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The maximum flow rate for the unit hydrograph is 264.00 cfs between the hour <br /> intervals of 14 and 18 of the 1,000-year 24-hour storm event. <br /> HYDRAULICS <br /> From the hydrology and unit hydrograph studies, the desilting basin was designed to <br /> handle the Q,000 of 278.34 cfs. The desilting basin has a volume of approximately <br /> 11.1 acre-feet. <br /> The riser, culvert, and spillway were designed to accommodate the Q,000• To <br /> calculate the flow rate for the culvert, riser, and spillway the Manning, Orifice, and <br /> Weir equations were applied. The flow rate for the culvert capacity using the <br /> Manning's equation is 81.66 cfs. The flow rate for the riser using the Orifice's <br /> equation is 159.59 cfs. The spillway flow rate using the Horizontal Crested Weir's <br /> equation is 295.59 cfs. (See Attached Calculations). <br /> BASIN ROUTING <br /> Basin routing utilizes the Advanced Engineering Software Program Flood, by <br /> extending the unit hydrograph study. The basin routing process uses the Modified <br /> Plus and Finite Difference Method. The peak inflow at the 16.5 hour of the study is <br /> 264.00 cfs with an effective volume of 12.9 acre-feet and the peak outflow at 16.6 <br /> hours is 242.10 cfs with an effective volume of 12.8 acre feet. (See Attached <br /> Calculations. <br /> SOIL SEDIMENTATION <br /> The erosion and sediment control equation was used to calculate the settling <br /> velocity. The flow rate use was estimated for the 2-year, 24-hour storm, where the <br /> QZ = 140 cfs. For the surface area, there are two alternatives: the first one is A, = <br /> 84,250 ftz; and, the second one is A, = 83,750 ft'. The results for both of these <br /> alternatives is V, = 0.002 ft/s. The basin should settle out the particles that are <br /> between fine sand and coarse silt (Table 8.1 in King's Handbook). (See Attached <br /> Calculations. <br /> SUMMARY <br /> The desilting basin is adequate for accepting the 1,000-year, 24-hour storm runoff in <br /> the developed condition. The culvert, riser, and spillway for the basin meet the <br /> appropriate design standard to adequately support the flow rate for the 1,000-year, <br /> 24-hour storm. The basin provides an effective 12.88 acre feet of storage based on <br /> the basin hydrology. However, if additional basin capacity is needed as the landfill <br /> develops, the basin may be excavated to provide additional detention capacity, <br /> provided a storm water pump is added. <br /> 2 BRYAN A.sTiRRAT AND ASSOCIATES <br /> L:\Allied\Forward Landfill\FU-03 Basin\Design Rpt-LE\Forward Hyd Report 06-12-03.Doc <br />