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I I II I I I I I I I I I I I I I I I I 3.8.2 Comparison to Previous Studies The results of this study for existing development conditions were compared to previous hydrologic analyses of Box Elder Creek and nearby watersheds. These analyses included the COE Section 22 Study upstream of Bootleg Reservoir (COE, 1989), the Adams County FIS (FEMA, 1989), the Arapahoe County FIS (FEMA, 1993), the Running Creek study upstream of Elizabeth (WRC, 1979), and the Beebe Draw Study (WWE, 1992). The comparison of existing development condition lOO-year discharges is shown in Appendix Q. One-hundred-year existing development discharges estimated in this study compare very favorably to the discharges developed in the COE study (COE, 1989) and the Beebe Draw Study (the Beebe Draw results pertain only to tributary areas less than 2000 acres) (WWE, 1992). The results of this study and the COE study for Box Elder Creek are lower than the discharges for comparable areas identified in the Adams and Arapahoe County FIS reports and the Running Creek report, as shown in Appendix Q. Besides this study, the COE study was the only analysis that used detailed rainfa11Jrunoff modelling procedures (COE, 1989). The COE analysis used a modified version of the Storm Water Management Model (SWMM) runoff block to generate and route hydro- graphs from 238 individual subwatersheds. COE applied a uniform, area11y adjusted 6-hour design storm similar to the design storm used in this study. COE also conducted a sensitivity analysis regarding storm size and centering. The COE report notes that regional approaches may not be directly applicable to Box Elder Creek because of its unusual (long and narrow) watershed shape. The PIS reports in Adams and Arapahoe Counties were based on regional approaches, primarily on USGS Water Supply Paper 1680 (uSGS, 1968). This simplified approach provides for flood estimates in different regional aieas of the Missouri River Basin below Sioux City, Iowa. The approach, in Region 10 (where Box Elder Creek is located), was based on discharge records from 6 to 22 years (from 1940 to 1962) for five gages on Cherry, Kiowa, and Bijou Creeks (none of these watersheds are as narrow as the Box Elder Creek watershed). Discharges are shown to vary directly with watershed area, a phenomenon that conflicts with this study's results and the COE results for the narrow Box Elder Creek watershed. If the method is applied based on the adjacent hydrologic region to the west (Region 9, where Plum Creek is located), the l00-year discharge for the 200 square mile Box Elder Creek watershed is 6000 cfs, compared to 25,000 cfs. In other words, the procedure recommends significantly different results for creeks like Plum Creek and Cherry Creek, even though the creeks are adjacent and both drain from the Palmer Divide. DENlOOI5186.WP5 3-9