Monday, 17 November 2003
Post-wildfire flash flooding: An analysis of Colorado wildfires and the North American monsoon from 1995–2002
The hydrologic responses to convective rainfall in areas recently burned by wildfire can include enhanced soil erosion, debris flow, and flash flooding. Meteorologically, these responses are influenced by rainfall patterns, rainfall intensity, and storm-relative total rainfall in the post-wildfire environment. This paper addresses the seasonal fire distribution for the period of record (POR) 1995-2002 for the state of Colorado. The study includes 152 fires that burned during the POR and 284 flash floods for the eight-year period. The paper then presents a method to assess the intensity of North American Monsoon (NAM) rainfall across Colorado for summers during the POR. Flash flooding and debris flow events for the POR are identified and statistical relationships are developed for NAM intensity, fire intensity, and hydrologic response in four sectors of Colorado (National Weather Service Forecast Areas). A case study is then presented detailing the storm relative cloud-to-ground lightning, storm-relative rainfall, and hydrologic response at the Coal Seam wildfire site in Garfield County, Colorado during the summer of 2002. This case study presents analyses of convective events that occurred from the time of fire containment (09 July 2002) to 30 September 2002. USGS rainfall data at the Coal Seam site provided very high temporal and spatial resolution coverage of convective rainfall at the burn site and in adjacent areas. On 05 August 2002 5-minute rainfall intensity of 7.4mm was recorded at the burn site. On 12 September storm-total rainfall of 3.2mm was recorded at the site. Both of these rainfall events generated flash flooding and/or debris flows at the Coal Seam site. Cloud-to-ground lightning data from the National Lightning Detection Network revealed that the 05 August and the 12 September episodes were much more intense than nine prior convective episodes. The cloud-to-ground lightning patterns over an area 100x100km surrounding the burn site, suggest that flash density and flash frequency peaked approximately 20 minutes prior to the heaviest rainfall and subsequent debris flow/flash flood on 05 August and 12 September respectively.