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