Forecasting the Frequency and Magnitude of Postfire Debris Flows Across Southern California

Southern California has a long history of damaging debris flows after wildfire. Despite recurrent loss, forecasts of the frequency and magnitude of postfire debris flows are not available for the region like they are for earthquakes. Instead, debris flow hazards are typically assessed in a reactive manner after wildfires. Such assessments are crucial for evaluating debris flow risk by postfire emergency response teams; however, time between the fire and first rainstorm is often insufficient to fully develop and implement effective emergency response plans like those in place for earthquakes. Here, we use both historical distributions of fire and precipitation frequency and empirical models of postfire debris flow likelihood and volume to map the expected frequency and magnitude of postfire debris flows across southern California. We find that at least small debris flows can be expected almost every year, while major debris flows capable of damaging 40 or more structures have a recurrence interval between 10 and 13 years, a return interval that is comparable to a magnitude 6.7 earthquake. A sensitivity analysis to possible future changes in current fire and precipitation regimes indicates that debris flow activity in southern California is more sensitive to increases in precipitation intensity than increases in fire frequency and severity. Projected increases in rainfall intensity of 18% result in an overall 110% increase in the probability of major debris flows. Our results, in combination with an assessment of exposure, can be used to prioritize watersheds for further analysis and possible prefire mitigation. Plain Language Summary Southern California has an especially high risk of debris flows after wildfire. While debris flow hazard assessments are routinely conducted after wildfires, time between the fire and the first rainstorm is seldom enough to use this information to fully develop and implement effective emergency response plans. Here, to identify these hazards before a wildfire occurs, we map the frequency and magnitude of postfire debris flows across southern California in a somewhat similar manner as earthquake hazards are already mapped for the state. The maps are developed using both historical distributions of fire frequency, fire severity, and rainfall intensity and the same models to assess debris flow hazards after real fires. We also evaluate the sensitivity of debris flow frequency and magnitude to possible future changes in fire behavi