CHARACTERIZING RISK FROM POST-FIRE DEBRIS-FLOW HAZARDS

Prediction of post-wildfire debris flow hazards and risks have been a constant theme of geoscientific research. The hazard and risk have been complicated by climate change, which has heightened the need for scientists and engineers to rapidly advance our understanding of post-wildfire debris flow initiation, runout, and inundation. Here, we use debris flow modeling to investigate potential risks across the entire burn area immediately following nine different wildfires occurring between 2020 and 2021 in Colorado. Publicly available data sets are used to develop a credible post-wildfire debris flow scenario to guide the modeling and assess the debris flow hazard in space and time. A calibrated model using a Monte-Carlo simulation captures the potential variability in the runout, maximum depth, volume, and probability within each wildfire. Models runs consist of five different sets of randomly assigned initiation points. Each set of initiation points generates 50 debris flows to produce a total of 250 debris flows within watersheds from the nine wildfires. Hazard assessment data are integrated with consequence data to evaluate a risk cost to structures and a risk cost proxy for roadways in the nine wildfires. Hazards and risks identified in the modeling for roadways in the Grizzly Creek and East Troublesome Fires corresponded well with known debris flows impacting roads in the 2021. High correspondence indicates scenario development, modeling, and risk analysis would have properly identified the debris flow risk along the roadways. Numerous buildings were impacted by potential debris flows in the East Troublesome and Cameron Peak Fires. Identified risk categories ranged from lowest to highest in the Cameron Peak Fire and from the lowest to the second highest ranking in the East Troublesome Fire. Modeled debris flow volumes when compared USGS volumes for the same locations showed the USGS volumes were consistently higher. Volume disparities were consistent across different size watersheds and between the different fires. In summary, a valid and comprehensive approach to rapidly evaluating debris flow risk following wildfire has resulted from this study.