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Page 6 <br />Proposed Inlet/Outlet Configuration <br />July 22, 2008 <br />Previous discussions have indicated that the currently proposed inlet consists of a pipe inlet rather than <br />the previously proposed open-channel Texas Crossing. The reasoning behind this proposed <br />modification is as follows: <br />The originally proposed channel inlet consisted of a 100-ft wide open channel with 3-ft <br />horizontal to 1-ft vertical side slopes. At higher river flows, this channel will induce larger <br />volume and flow velocities through the inlet opening, through the pit and out the outlet <br />channel. For example, during a 100-year river flow event, it is estimated that such an.inlet <br />channel would divert on the order of 10,000 cfs (over 13 percent of the total river flow) <br />through the inlet opening and create velocities as high as 15 ft/sec. It would be very difficult <br />and costly to harden such a channel to prevent it from enlarging due to the scour forces. <br />The potential for flushing previously deposited sediment out of the pit will be reduced. <br />Design and construction for both the inlet and outlet will be significantly easier and less costly. <br />4. The requirements for energy dissipation will be reduced as will the potential for developing <br />large plunge pools either at the inlet or the outlet. <br />In conjunction with a dike, the potential for river capture by the pit will be substantially <br />reduced. <br />6. The amount of flow diverted from the river is greatly reduced, addressing one of Mr. Miller's <br />concerns that the proposed action will adversely impact the flow through, and around, his <br />island. <br />Mr. Miller has stated a personal preference for use of a pipe inlet in order to address his <br />concerns. <br />The design criteria for the inlet and outlet are as follows: <br />To assure that water will flow through the pit during the 1 year river event and all higher flow <br />events. <br />To reduce adverse erosion and scour impacts. <br />To minimize maintenance requirements. <br />The inlet is proposed to be located at approximately river station 2.47 (see Figure 1). The normal pond <br />level is established by the river stage at the inlet during the 1 year event- elevation 4499.2. In order to <br />provide one ft of head for the inlet under a 1 year flow stage, the normal pond level was set at elevation <br />4498, which will be the bottom of the control section for the outlet. The outlet structure will consist of <br />an armored open channel located at about river station 1.52 which will extend from the inside edge of <br />the pit to the river channel and be between 10 and 20 ft wide. It will be designed to be capable of <br />discharging at least 50 percent greater flow than the maximum inlet capacity. <br />The inlet will consist of three, 24-inch diameter pipes approximately 250 ft long. These relatively small <br />diameter pipes were selected to minimize the potential for trash collection. For example, during river <br />flow events larger than the 1 year, the top of the pipes will be covered and floating debris will be <br />carried downstream over the top of the pipes. The bottom of the pipes at the inlet will be set at <br />elevation 4,498 ft. The pipes will slope downward and daylight to just below the mean pit water