Temporal changes in rainfall intensity-duration thresholds for debris flows in a recovering burned area

Post-fire debris flows are often observed during the first rainy season following a wildfire, but it is unclear how long the elevated threat of debris flow persists. We constrained a hydrologic model using field and remotely sensed measurements of soil-infiltration capacity, vegetation cover, runoff, and debris-flow activity. We applied this model to estimate rainfall intensity-duration (ID) thresholds for debris-flow initiation within three burned areas in the southwestern United States over a post-fire recovery period of 3-4 years. Modeling suggests ID thresholds are lowest immediately following the fire (below a one-year recurrence interval storm) and increase with time, such that a 10- to 25-year recurrence interval storm would be required to generate a debris flow after three years of recovery. Results improve our ability to assess temporal changes in postfire debris-flow potential, highlight how site-specific factors may alter the persistence of postfire debris-flow hazards, and provide additional constraints on the timescale of recovery following wildfire.

Hoch (2022) Temporal changes in rainfall intensity-duration thresholds for debris flows in a recovering burned area. USGS Landslide Hazards Seminar, 17 August 2022.