FLOOD10311 (2) - Laserfiche WebLink

Home Browse Search

Using a drainage area of 17.7 mi" and average precipitation value of 25.8 inches, the 100-year peak discharge for the entire Willow Creek basin was developed for natural, unburned conditions. The 100-year peak discharge of 444 ft'/s was used as target discharge for model calibration. Drainage area and average precipitation for each subbasin were used to calculate a peak discharge target for model calibration for each subbasin. The 100-year, I-hour storm, as determined from the NOAA Atlas 2 Volume IIl-Colorado (Miller and others, 1973), was 1.60 inches for most areas of the Willow Creek basin and was selected for rainfall- runoff modeling in this study. The USDA Forest Service BAER team used a 25-year I-hour rainfall depth (1.25 inches) for the design purposes of the Million fire burned-area report (USDA Forest Service, 2002). Rain gages were operated in the area of the Million fire by the USGS in 2003 and 2004. The data from the local rain gages was analyzed for a storm distribution that could be used in the rainfall-runoff modeling. No short storms with rainfall in the range of the 100-year depth were found. Instead, a measured I-hour rainstorm distribution used for rainfall-runoff modeling in the Glenwood Springs area of Colorado (U.S. Army Corps of Engineers, 1998) also was used in the present study to represent a short, intense rainstorm in a mountain region. The SCS lag-time equation (Soil Conservation Service, 1985) was used to compute lag time, from which other required parameters also can be computed. The HEC-HMS drainage basin model (U.S. Army Corps of Engineers, 200 I b) was used to simulate the pre-burn rainfall-runoff processes in tributaries of the Million fire (fig 3). An objective calibration method using non-linear regression was used to calculate optimal pre-burn rainfall-runoff parameters (Elliott and others, 2005) from target 100-year peak discharges for each Willow Creek subbasin. Resulting parameter sets (RCN, initial abstraction, and lag time) (table 2) then were analyzed using HEC-HMS to compute the routed peak discharge at the mouth of Willow Creek. The routed peak discharge of 359 ft'/s differed from the target discharge for the entire Willow Creek basin by about 19 percent. Flow routing at Willow Creek used the Muskingum method. Values of Muskingum K = 0.3 to 0.4 hour (defines the time of travel for a flood wave traversing the basin) and X = 0.2 to 0.4 (a weighting factor that describes the backwater storage effects of a channel) were selected within the range of commonly used values for channels in steep, upland drainage basins (McCuen, 1989; John Liou, Federal Emergency Management Agency, oral commun., 2002). A summary of pre-burn calibration data (RCN, abstraction, lag-time factors) for all subbasins in the Willow Creek drainage is provided in Appendix table la.