MODELING POST-WILDFIRE FLOOD, DEBRIS FLOW, AND EROSION HAZARDS USING GIS

Wildfires annually burn millions of acres in the Western United States and often create soil and vegetation conditions that can result in catastrophic flooding, loss of soil productivity, and debris flows that threaten life and property. The US Forest Service (Forest Service) Burned Area Emergency Rehabilitation policy requires the Forest Service to analyze post-wildfire hillslope and stream channel conditions and determine if a watershed emergency has been created by the wildfire. Immediately after a wildfire, burn severity is mapped using Landsat 7 images. To spatially model surface erosion, debris flow hazards and flood potential using ArcView GIS software, inputs including multiple soil properties, burn severity, hydrologic runoff curve numbers, slope, and vegetation layers are intersected to produce polygons with unique hydrologic response and soil erosion characteristics. To account for the difference in pre- and post-fire conditions, adjustments are made to soil parameters in the Revised Universal Soil Loss Equation and runoff curve numbers based on burn severity. Potential slope treatment areas that will minimize flooding and soil loss are identified spatially in the model and then prioritized based upon effectiveness and cost-risk analysis. Examples from 2001 and 2002 wildfires in Colorado and California will be provided.

Poster visuals will include: 1) photos of burned and unburned watersheds at low, moderate, and high burn severity, 2) ArcView generated layouts depicting spatially modeled pre- and post-fire erosion, potential debris flow source areas, and flood hazard zones, and 3) photos of debris flows, erosion, and flooding to illustrate the potential for destruction after wildfires.