INVESTIGATIONS OF DEBRIS FLOWS FOLLOWING THE 2002 MISSIONARY RIDGE WILDFIRE

The Missionary Ridge fire burned 70,085 acres from June 9 to July 17, 2002. The first large rain event after the fire initiated a debris flow on July 23, 2002. Debris flows continued to occur during the months following the fire. Debris flow peak discharge rates were estimated to be up to 160 m3/s in some basins affected by the fire. Newly activated areas of debris fans overflowed existing channels and deposited debris in zones that were previously assumed to be safe. Numerous studies were conducted to determine the level of hazard associated with post-fire debris flows. These included an initial response by the USFS Burned Area Emergency Response (BAER) team to determine the debris flow hazard, a study by the USGS and CGS to assess the potential debris flow peak discharges for burned basins, a hydrologic hazards study by the USGS using the FLO-2D model, a study by the USFS and CGS to determine new potential snow avalanche paths within the burn area using GIS, an evaluation of rapid, stream channel variation and a study of the basin response and the history of wildfires by students at Fort Lewis College. These investigations were conducted at varying times following the fire. The level of accuracy of the investigations increased with time; however, there was a trade off between timely response and accurate information, both of which were important to local emergency managers. An ongoing investigation by the CGS includes a comparison of pre- and post-fire aerial photos to determine morphologic changes to debris fans in areas affected by the Missionary Ridge Fire.