Extreme precipitation reveals impacts of a low severity wildfire on debris-flow processes

Abstract:

Wildfire alters soil and vegetation properties in ways that promote increased runoff and erosion, which can lead to hazardous floods and debris flows. It is well established that the impacts of fire on hydrologic and geomorphic systems depend on soil burn severity. Post-fire debris-flows are most common in areas burned at moderate-high severity and, therefore, many studies of post-fire debris flows focus on these areas. Here, we take advantage of a natural experiment following the 2019 Woodbury Fire in central Arizona, USA, to examine differences in debris-flow processes between an area burned primarily at low severity and a nearby unburned area. Several months after the fire, our study area was impacted by a rainstorm that produced over 100 mm of rainfall in less than a 6-hour period, leading to widespread debris flow activity within and outside of the burn scar. Despite similar rainfall intensities, debris flows initiated in 22 out of 26 (85%) burned watersheds whereas only 26 out of 47 (55%) unburned watersheds produced debris flows. Unburned watersheds that produced debris flows had basin areas that were generally smaller, had greater median slopes, and higher Melton ratios compared with burned watersheds that produced debris flows. Several debris flows were mobilized from shallow landslides in the unburned area, but the vast majority of all debris flows in both burned and unburned watersheds were initiated by runoff. Channels scoured by debris flows within the burned area were several times wider than debris flow channels in the unburned area, suggesting that debris flows in the burned area tended to have greater volumes. Despite these differences, pebble counts of debris flow deposits did not suggest any systematic change in the grain size distributions of debris flows initiating from burned and unburned areas. Results help to expand our ability to predict post-wildfire debris-flow activity across a wider range of burned settings, including the Sonoran Desert vegetation community where the hydrologic and geomorphic impacts of fire are not well constrained, and suggest that low severity fire can be sufficient to reduce thresholds for debris-flow initiation.