Post-wildfire Landslide Engineering: Hazard Assessment to Mitigation in the Cameron Peak Wildfire, Colorado, USA

Larger wildfires and more intense rainfall in the western United States are placing communities at greater risk of debris flows. The Cameron Peak Wildfire burned extensive forest lands throughout Larimer County, Colorado, USA and caused the declaration of a major disaster by Federal Emergency Management Agency. Recognizing the link between post wildfire rainfall, floods, and debris flow hazards, the Natural Resources Conservation Service Emergency Watershed Protection Program sought to assess risks to communities and infrastructure within the county and provide options for mitigating the hazards. Debris flows were modeled from high intensity burns at four study areas within the county and the potential impact determined using DebrisFlow Predictor. Debris flow scenarios were based on mapped high, medium, and low burn severity in the watersheds, and 500 model simulations produced runout pathways, debris flow inundations, depths, and volumes onto occupied fans. Model runs permitted the calculation of expected maximum and average debris flow volumes while velocities were estimated based on existing literature and measured velocities from field sites in North America and internationally. Debris-flow impacts, and loads were calculated using nine different debris flow impact models and run-up heights were also calculated to assist with wall height designs. The primary protection at many locations was a gabion wall system. Larger concrete and steel structures were considered for channels with reasonable access to build this style structure. The results herein provide a comprehensive approach to identifying post wildfire debris flow hazards, assessing the potential impacts from debris flows, and designing structures to mitigate those impacts.