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December 2007 Georgetown – Final Alternatives Analysis Report <br />page 3 <br />There are several options regarding operation of the new slide gate. The simplest and least expensive <br />option includes a rising stem with manual operation. Electrical actuation is another option that can be <br />incorporated along with the manual operator. Both the manual and electrical options include a gate stem in <br />the flow path of water over the spillway. Any obstacles in the flow path reduce the efficiency of the spillway <br />design, and introduce an obstacle that could catch trash and debris. An option which eliminates obstacles <br />in the flow path includes a submerged hydraulic operator. If the submerged hydraulic operator option is <br />employed it is recommended that the existing gate be operated with a similar unit, as both operators could <br />be driven by the same hydraulic power unit.2 The submerged operator will eliminate obstacles in the flow <br />path over the spillway, whereby assuring the maximum spillway capacity. The engineer’s opinion of <br />probable cost includes hydraulic gate operation. Figures 1A and 1B in Appendix A illustrate the slide gate <br />alternative. <br />4.2 Alternative 2 – By-Pass Pipe Through Dam <br />The by-pass pipe alternative through the dam involves installing a new pipe through the left (looking <br />downstream or West) abutment of the spillway. A 48-inch diameter pipe could discharge the maximum flow <br />increment of 225 cfs. A 42-inch diameter gate valve would be included on the downstream end of the pipe <br />to regulate flows through the pipe, and a 48-inch square slide gate would be included on the upstream end <br />of the pipe to act as a bulkhead. The invert of the new pipe would be located approximately thirteen feet <br />below the crest of the spillway with an inlet located on the left abutment wall (See Figures 2A and 2B in <br />Appendix A). <br />Installation of the pipe in the existing embankment involves trenching through the embankment, installing <br />the pipe, then backfilling and compacting the embankment over the pipe. The existing spillway structure <br />concrete and embankment dam were designed for a specific application, and considerations should be <br />made if the design is altered during construction of the new pipeline. Drawings 3 of 9 and 4 of 9 of the <br />Georgetown Lake Dam reference drawings dated September 1, 1971 show the walls and foundation of the <br />existing structure. If this option is adopted a structural evaluation of the retaining walls and foundation <br />would have to be performed to check the stability of these walls during all stages of construction. An <br />analysis of these items would be performed during final design of the alternative. <br />A 48-inch bulkhead gate at the inlet and a 42-inch gate valve at the outlet would be included to operate the <br />by-pass line. It is assumed that the inlet bulkhead gate would be installed in the dry with the use of a steel <br />fifteen-foot diameter half-pipe temporary bulkhead, similar to the dewatering method used in Alternative 1 <br />above. The regulating gate valve would be installed in a dry condition. Operation of the gate and valve <br />would be by manual / electric operators with rising stems. Submerged hydraulic operation of the upstream <br />gate is not required since the gate stem would not be located on the spillway crest. <br />The concrete abutment walls would be penetrated at the inlet and outlet. For a 48-inch diameter pipe, the <br />concrete core should be made slightly larger than 48-inch to accommodate concrete reinforcement. The <br />engineer’s opinion of probable cost includes concrete coring for two 54-inch diameter penetrations. Coring <br />can occur from either side of the abutment wall. <br />Diversion flows to the hydro plant downstream could be accomplished with a modified version of this <br />alternative by provision of a bifurcation to the hydro plant. The by-pass pipe would extend past the <br />downstream abutment wall into a valve chamber. The valve chamber would contain a regulating gate valve <br />for the by-pass flows, and a guard valve on a diversion line to the hydro plant. The engineer’s opinion of <br />probable cost does not include this modification. <br />x 2 The Engineer’s Opinion of Probable Cost includes the cost of submerged hydraulic operation <br />(hydraulic cylinder and hydraulic power unit) of the new gate. The cost of an additional submerged <br />hydraulic cylinder to operate the existing gate is about $15,000. The hpu would be used to operate <br />both cylinders.