urban expansion into mountainous regions continues to increase around the world, an improved understanding of the hazards facing communities “protected” by infrastructure is important to better characterize the spatial distribution of risks across populations, increase risk awareness, and inform sustainable adaptation and resilient land development practices.