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Why the UDFCD rainfall-runoff model is superior for Box Elder Creek 1. The UD FCD rainfall-nmoff model represents the long, narrow shape of the Box Elder Creek watershed. ceNlClCI.lRHYO.CCC 4 I I I I I I I I I I I I I I I I I I The UDFCD rainfall-runoff model was applied to the Box Elder :reek watershed using a thorough discretization of sub watersheds and flow paths. A totll of 633 subwatershed areas were independently analyzed and accumulated into the overall model. This level of discretization provided a model that represented the unique hydrologic characteristics of the narrow Box Elder Creek watershed. In addition, the model represented the soils and infiltration rates, slopes, and other basin-specific parameters of the Box Elder Creek watershed more accurately than rei5"ession equations could. The FE?vlA letter notes that some of the subwatersheds at the downstream end of the study area (Watershed 3) seem to have a length to width ratio greater than 4, suggesting that this may explain why the UDFCD predicted discharges are lower th,m the eff~ctive FIR.YI discharges. This portion of the watershed, being extremely narnw and at the very downstream end of the study area, would not impact the hydro!;raph peak on mains tern Box Elder Creek. Therefore, some narrow subwatersheds in Watershed 3 do not explain why the submitted discharges are lower than the effective FIR.yI discharges. On the other hand, FE?vlA's notion about the hydrologic effect of long, narrow watersh~d shapes is, in general, valid, especially if applied to the narrow Box Elder watershed as a whole. 2. The UDFClJ rainfalI-runcff model compares favorably with the results of the us Anny Corps of Engineers Section 22 Study. Like the UDFCD study, the Corps of Engineers (COE) selected a detailed rainfall-runoff modeling approach to represent the long, narrow shape of the BJX Elder Creek watershed. The Section 22 analysis used a modified version of the Storm Water Management Model (SWIvINI) runoff block to generate and route hydro graphs from :~8 individual subwatersheds. The Corps of Engineers (COE) applied a uniforn, arealy adjusted 6-hour design storm similar to the design storm used in the UDFCD study. COE included a sensitivity analysis regarding storm size and centering. As in the l.JDFCD study, 24- or 48- hour storms were not used because storm data show that flood-producing rainfall events in the region are relatively short and intense. COE predicted a lOa-year discharge of 7,900 cis at 1-70 compared to 11,800 cis in the UDFCD study. 3. The UDFCD rainfall-runoff model compares ftroorably with the 1973 flood of record. UDFCD estimated that the peak discharge of the 1973 event, the flood of record, may have been on the order of 6000 cis at Bootleg Reservoir. Bootleg Rese,:voir is located approximately 15 miles north of the study area of the UDFCD Outfall Systems Planning Study. The 1973 flood discharge estimate was based on observations of reservoir filling and emptying rates. Estimates of rainfall for the 1973 event ranged from 5 to 6 inches in 24 hours, approximating the magnitude of the laO-year predpitaticn identified in the National Oceanographic and Atmospheric Administration (NOAA) Atlas for Colorado. In other words, a storm event on the order of a 100-year flood occurred on Box Elder Creek and yielded a peak discharge of approximately 6000 cis at BOOtll!g Reservoir. This discharge estimate compares more favorably with the submitted UDFCD discharge than the peak discharges estimated using regional regression equations.