LEARNING TO ROLL WITH THE ONE-TWO PUNCH OF FIRE AND DEBRIS FLOW: LESSONS LEARNED FROM THE JANUARY 2018 DEBRIS-FLOW DISASTER IN MONTECITO, CA

Every year, millions of acres of steep terrain in the U.S. are left vulnerable to debris flows following wildfire. Small, yet damaging debris flows can be triggered in the burn area by modest rain storms with less than one-year recurrence intervals. More intense rainfall can generate large, highly destructive debris flows, such as the 9 January 2018 debris-flow event in Montecito, CA that followed the Thomas Fire and caused 23 fatalities and damaged over 400 homes. Current trends in climate and population suggest the threat of post-fire debris flow is growing: Wildfires are increasing in size and severity, rain storms are projected to increase in intensity, and more people are living near steep, fire-prone terrain. Despite this growing threat, we currently lack a complete framework for evaluating the risk of post-wildfire debris flows. While tools are available for estimating the likelihood and potential volume of the flows after wildfire, methods to rapidly assess where the material will go and what the impacts might be are not fully developed. Here, we use detailed field observations of the inundation and damage in the recent Montecito debris-flow disaster to better understand the challenges of predicting runout and loss after a wildfire. The observations provide rare spatial and dynamic constraints for testing debris-flow runout models including measurements of the peak flow depth, deposit thickness, grain size, and velocity along the runout paths. The complexity of the runout paths on the urbanized alluvial fans makes the event a particularly challenging test case. We pair the observations of inundation with a unique database on building damage at Montecito to define debris-flow fragility curves following the FEMA Hazus tsunami model. This connection provides a framework for translating the results of inundation models into estimates of economic loss which can help communities better assess their risk to the one-two punch of fire and debris flow.