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<br />Chapter II Description of Alternatives 19 <br /> <br />This concept, using underwater construction methods, removes the top 72 feet of the <br />trashrack structure which would then be at elevation 3,580. To accommodate all of the <br />equipment and provide trashrack slots up to the surface, a hoist deck would be provided at <br />elevation 3,715, and supported by steel columns which connect to the top of the trashrack <br />structure. The columns would also have guides attached for extending the existing trashrack <br />guides up to the surface. Trashracks would then be provided in these slots all the way to the <br />surface. <br /> <br />An alternative to this trashrack arrangement would be to provide a smaller semicircular <br />trashrack mounted to the top of the control gate. The trashrack would travel with the control <br />gate. This guide system, which runs from the surface down to the bottom of the structure, is <br />provided for the relief and control gate, <br /> <br />To force the flow through the top, the top 37.5 feet of the existing trashrack opening would <br />be closed and the lower 50 feet blocked by a relief gate, the same as proposals 2A and 28. <br />To close the upper portion of the trashrack opening, plates would be installed over the <br />existing trashracks. The relief gate, which would be 50 feet high and semicircular in shape, <br />would be installed in the new guides, The relief gate would be designed with relief panels to <br />protect the system and trashrack structure from excessive differential head. A control gate <br />which acts like an adjustable weir would also be installed in the new guides. It would be <br />semicircular in shape, 80 feet high, and operated by a hoist. <br /> <br />With the bottom of the existing trashrack structure blocked, and the top of the control gate set <br />at the proper submergence, the warm surface water would be pulled from the top of the <br />reservoir and into the semicircular control gate and down the trashrack structure. As the <br />reservoir drops, the control gate is lowered to maintain the proper submergence. If 40 feet of <br />submergence is necessary, the system would be fully effective down to reservoir elevation <br />3,620 providing an effective operating range of 80 feet. During periods when selective <br />withdrawal is not needed, the relief gate would be brought up, using a lifting frame and <br />mobile crane, and stored near the surface to conserve warm water in the lake and minimize <br />head loss. <br /> <br />Proposal No.4 - External frame structure with three flat gates. This proposal was <br />brought forward for feasibility cost estimates to define the upper limit of the options <br />available, The feasibility cost for this proposal was $148,500,000. This method was found <br />to be nearly ten times more expensive and yet functionally similar to proposal 2A. This <br />alternative had slightly less impact on power production and unneeded flexibility. It did not <br />offer significant operational benefits over less expensive modifications. <br /> <br />The exterior frames concept consists of a steel truss structure attached to the face of the dam. <br />The structure would encompass the existing trashrack structure. Each structure would be <br />approximately 50-ft wide (cross canyon direction), 50-ft deep (stream direction) and 265-ft <br />high. Each structure would be suspended from a rigid frame attached to the face of the dam <br />by a rigid frame (knee-braced support) at a centerline elevation of3,7l0. A separate structure <br />would be provided for each unit. This would allow for staged construction. The structures <br />would be connected to the dam at various elevations as required to resist water hammer, <br />