QUANTIFYING THE EFFECTS OF POST-WILDFIRE DEBRIS FLOWS ON LANDSCAPE EVOLUTION FROM UAV AERIAL IMAGES – AN EXAMPLE FROM THE SOUTHERN WASATCH MOUNTAINS

Debris flows are a common, and often destructive, phenomena often associated with wildfire-affected landscapes throughout Western North America. As climate change increases the frequency and severity of both wildfires and extreme precipitation events and as the wildland-urban interface continues to expand, we expect the risks posed by post-wildfire debris flows to increase throughout the West as well. One method for rapidly investigating the past, present, and future effects of debris flows on landscape evolution is using unmanned aerial vehicles (UAVs) to acquire aerial images of the landscape. The aerial images can be quickly processed to obtain quantitative information about landscape evolution before, during, and after debris flow events. Here, we present the results of a preliminary investigation of a prominent debris flow fan 4 miles south of Santaquin, Utah. The fan deposits record a long history of successive debris flows with pervasive charcoal, suggesting a strong influence from wildfires on the initiation of past debris flows. The area was burned during the William Fire in September of 2020, and we began our initial survey of the site in early Summer 2021. Following an intense rainstorm on July 30th, we returned to the field in early August to find a new debris flow that both incised through portions of the debris flow fan and deposited new material further down the fan. We acquired UAV aerial images of the fan both prior to and after the July 30th debris flow. We generated digital terrain models (DTMs) from the aerial images and analyzed the DTMs to highlight patterns of erosion and deposition for the July 30th debris flow. These patterns of erosion and deposition provide important insight into the mechanics driving these dangerous flows. We also calculate hillslope parameters to identify paths most likely affected by future debris flows. The populations of nearby communities increased over 500% in the past 30 years, and the population is projected to grow another 300% over the next 30-40 years. As these growing communities continue to sprawl further into the foothills where debris flows are common, we must be able to provide a more complete understanding of future debris flow hazards in light of a changing climate.