DEBRIS-FLOW INITIATION PROCESSES FROM BASINS RECENTLY BURNED BY WILDFIRE, WESTERN USA

Debris-flow initiation processes in basins recently burned by wildfire differ from those generally recognized on unburned, vegetated hillslopes. These differences result from burn-induced changes in the hydrologic response to rainfall events. Effective hazard assessments following wildfires require recognition and evaluation of the appropriate initiation mechanism. In this study, we document four different processes that can lead to the generation of debris flows from recently burned basins. Field observations, measurements, and experiments in watersheds impacted by wildfire are used to evaluate the different processes for debris-flow generation. At Storm King Mountain, Colorado, post-wildfire debris flows initiated through a process of progressive bulking of storm runoff with material eroded primarily from burned hillslopes. A similar process was observed in the Bitterroot Range in Montana, but these debris flows were comprised of material eroded primarily from channels. In both study areas debris-flow initiation occurred at threshold values of upslope contributing area and gradient. At these locations sufficient eroded material had been incorporated, relative to the volume of contributing surface runoff, to generate debris flows. At the Cerro Grande fire in New Mexico, wildfire-related debris flows initiated as levee-lined rills on steep, gravel-mantled hillslopes in response to storm rainfall with approximately 2 year recurrence. The materials entrained by this storm contained a higher proportion of silt- plus clay-sized materials in the <2mm fraction than the materials collected from storms that produced comparable sediment-runoff concentrations, but not debris flows. This difference in materials indicates that the availability of fine-grained wood ash mantling the hillslopes is a critical factor in the generation of debris flows by this process. And last, post-wildfire debris flows that initiate by rainfall-infiltration triggered landsliding were observed on burned hillslopes in Colorado and southern California. The material derived from the landslides accounted for only about 7% of the total volume of material deposited at canyon mouths, suggesting the importance of runoff-dominated, rather than infiltration-dominated processes on recently burned hillslopes.