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4. Definition of all drainage basin parameters which affect the rainfall/runoff relationship. surface profile model were followed to compute water surface elevations at each cross section. 5. Utilization of two models, the Colorado Urban Hydrograph Procedure (CUHP) and the Stormwater Management Model (SWMM) to calculate storm runoff flow and volume for a variety of storm events. DETENTION 6. Utilization of the HEC-2 computer model to calculate the 100-year floodplains on selected creeks in the study area. A special feature of the subject Master Plan was to perform all baseline hydrologic and hydraulic work on the basic assumption that all ponds and reservoirs were full at the beginning of all storms throughout the full range of frequencies. 7. Utilization of CUHP and HydroCAD computer models to calculate flood flows and volumes in the developed Core Area of the plant. Through this model, Core Area drainage system deficiencies were defined. This has two advantages. The first is that it represents a worse-case scenario, thus providing a good level of conservatism. However, the main reason for this assumption is that it allows planning freedom to potentially add or remove detention ponds when studying alternative options. FWOD HYDROLOGY TABLE II-I Procedures for flood hydrology for this study were adopted directly from the UDFCD Manual (Wright McLaughlin, 1969) to avoid questions regarding applicability ofthe detailed work product to the Rocky Flats area. These procedures included use of CUHP and SWMM. More important, however, was the careful selection and input to the models of reasonable and appropriate parameters reflecting the actual topography and other physical conditions of the basin. The significant aggregate parameters representative of the four main drainage basins delineated are shown in Table II-I. MODELLING PARAMETERS FOR EXISTING CONDITIONS OF MAIN DRAINAGE BASINS Six runoff event frequencies were analyzed: the 2-, 5-, 10-, 25-, 50-, and 100-year events. This range of frequencies is adequate to characterize the hydrology of the area as it pertains to drainage planning. It is consistent with national practice and local standards. .. I UpperBlgDry R6ck Creek ... Walnut Creek at Woman Creek at Creek at at State ... Parameterslll . . . Indiana Street Indiana Street .. Indiana Street . ... HlghWli)' 128 . Area 3.71 sq mi 4.51 sq mi 4.68 sq mi 2.91 sq mi Basin length 5.70 mi 5.68 mi 7.19 mi 3.98mi Length to Centroid 2.21 mi 2.27 mi 3.17mi 1.96 mi (Lo) Basin slope 0.027 ft/ft 0.028 ft/ft 0.031 ft/ft 0.030 ft/ft Impervious existing 14 percent 2 percent 2 percent 9 percent Pervious retention 0.49 in 0.52 in 0.46 in 0.54 in Impervious retention 0.10 in 0.10 in 0.10 in 0.10 in Initial infiltration 3.75 in/hr 3.64 in/hr 2.25 in/hr 4.06 in/hr Final infiltration 0.55 in/hr 0.55 in/hr 0.52 in/hr 0.56 in/hr FLOODPLAIN DELINEATION Main drainages and tributaries for floodplain delineation were selected to be consistent with the stated planning objectives. General stream characteristics were reviewed after which locations were selected for hydraulic cross sections that were tied to overall channel thalweg stationing. Standard procedures for the U.S. Army Corps of Engineers' HEC-2 water (I) A complete description of modelling parameters is presented in Section III.