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FLOOD HYDROLOGY The South Boulder Diversion Canal is significant to Rocky Flats Plant site flood planning and floodplain management because it has historically accepted overland runoff from the west for a portion of its length across Rocky Flats. The use of this canal to intercept stormwater has been approved in writing by the Denver Water Department with certain qualifications, and would bc particularly significant in bypassing flood runoff west of the canal during the 100-year storm event. To determine the flood hydrology characteristics of the Rocky Flats Plant, the Master Plan utilizes two models, the Colorado Urban Hydrograph Procedure (CUHP) and the Stormwater Management Model (SWMM). Thc CUHP model is based on the unit hydrograph procedure defined as the hydrograph of 1 inch of direct runoff from the tributary area resulting from a unit storm. SWMM accepts the individual basin hydrograph information calculated by CUHP and routes the hydrograph downstream through conveyance elements (e.g., channels, culverts, detention basins). These models were used to calculate the rates of flow and volumes of storm runoff from precipitation durations of 2 and 6 hours, and frequencies of 2-, 5-, 10-, 25-, 50- and 100-year storm events. A transfer of the Kinnear Ditch to Last Chance Ditch for carriage purposes would remove Kinnear Ditch water from Woman Creek. The results of these two models were verified by an internal check of data and by comparison with data contained in published reports of the U.S. Army Corps of Engineers (USACE), Advanced Sciences, Inc. (ASI) and UDFCD for the same hydrological design points. Moreover, to avoid potential conflicts with land use by the City of Arvada, the planning is compatible with future basin development projects of the Jefferson Center Metropolitan District, a planned development west of the Rocky Flats Plant. CHANNEL HYDRAULICS AND FLOODPLAIN DELINEATION The Master Plan uses the HEC-2 computer model to calculate the floodplain for the 100- year, 6-hour storm event with present channel and basin development conditions for selected major drainages in the Rocky Flats Plant. The HEC-2 program is capable of computing water surface profiles in irregularly shaped channels by backwater analysis. It is particularly appropriate for use in areas like Rocky Flats where high sediment and debris loads are not expected. The floodplains delineated by the Master Plan will assist in analyzing facility siting, possible facility inundation, and flood effects on locations of interest (e.g., operable units). Long-term storm events (greater than 6 hours) were also analyzed in the Master Plan. Generally, such storms are not analyzed in master drainage plans in the Denver area, because experience has shown that short storm events are more severe and produce more runoff. CORE AREA DRAINAGE WATER RIGHTS AND DITCHES The Master Plan analyzes ditch flood potential, administration, diversions and other related issues which could affect drainage and flood control within the study area. Based on this analysis, the principal conclusions are that: The "Rocky Flats Plant Drainage and Flood Control Master Plan" includes a detailed analysis of the existing drainage system of the Core Area. As in the model developed for the overall study area, the CUHP model was used for calculating runoff hydrographs. The HydroCAD model was then used for routing the hydrographs through the swales, culverts, detention basins and storm sewers of the Core Area drainage system. The HydroCAD model provides data in a more user-friendly form than SWMM, and offers greater flexibility in designing future drainage improvements required in the Core Area. Based on this analysis, the Master Plan delineates the deficiencies in the existing drainage system. . The Coal Creek ditches do not add to the flood potential of Rocky Flats. During major flooding, the ditches which divert from Coal Creek likely will be subject to siltation, debris and washout damage. Flood water will not be transported out of the Coal Creek basin in significant quantities.