ASSESSMENT OF POST-WILDFIRE DEBRIS-FLOW HAZARDS IN POPULATED ENVIRONMENTS: A CASE STUDY OF THE JANUARY 2018 MONTECITO, CALIFORNIA DEBRIS FLOWS

Post-wildfire debris flows are a known threat in mountainous communities that are subject to wildfire. Researchers at the U.S. Geological Survey have analyzed a large number of historically observed debris flows in burn areas, and subsequently developed empirical approaches for predicting the probability and volume of debris flows in response to design rain storms. However, less work has been done to map and model the extent of debris flow inundation in downstream areas. Here we present the results from a rapid response mapping effort following the January 2018 debris flows in Montecito, CA, where there were 21 fatalities and more than 400 homes damaged or destroyed. Through field mapping we quantified the lateral extent of debris flow movement, vertical inundation height, depth of deposited sediment, and the approximate location where deposition changed from boulder-dominated to mud-dominated. In addition, we also characterized the infiltration rates of soil in the debris flow source areas. The field mapping effort serves as a first step toward future modeling efforts of debris flow deposition, and highlights the importance of accounting for anthropogenic infrastructure in routing debris flows. For example, large amounts of sandy mud flowed across a highway overpass bridge (parallel to the flow path) and continued downstream along roadways for approximately a kilometer, spreading laterally. Moreover, bridges that were perpendicular to channelized drainages significantly influenced channel avulsion by redirecting flow onto the floodplain after they were plugged with debris. In addition, the source area for the debris flows at this site appeared similar to prior post-wildfire debris flow initiation observations. That is, we did not observe large mass-movements, and most of the material appeared to originate from hillslope rills or steep mountain channels. Furthermore, low infiltration rates in the source area suggest that the proximate cause of debris flows was infiltration-excess overland flow in burn areas. Therefore, our initial observations from field mapping provide new insights and strengthen existing theory regarding post-wildfire debris flows.