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For this analysis, the basin areas and centroids were determined by using a Graphic Design System (GDS) computer drafting program and 1" = 500' scale maps (Scharf & Associates, 1987). The basin lengths and land slopes were hand-measured on the same mapping. The existing condition percent imperviousness was estimated by examination of 1" = 100' scale maps for the plant site (Scharf & Associates, 1987) and 1" = 2000' maps (USGS, 1965) for the areas outside the plant site. The mapped data were verified by site observations. The future condition percent imperviousness was taken from the approved UDFCD Big Dry Creek master drainage plan (Greiner Engineering, 1986). The value of 98 percent imperviousness for the future condition of the Core Area is conservative, but considered appropriate for the future condition master planning effort. SWMM was developed by the Environmental Protection Agency and modified by Missouri River Division of the USACE. A portion of the model was later modified to be specifically used with CUHP. This version is called UDSWM2-PC by UDFCD; however, the generic term "SWMM" will be used in this report. SWMM accepts the individual basin hydrograph information calculated by CUHP and routes the hydrograph downstream through conveyance elements (e.g., channels, culverts, detention basins). The routing is based on a kinematic wave approach using Manning's equation. The specific input to the SWMM model includes sub-basin hydrographs generated by CUHP; the connectivity of the conveyance elements, shown on Figure IV -1; and the length, bottom width, side slopes, channel slope, and Manning's "n" of channel elements, shown in Tables IV-3 and IV-4. The SWMM model element locations are shown in Figure IV-2, sheets 1 and 2. The soil infiltration characteristics were determined by examination of Soil Conservation Service soil maps (SCS, 1975) and UDFCD recommended values. The soils are of three general types; soils in the upland areas west of the Core Area have high infiltration rates, soils in the steep valleys that drain to the creeks have low infiltration rates, and soils in the creek bottoms have medium infiltration rates. The SWMM output consists of tabulated storm hydrographs and flow depths for channel elements, and tabulated outflow hydrographs and storage volumes for detention elements. The channel element length and slopes were calculated using 1" = 500' maps (Scharf & Associates, 1987). The surface retention characteristics were estimated using the recommended UDFCD range of values as a guide. Flat areas were assigned a higher retention value than steep areas. CUHP basin characteristic input variables are listed in Table III-l for all sub-basins. The channel configurations were categorized into six general types. The configurations are not exact, but rather are an idealized trapezoidal representation of the type of channel. SWMM Model The detention basin storage information was taken from preliminary stage-storage curves calculated by Merrick & Company (Merrick & Company, 1991). The discharge characteristics were calculated using a broad-crested weir equation with spillway configuration information taken from the same mapping. The original procedure for use of CUHP was to repeat the hydrograph derivations at each basin design point. While generally satisfactory in its application, CUHP did not necessarily differentiate between narrow channels and wide channels where floodplain storage took place. Imnlementation of Rocky Flats Plant COOP ISWMM Models UDFCD subsequently adopted the SWMM (UDFCD, 1986) for routing of sub-basin hydrographs downstream. Here, channel flow velocities and floodplain storage characteristics are reflected. SWMM routing of the sub-basin hydrographs was used for the Rocky Flats basin. The hydrologic modelling of the drainages crossing Rocky Flats was done by separating the area into three models - one for the Great Western Reservoir including Walnut Creek drainage basin, another for the Standley Lake drainage basin including Woman Creek and Upper Big Dry Creek, and the third for Rock Creek.