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The individual sub-basins ranged in size from 7 acres to 1734 acres. The selected sub-basins lying on the plant site were smaller in size than those outside the plant site to provide opportunity for a more detailed definition of flows. It was assumed that the Walnut Creek Diversion Dam, the Walnut Creek Diversion Canal, the South Interceptor Ditch, and the Woman Creek Diversion Dam would have sufficient hydraulic capacity to convey the runoff from the 100-year, 6-hour precipitation event. This was verified as correct after comparison of the calculated peak flow at these locations to the calculated hydraulic capacity of each of the facilities. 6. The following criteria were used in the development of the models: 1. Natural drainage channels and man-made facilities specifically constructed for the purpose of water management or flood control were modelled. The Walnut Creek Diversion Canal, the South Interceptor Ditch, and an unnamed ditch immediately north of the East Entrance Road were modelled. Other man-made ditches were ignored in the modelling. To allow for flexibility in the modelling effort, the A-, B- and C-series ponds were assumed full at the start of the precipitation event with no attenuation of flow through the ponds. The results of the hydrologic analysis described in this section are presented in tables and figures for ready reference and comparison of rates of flow and volume of runoff for each frequency of occurrence. Individual hydrological design points are located throughout each major basin from the upper portion of each basin easterly to Indiana Street. Results 2. Ponds, lakes, and depressions other than the A-, B-, and C-series of ponds, the Landfill Pond, Great Western Reservoir, and Standley Lake were not separately modelled. They were included in the characteristics of the basin in which they are located. The range (2- to lOO-year) of peak flood flows for each sub-basin is based upon the 6-hour precipitation distribution. The 2-hour and the 6-hour distribution resulted in the same peak rate of runoff for each sub-basin because they are identical for the first 2 hours. The volumes of flood runoff are larger for the 6-hour precipitation distribution for developed basins such as in the Core Area. For purposes of drainage master planning at the Rocky Flats Plant site, the 6-hour design storm is utilized. 3. There is an insignificant amount of water spilled from Coal Creek to ditches that cross the plant site. This is consistent with field observations of the ditch headgates and evaluation of geomorphological conditions and ditch banks (further discussed in Section V) and analysis of Coal Creek floodplain hydraulics. The 24-hour, 4-day and lO-day upslope storms were analyzed. The analysis demonstrated that the long-duration storms were not critical to the rate of runoff. The long duration storms yield a higher volume of runoff only for basins with high percentage of imperviousness; other basins yield a lower volume of runoff. 4. For the purposes of master drainage planning, the Core Area was divided into main sub-basins according to the natural overland flow path of runoff water. Minor system storm sewers and small culverts were not modelled as part of the major drainage system. A detailed analysis of the Core Area that includes the initial (local) drainage system elements is included in Section VII of this Master Plan. All given flow rates and hydrographs assume that ponds and reservoirs are full and that there is no diminution of flow caused by the ponds and reservoirs. This assumption, while conservative, provides the basis and opportunity to analyze ponds and reservoirs independently and under different operating scenarios. 5. For the purposes of master planning, the downstream end of the study area is Indiana Street. Inflow hydrographs to Standley Lake and Great Western Reservoir were computed and found to be similar to the approved UDFCD master drainage plan for Big Dry Creek (Greiner Engineering, 1986). CURP Model Results. The peak flow, runoff volume, peak flow per unit area, and unit runoff generated by CURP for present development conditions for each of the delineated sub-basins are presented in Appendix IV-B. The peak flow and runoff volumes are significantly dependent on the percentage imperviousness and soil infiltration characteristics.