Rheologic Effects of Ash in Post-Wildfire Debris Flows in the Southern Wasatch Front

Post-wildfire debris flows commonly occur throughout the Western United States, and the hazards they pose to populations and infrastructure will only increase in the future. Drought in the West, combined with increased variability and intensity of rainfall events, will produce more fire prone landscapes with increased chances of debris flows. Developing a comprehensive predictive model will ultimately increase the quality of hazard assessment and decrease risk posed to communities. Wildfire-produced ash greatly impacts hydrological responses during subsequent rainfall events. Ash particles can weld together creating an impermeable surficial layer atop charred soils. Decreasing infiltration and increasing surface scouring, the ash layer promotes the transport and accumulation of sediment downslope. Conversely, ash that has been integrated into soil can increase water infiltration, which could lead to oversaturation and destabilization of the soil. In this study, we seek to understand the effects of fine organic ash on the rheology of wildfire-affected soils and debris flows. To accomplish this, we collected samples from the Mendenhall Creek Fan, the site of the 2020 William Fire, near the southern end of the Wasatch Mountains and five miles south of Santaquin, Utah. Samples are sieved down to 1 mm, and the organic content of the <1mm components is determined using loss-on-ignition techniques. A Sympatec QICPIC measures the grain size distributions down to 5 µm and characterizes particle shape, both of which correlate to burn severity. Lastly, infiltration and runoff experiments investigate the effects of burn severity and ash content on debris flow initiation and intensity. Our results demonstrate the important effects of fine ash content and characteristics on post-wildfire debris flows, which ultimately must be accounted for in models used to determine the hazards posed by these dangerous natural phenomena.