Hydrogeomorphic Recovery and Temporal Changes in Rainfall Thresholds for Debris Flows Following Wildfire

Abstract

Wildfire-induced changes to soil and vegetation promote runoff-generated debris flows in steep watersheds. Debris flows are commonly observed the first wet season following a wildfire, but it is not clear how long the elevated threat of debris flows persists and why debris-flow potential changes in recovering burned areas. This work quantifies how rainfall intensity-duration (ID) thresholds for debris-flow initiation change with time since burning and provides a mechanistic explanation for these changes. We constrained a hydrologic model using field and remote sensing measurements of soil infiltration capacity and vegetation cover as well as hydrologic monitoring data of flood and debris-flow activity. We applied this model to estimate rainfall 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 thresholds are lowest immediately following the fire (below a 1-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 3 years of recovery. Modeled changes in rainfall ID thresholds can be attributed to increases in soil infiltration capacity, canopy interception, hydraulic roughness, and D50 grain size of sediment entrained in an incipient debris flow. The importance of each of these factors varied between the three sites. Results improve our ability to assess temporal changes in post-fire debris-flow potential, highlight how site-specific factors may alter the persistence of post-fire debris-flow hazards, and provide additional constraints on the timescale of geomorphic recovery following wildfire.