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3.5.5 Retention Storage Losses 3.6.4 Routing Connectivity Retention storage losses were determined according to Table 2,1 of the Runoff chapter of the USDCM (UDFCD, 1969). For rural conditions (future imperviousness less than 20%), retention storage losses were assumed to be 0.05 inch for impervious areas and 0.40 inch for pervious areas. For urban conditions (future imperviousness greater than 20%), the retention storage losses were asswned to be 0.05 inch for impervious areas and 035 inch for pervious areas, The routing connectivity of the conveyance elements, design points, and subwatershed hydrographs was incorporated into the UDSWM2 models according to the connectivity diagrams provided in Appendix G. 3.7 Results 3.5.6 Infiltration Losses 3.7.1 Peak Flows The infiltration parameters (initial infiltration, final infiltration, and decay coefficient) were computed from Table 2,2 of the Runoff chapter of the USDCM based on a weighted average of hydrologic soil type (UDFCD, 1969). The weighted average of hydrologic soil type for each subwatershed was computed based on superimposing hydrologic soils maps onto subwatershed maps. A summary table of the peak discharges for the 10" 50" 100" and 500'year events is provided in Appendix R The information shown in the summary table was organized so that it only includes hydrologic results pertaining to the appropriate area adjustment shown in Table 3,1. 100,year peak discharges at the downstream ends of Oak Gulch, the Unnamed Tributary, and Lemon Gulch are shown in Table 3-4. 3.6 UDSWM2 Modeling Table 3-4 IOO-Year Peak Discharges at Selected Locations Drainage IOO-year Peak Stream Area Discharges (cfs) (sq. mi.) Oak Gulch at confluence with Cherry Creek 1.784 2100 Unnamed Tributary at confluence with Cherry Creek 1.037 1540 Lemon Gulch at confluence with Cherry Creek 7.530 6570 A UDSWM2 model was created to route the CUHP generated hydrographs through the existing stream network Conveyance element parameters required for the UDSWM2 model include the conveyance element identifier, channel bottom width, length, invert slope, side slopes, hydraulic roughness, and routing connectivity. A summary of these characteristics is shown in Appendix F and the methods for estimating these characteristics is described in the following sections. 3.6.1 Conveyance Element Identification Number Conveyance elements were nwnbered from downstream to upstream for each stream. All conveyance elements were identified with even nwnbers. Design points were located at the upstream and downstream ends of each conveyance element nwnber and identified by the appropriate odd nwnbers. 3.7.2 Comparison to Other Studies All streams were modeled as trapezoidal cross sections. Bottom widths, lengths, invert slopes, and side slopes were estimated from the I :24,000 USGS quadrangle maps and field observations. In some cases, streams were represented by a base flow channel and an overflow floodplain channel. The results of this study for future development conditions were compared to regional regression equations and paleoflood estimates. Table 3,5 shows a comparison of 100,year discharges determined from U.s. Geological Survey (USGS, I 987a) and Colorado Water Conservation Board (CWCB, 1997) regression equations. Table 3,5 also includes paleoflood estimates for the three tributaries that were developed in a recent USGS study (USGS, 1998). Note that the regression equations were developed for undeveloped watersheds and that the results for this study are based on developed conditions. The study results are roughly one and a half times greater than discharges developed from the CWCB regression equation, but very close to the discharges developed from the USGS regression equation. The study unit discharges (ratio of peak discharge to watershed area) vary from 1.4 cfs/acre to 2.3 cfs/acre, which is a reasonable range for this area. 3.6.2 Stream Geometry 3.6.3 Roughness The roughness or Manning's "n" values for the conveyance elements were determined according to field observations and then increased by a factor of 1.25 as recommended in the UDSWM2 Users Manual (UDFCD, 1995b). The Manning's "n" values used for the conveyance elements in the UDSWM2 analysis ranged from 0.045 to 0.084, 5 Oak Gulch and Lemon Gulch FHAD.OOC