How Long Do Runoff‐Generated Debris‐Flow Hazards Persist After Wildfire?

Abstract

Runoff‐generated debris flows are a potentially destructive and deadly response to wildfire until sufficient vegetation and soil‐hydraulic recovery have reduced susceptibility to the hazard. Elevated debris‐flow susceptibility may persist for several years, but the controls on the timespan of the susceptible period are poorly understood. To evaluate the connection between vegetation recovery and debris‐flow occurrence, we calculated recovery for 25 fires in the western United States using satellite‐derived leaf area index (LAI) and compared recovery estimates to the timing of 536 debris flows from the same fires. We found that the majority (>98%) of flows occurred when LAI was less than 2/3 of typical prefire values. Our results show that total vegetation recovery is not necessary to inhibit runoff‐generated flows in a wide variety of regions in the western United States. Satellite‐derived vegetation data show promise for estimating the timespan of debris‐flow susceptibility.

Rainfall intensification amplifies exposure of American Southwest to conditions that trigger postfire debris flows

“…2, 3). An increased frequency of exceedance indicates that we may expect more opportunities for sufficiently intense rainfall to initiate debris flows before postfire soil-hydraulic recovery and/or vegetation regrowth can reduce susceptibility to flash floods and runoffgenerated debris flows 39,[50][51][52][53] . Temporal changes in sediment sourcing (e.g., gradual entrainment from hillslopes, mass failure of the channel bed, and shallow landsliding) have been observed to fuel multiple postfire debris flows over the course of a rainy season in the first year after fire 40,54 .…”

A Nature-Based Approach Using Felled Burnt Logs to Enhance Forest Recovery Post-Fire and Reduce Erosion Phenomena in the Mediterranean Area

“…These results suggest that the natural regeneration of the forest in our research is still at an early stage, which is insufficient to allow infiltration dynamics comparable to the pre-fire period. According to several research findings, the infiltration excess runoff threshold increases over long periods of time [98][99][100]; therefore, it requires years after a forest fire to minimise surface runoff. Such hydrologic effects emphasise the relevance of actions to rehabilitate hillside properties to minimise the risks of flash floods and debris flows arising from the generation of excessive runoff [99,100], as well as water pollution processes [101].…”

Deciphering Controls of Pore‐Pressure Evolution on Sediment Bed Erosion by Debris Flows

“…Debris flows, composed of debris grains and interstitial slurry, are among the most destructive and dangerous mass movements in mountainous regions worldwide (Dietrich & Krautblatter, 2019; Iverson, 1997). Debris flows are frequently triggered by extreme rainstorms (Bollschweiler & Stoffel, 2010; Graber et al., 2023), cliff failures (Rengers et al., 2020; Stoffel et al., 2014) and permafrost degradation (Damm & Felderer, 2013; Sovilla et al., 2006).…”