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December 2007 Georgetown – Final Alternatives Analysis Report <br />page 4 <br />4.3 Alternative 3 – By-pass Pipe Around Dam <br />The by-pass pipe alternative around the dam involves installing a new 54-inch diameter pipe around the left <br />(West) abutment of the dam. Based on the topography, an inlet structure would be located approximately <br />120-ft upstream of the dam on the West side of the reservoir. The pipe would discharge into the creek <br />approximately 100-ft downstream of the existing spillway. (See Figures 3A and 3B in Appendix A) Routing <br />the pipe completely around the dam may reduce or eliminate regulatory agency requirements. <br />In order to route the pipe completely around the dam, the pipe would need to be located in Alvarado Rd. <br />The engineer’s opinion of probable cost assumes that the invert of the new pipe would be located <br />approximately 20 feet below the eastbound lane of the road. It is likely that utilities currently exist along the <br />proposed pipe route. Therefore, it is assumed that a utility survey would be performed before design of this <br />alternative in order to avoid any conflicts. <br />A 15-inch diameter concrete sewer pipe is known to run along the West side of the reservoir from <br />Georgetown to the sewer plant downstream of the dam, and is located approximately 6-feet below the <br />spillway crest elevation. As shown on Figure 3A, the intake channel for a new intake structure directly <br />conflicts with the existing sewer line. It is our opinion that approximately 60-feet of the sewer line would be <br />rerouted at this location. On the downstream side of the dam, it is anticipated that the new pipe would <br />cross under the existing sewer line. <br />Regulation of flow through the by-pass pipe would be with a 42-inch diameter regulating gate valve at the <br />downstream end of the pipe. The valve would be housed in a valve vault and would discharge into a <br />concrete discharge structure on the bank of the creek downstream of the existing spillway. <br />A bulkhead gate at the intake structure would allow drainage of the pipe for inspection. Based on the <br />topography, it appears that the intake structure could be located between the West side of the reservoir <br />and the East side of Alvarado Road and would consist of a trashrack, the bulkhead gate, and a bubbler <br />system to prevent ice formation at the stem. <br />Depending on the actual location of the intake structure, construction of this alternative could be performed <br />the following ways: <br />x If the topography requires that the intake be located in the reservoir, a coffer dam would be <br />required for construction in the dry. <br />x If the intake structure can be located on the bank of the reservoir, construction of the structure <br />could take place in a dry pit on the bank, and then an intake channel would be excavated. <br />x Excess trench dirt from the pipeline installation could be placed in the reservoir at the location of <br />the intake structure. This would allow extension of the structure into the reservoir without the use <br />of a coffer dam. <br />The engineer’s opinion of probable cost assumes that the pit would be dug for construction of the intake <br />structure, then the dirt plug would be excavated to create the intake channel. <br />Diversion of water to the hydro plant could be accomplished with this alternative by providing a bifurcation <br />and suitable valves. A detailed hydraulic analysis would need to be performed to determine an <br />economical diameter of pipe with and without hydro plant flows. The engineer’s opinion of probable cost <br />does not include provisions for a hydro plant diversion.