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