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Page 7 July 22, 2008 <br />surface. The discharge end of the pipes will be placed at this relatively high elevation to minimize the <br />potential for both sediment plugging and plunge pool scouring. This will place the pipe outlets at <br />approximately elevation 4,496 ft. An armored energy dissipating plunge pool will be constructed at the <br />end of the pipes. The pipe inlet will consist of a reinforced concrete headwall sloped to conform to the <br />natural slope. The area around the inlet headwall will be armored to minimize the potential for <br />undermining. The pipe entrance will be mitered to conform to the slope of the headwall. A trashrack <br />will cover the pipe entrances with primary trash bars oriented in the direction of flow in order to <br />minimize the potential for collection of debris. The pipes will be installed with cutoff collars to <br />minimize the potential for washout due to erosion along the outside of the pipes. <br />Figure 3 shows the estimated inlet flow at various river flows and return intervals. The flow capacity <br />varies from about 13 cfs during a 1 year event to about 90 cfs during a 5 year event. For all events <br />greater than the 5-year return interval, it is seen that the inlet capacity stays roughly the same (actually <br />decreases slightly for higher river flows). This condition is due to the fact that, for river flows greater <br />than the 5-year event, there is backflow from the river into the pit at the downstream end of the pit <br />which reduces the head across the inlet. <br />At 90 cfs pit flow-through, the estimated pit detention time is 4.5 days, not considering progressive <br />sedimentation. Sedimentation processes will reduce this detention time. <br />WWL would appreciate a written response from you regarding the proposed conceptual design. We <br />will be glad to address any comments or questions you may have. <br />Sincerely, <br /> <br />Bruce D. Smith <br />'Principal Hydrogeologist <br />cc: Project file