DRIVERS OF THE VOLUME OF POST-FIRE DEBRIS-LADEN FLOWS

As human infrastructure and the wildland-urban interface in the United States grow, debris-laden flow hazard following wildfires is becoming a greater concern. The volume of a debris flow is used as a common proxy for hazard. Relatively few records of debris-laden flow volumes exist in the published literature, however; we combined these records with unpublished work and our own field data to create a catalog of debris-laden flow events. We compared the frequency-magnitude distributions of post-fire and non-fire-related debris-laden flows, and found that post-fire debris-laden flows tend to be smaller. We then investigated the role of fire severity and basin topography in determining debris-laden flow volume. While fire severity has been shown to be a strong driver of debris-laden flow probability, fire severity was not related to debris-laden flow volume. Topographic factors, however, explained more than 50% of the variability in volume. Precipitation and time since last debris-laden flow are also expected to be strong drivers of volume, but data were not available for testing their role. The increasing probability of debris-laden-flow generation with increasing fire severity is likely due to a number of changes in the post-fire environment, including removal of overstory vegetation and litter and duff, which may lead to increased runoff. Future work can combine fire severity as a proxy for probability with basin topography as a proxy for volume in order to derive risk to highly valued resources such as municipal water supplies following wildfire.