POST-WILDFIRE DEBRIS FLOWS IN THE SOUTHERN WASATCH MOUNTAINS

Debris flows are powerful natural events consisting of a mixture of water and sediment that travel downhill at speeds exceeding 55 km/h. These destructive phenomena occur rapidly in areas with high precipitation storm events and unconsolidated sediment. Debris flows can quickly entrain material, including large boulders, resulting in increased flow velocities and runout distances. Wildfires can destabilize soils through the destruction of vegetation, increasing the chance of local sediment being entrained in surface runoff following heavy rainfall and thus increasing the likelihood of large debris flows. With the recent proliferation of housing development along the Wasatch Front and the effects of climate change on precipitation patterns and wildfire frequency, the risk of destructive debris flows in the region will increase in the future. In this project, we investigated locations of wildfires that occurred in the 2020 and 2021 summers including the Williams Fire, the Ether Hollow Fire, and the East Canyon Fire. Following our initial site investigations, we chose a debris flow fan deposit in the proximity of the Williams Fire, ~4 miles south of Santaquin, Utah, as the main study area. We chose this site because the deposits reveal a long history of debris flows, and we also witnessed the results of a significant debris flow that occurred on or around July 31st, 2021, in the middle of our field sampling campaign. We collected samples at 10 locations with multiple samples from different depths at each location. We then sieved the collected samples to obtain grain size data incrementally down to 0.125 mm or 3 phi and weighed samples to gather granulometry data. The trends in our data indicate that transport capacity of the local debris flows increases as the stratigraphic horizon moves upwards, suggesting that the magnitude of debris flows in this area has increased through time. Our main goal in this project is to assess the frequency and magnitude of past debris flows and the risk of future events and then communicate potential hazards to growing communities along the Wasatch Front and analogous Mountain West communities.