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<br />. <br /> <br />. <br /> <br />. <br /> <br />ABSTRACT <br /> <br />FLOODS FOLLOWING FIRE <br /> <br />Prepared for 1997 CASFM Conference in Vail, Colorado <br /> <br />by <br />Tbomas Browning, Colorado Water Conservation Board <br />Robert Jarrett, U.S. Geological Survey <br /> <br />Forest fires, which can be triggered naturally or as a result of human activity, are a common occurrence <br />in many areas of the United States. In Colorado, large portions of the Front Range as well as much of the <br />high country in western Colorado face very high forest fire danger (witness the deadly Storm King <br />Mountain fire in 1994 and the Buffalo Creek fire in 1996). Much of Colorado's population growth is <br />occurring in these fire-prone areas. The critical issue for floodplain managers, hydrologists, forest <br />managers, and meteorologists is to quantitatively identify the increased flood and debris flow risks <br />following wildfires. <br /> <br />On May 18-19, 1996 an intense wildfire burned approximately 12,000 acres offorested land in the <br />Buffalo Creek area. Vegetation and soil conditions in the severely burned areas of the watershed were <br />changed dramatically from their pre-fire conditions. The resulting hydrophobic (water repellent) soils <br />and the loss of vegetative cover from the fire greatly changed the hydrolic response of the affected <br />watersheds. This change in watershed conditions was partly responsible for the deadly Buffalo Creek <br />flash flood event less than two months after the fire. <br /> <br />On the night of July 12, 1996, the Buffalo Creek flash flood occurred within an unicorporated area of <br />Jefferson County, Colorado. The rainfall information for the July 12, 1996 flash flood has been <br />reconstituted by Henz Meteorological Services in conjunction with other available data. A thunderstorm <br />in the area produced at least 3 inches of rain in about an hour: the 100-year, I-hour rainfall is reported to <br />be 2.1 inches in an hour for this part of Jefferson County. The ensuing flash flood took the lives of two <br />people in addition to causing major damage to roads, bridges, utilities, public facilities, homes and <br />vehicles. The Buffalo Creek flood event was truly a disaster, with total flood related damages estimated <br />to be over 5 million dollars. Several other rainstorms produced flash flooding in 1996. <br /> <br />The U.S. Geological Survey (USGS) and the Colorado Water Conservation Board (CWCB) have been <br />collecting field data and preparing rainfall-runoff models for the Buffalo Creek area to demonstrate the <br />hydrologic response of natural and burned watersheds in forested areas. The runoff values for the July <br />12, and other storms were estimated by the USGS and CWCB for streams having unburned, moderately <br />burned, and severely burned watersheds for over 100 locations in the area. The rainfall reconstitution <br />was used in conjunction with estimated runoff values in order to develop and calibrate rainfall-runoff <br />computer models for several watersheds in the Buffalo Creek area. The results of these models provide <br />clues for the quantitative effects that fires have on hydrology. <br /> <br />The results of the rainfall-runoff modeling efforts, along with other aspects of "floods following fire", <br />will be presented. In addition, an overview of the partnering process (residents, local officials, state and <br />federal officials) for risk identification, hazard mitigation and flood warning will be presented, <br />