Four Processes For Debris-flow Initiation From Basins Recently Burned By Wildfire, Western U.s.a.

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 basin hydrologic response to rainfall events. Effective hazard assessments following wildfires require recognition and evaluation of the ap- propriate initiation mechanism. In this study, we document four different processes that can lead to the generation of debris flows from recently burned basins in the western United States. Field observations, measurements, and experiments in water- sheds impacted by wildfire are used to describe and 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 of runoff bulking leading to debris flow was observed in the Bitterroot Range in Montana, but these debris flows were comprised of material eroded primarily from channels, with very little contribu- tion from the hillslopes. In both of these study areas debris-flow initiation occurred at a threshold value of upslope contributing area and its gradient. At these locations suf- ficient eroded material had been incorporated, relative to the volume of contributing surface runoff, to generate debris flows. The location of the threshold reflects vari- ations in sediment supply relative to gradient, in that steeper hillslopes and channels provide more erodible material per unit area. At the Cerro Grande fire in New Mexico, wildfire-related debris flows initiated as levee-lined rills on steep, gravel-mantled hill- slopes in response to storm rainfall with approximately 2 year recurrence. Sediment- runoff concentrations of between 0.23 and 0.81 kg/liter (with a mean of 0.42 kg/liter) were measured in hillslope traps following the debris-flow producing storm. The ma- terials 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 hill- slopes is a critical factor in the generation of post-wildfire debris flows by this pro- cess. 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 7Ongoing research

1 focuses on documenting the rainfall, material, and morphological controls on these different processes.

2