The Effect of Wildfire Events on the Frequency-Area Distribution of Debris-Laden Flow Contributing Areas

This project investigates how wildfire events affect the size and frequency of debris-laden flows in the northern Rocky Mountains. Previous workers have established that the occurrence of debris-laden flows increases following wildfires, but quantitative evaluations of this increase are lacking. In regions with short-duration high-intensity rainstorms, this increase is likely attributable to increased surface runoff, caused by wildfire's consumption of vegetation and duff, hydrophobicity, and ash filling soil interstices. Post-fire debris-laden flows commonly initiate through progressive bulking, during which increased surface runoff entrains sediments in small rills which feed into larger gullies. Sustained downcutting begins within a gully at the debris-laden flow initiation site. The contributing area, or the total hillslope area above the debris-laden flow initiation site, can be determined by mapping flow lines upslope from the initiation site to the ridgeline. Two burned areas in western Montana are studied: the Valley Complex of 2000 in the Sapphire Mountains, and the Red Eagle Fire of 2006 in Glacier National Park. The perimeters of debris-laden flow contributing areas in each burn will be mapped on aerial photos and high-resolution satellite imagery, and their areas calculated. Debris-laden flow contributing areas will also be mapped in an adjacent unburned area of equivalent acreage. Frequency-area distributions will be plotted and parameterized, and the distributions for the burned and unburned areas compared. We hypothesize that the frequency-area distributions will exhibit a power-law relationship, as this relationship has been documented in related natural phenomena including landslide and fire sizes. We expect that the distribution of debris-laden flows in recently burned areas is significantly different from that of adjacent unburned areas, with different slope and/or intercept parameters. By parameterizing the frequency-area distribution, the probability of a debris-laden flow of a certain size can be predicted, aiding in assessment of hazards to nearby resources and populations.