Spatial and Temporal Analysis of a Single Post-Wildfire Debris Flow from August 2021 in the Southern Wasatch Mountains

Debris flows are powerful and potentially destructive natural events consisting of water, sediments, and organic material that affect natural landscapes and can destroy infrastructure as they travel downslope. These destructive phenomena occur in areas with high intensity rainfall events and heavy loads of unconsolidated sediment. Wildfires contribute to debris flows through the destabilization of soils, the destruction of vegetation, and increasing surface runoff following heavy rainfall. In this project, we performed an in-depth analysis on a single post-wildfire debris flow that occurred in early August 2021 on the Mendenhall Creek Fan, located near the southern end of the Wasatch Mountains. Prior to the debris flow that occurred in early August, our team first examined the Mendenhall Creek Fan in early Summer 2021, when we collected UAV aerial imagery of the fan and samples of historic debris flow deposits. Following the new debris flow, we again collected UAV aerial imagery in August 2021 (within a few days of the debris flow) and again in July 2022. The three sets of imagery are each composited into separate Digital Elevation Models (DEM) using WebODM photogrammetric software. We then use QGIS software along with open-source data from the USGS, USDA, and NOAA to model the estimated probability and volume of debris flows. We refine the model using detailed spatial data specific to our field site, including burn severity, clay content, and rainfall intensity. Additionally, we analyze patterns of erosion and deposition and overall landscape evolution within our field site using the set of three DEMs. Finally, the results from this assessment are compared to the 2020 William Fire USGS Emergency Assessment of Post-Wildfire Debris Flow Hazards, which are hazard maps rapidly produced following a wildfire to assess the risk of post-wildfire debris flows. Our goals are to assess the effectiveness of the USGS model in this area of the Wasatch Mountains and ultimately improve our ability to accurately model the hazards of post-wildfire debris flows.