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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 eleyations 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 yariety 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 yolumes in the deyeloped Core Area of the plant. Through this model, Core Area drainage system deficiencies were defined. This has two adyantages. The first is that it represents a worse-case scenario, thus providing a good leyel of conservatism. Howeyer, the main reason for this assumption is that it allows planning freedom to potentially add or remoye detention ponds when studying alternatiye options. FWOD HYDROLOGY TABLE II-I Procedures for flood hydrology for this study were adopted directly from the UDFCD Manual (Wright McLaughlin, 1969) to ayoid questions regarding applicability ofthe detailed work product to the Rocky Flats area. These procedures included use of CUHP and SWMM. More important, howeyer, 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 representatiye of the four main drainage basins delineated are shown in Table II-I. MODELLING PARAMETERS FOR EXISTING CONDITIONS OF MAIN DRAINAGE BASINS 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 oyerall channel thalweg stationing. Standard procedures for the U.S. Army Corps of Engineers' HEC-2 water Upper Big Dry .. .... .. ROCk Creek.. Walnut Creek at Woman Creek at Cr'eekllt 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 Six runoff eyent frequencies were analyzed: the 2-, 5-, 10-, 25-, 50-, and 100-year eyents. 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. FLOODPLAIN DELINEATION (I) A complete description of modelling parameters is presented in Section III.