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December 2007 Georgetown – Final Alternatives Analysis Report <br />page 2 <br />Figure 1: Existing Features <br />4.0 Alternatives <br />There are several methods of increasing flow through an existing dam. For the flow increments specified <br />above, the following five alternatives were studied in the preliminary analysis: <br />x Alternative A – Gated notch in the spillway section (Gated Notch) <br />x Alternative B – Additional slide gate with conduit through spillway section (Slide Gate) <br />x Alternative C1 – By-pass pipe through the embankment dam (By-pass pipe through dam) <br />x Alternative C2 – By-pass pipe around the dam (By-pass pipe around dam) <br />x Alternative D – Siphon over embankment dam (Siphon) <br />x Alternative E – Pumping station on embankment dam (Pumping Station) <br />The preliminary analysis resulted in a recommendation to study Alternatives B, C1, and C2 in more detail in <br />the final analysis. The three final alternatives are hereafter referred to as Alternatives 1, 2, and 3. <br />4.1 Alternative 1 – Slide Gate <br />The slide gate option involves installing a new slide gate adjacent to the existing 48-inch slide gate on the <br />upstream face of the spillway at approximately the same invert elevation as the existing gate. A new 48- <br />inch square gate can pass the maximum flow increment of 225 cfs, and can be throttled to reduce flow <br />through the opening to match the 75 cfs and 150 cfs flow increments. <br />Due to the shape of the spillway, about 19.5 linear feet of concrete would be removed to provide the new <br />conduit downstream of the new slide gate. The perimeter shape of the conduit depends on the method of <br />removal adopted. Diamond wire cutting would generate a square conduit, while coring would generate a <br />round conduit. A square conduit could pass more water due to the larger cross-sectional area, but <br />preliminary flow calculations were based on the smaller round conduit, so either option is acceptable for the <br />application. Wire cutting was assumed for the engineer’s opinion of probable cost included in Appendix B. <br />Wire cutting requires a dry condition upstream and downstream of the construction area. The report <br />assumes this would be accomplished by installing a fifteen-foot diameter half-pipe steel structure bolted to <br />the face of the spillway and the concrete apron. (See Figure 4 in Appendix A) Installation of the temporary <br />bulkhead would be by divers.