The Timing of Susceptibility to Post-Fire Debris Flows in the Western United States

Watersheds recently burned by wildfires can be susceptible to debris flow, although little is known about how long this susceptibility persists and how it changes over time. We use a compilation of 75 debris-flow response and fire-ignition dates, vegetation and bedrock class, rainfall regime, and initiation process from throughout the western United States to address these issues. The great majority (85 percent) of debris flows occurred within the first 12 months following wildfire, with 71 percent occurring within the first 6 months. Seven percent of the debris flows occurred between 1 and 1.5 years after a fire, or during the second rainy season to impact an area. Within the first 1.5 years following fires, all but one of the debris flows initiated through runoff-dominated processes, and debris flows occurred in similar proportions in forested and non-forested landscapes. Underlying geologic materials affected how long debris-flow activity persisted, and the timing of debris flows varied within different rainfall regimes. A second, later period of increased debris flow susceptibility between 2.2 and 10 years after fires is indicated by the remaining 8 percent of events, which occurred primarily in forested terrains and initiated largely through landslide processes. The short time period between fire and debris-flow response within the first 1.5 years after ignition and the longer-term response between 2.2 and 10 years after fire demonstrate the necessity of both rapid and long-term reactions by land managers and emergency-response agencies to mitigate hazards from debris flows from recently burned areas in the western United States.