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<br />Effects of the Buffalo Creek Wildfire <br /> <br />Two approaches were used to estimate the effect of the fire on flood runoff. First, July 12, 1996 flood <br /> <br /> <br />data for severely burned and unburned basins, which had similar rainfall amounts, were plotted against <br /> <br /> <br />contributing drainage area (fig. 3). Peak discharge from severely burned basins was 20-40 times larger <br /> <br /> <br />than for unbumed basins. Unbumed basins w~hin areas of maximum rainfall had minimal or no runoff, <br /> <br /> <br />which likely reflects rainfall interception by the duff in unburned areas; in add~ion, basin slopes in <br /> <br /> <br />unbumed areas generally are 20 percent or less. Slightly burned areas, which might have similar runoff as <br /> <br /> <br />prescribed-burn watersheds, had substantial flood and sediment runoff, but less than moderately- and <br /> <br /> <br />severely-burned basins. Since the fire, rainstorms have produced 9 floods (5 in 1996 and 4 in 1997) <br /> <br /> <br />larger than a 1 OO-year (pre-fire) flood (FEMA, 1986); most stonns were preceded by 5 to 10 mm of rainfall. <br /> <br /> <br />The largest flood on July 12, 1996 was about 10 times larger than the 100-year, pre-fire flood. For many <br /> <br /> <br />burned basins, the location of exposed bedrock or firm ground used to estimate discharge changed for <br /> <br /> <br />each storm (within +/- -100 m of other sites) due to shifting channels. Therefore, a fixed streamflow- <br /> <br /> <br />gaging station may not produce reliable records w~hout the use of a costly control (e.g., weir, flume) or <br /> <br /> <br />may need to be moved. <br /> <br />The second approach compared flood data for burned areas in Buffalo Creek w~h other Colorado Front <br />Range foothill extreme floods resu~ing from about 150 to 175 mm of rain in about an hour in similar basins <br />(slopes, soils, vegetation) but were unburned (fig. 3). Runoff from Buffalo Creek burned areas is similar to <br />floods in unburned basins in Colorado, but only a third to a half the rainfall (fig. 3). The combined peak <br />discharge for Buffalo Creek and Spring Creek is about 960 m3/s (-50 km2 contributing burned area), which <br />is greater than the 1976 flood of 883 m3/s in the Big Thompson River (McCain et aI., 1979) from a <br />contributing area of about 250 km2. A number of severely-bumed basins in areas near maximum rainfall <br />had unit discharges (peak discharge divided by drainage area) of about 60 m3/s/km2; the maximum un~ <br />discharge is about 40 m3/s/km2 for all Colorado floods (Jarrett, 1990). Clearly, the wildfire had the major <br />role in the severity of flooding in Buffalo Creek. Because the area of maximum rainfall was w~hin the <br />burned area, rainfall-runoff modeling is necessary to estimate potential flood runoff, but without the fire. <br /> <br />1 1 <br />