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Preliminary Design of Dam and Spillways 3) Downstream Slope, End of Construction 4) Upstream Slope, Rapid Drawdown 5) Upstream Slope, Pseudo-Static (Seismic) 6) Downstream Slope, Pseudo-Static (Seismic) PRIMARY OUTLET DESIGN The primary outlet regulates water releases from the Elkhead Dam to serve several purposes. Some of these are to deliver water downstream to meet the flow demands, handle emergency reservoir drawdown, carry water to produce hydroelectric power and pass frequent floods up to the mean annual flood without the use of a service spillway. The primary outlet and service spillway together will control the releases of floods up to the 100-year event. In accordance with the requirements of the Colorado State Engineer, however, safe handling of the SEF assumes the primary outlet is not passing water. The less frequent events will be handled by the service and/or emergency spillways. A 100-foot primary outlet tower, with a crest elevation of 6,418.5 feet, will be located in the reservoir approximately 260 feet north of the dam crest as shown in Figure 4-1. A 252-foot long by 4-foot wide footbridge will extend from the dam crest to the tower where electrical operators for the gates will be located. The bridge will be supported by the tower and 2 piers, one "A-frame" and one column, with spans of 84 feet. The "A-frame" pier is the higher of the two piers and has this shape for economic and structural reasons. The lowest elevation at which water will flow into the tower is 6,333.5 feet. A 5-foot square sluice gate will exist at this elevation. Two additional sluice gates, each 3 feet square, will be located on different faces on the outside of the tower. The location of these gates will depend on the water temperature release requirements which remain to be determined; as shown on Figure 4-3, the gates have tentatively been evenly spaced between 6,333.5 feet and 6,406 feet. All three of these gates will be guarded by trashracks. A fourth gate (guard gate) will be located on the inside of the tower at the entrance to the penstock. The penstock will be a 5-foot diameter concrete encased steel pipe and will be connected to the outlet tower at the elevation of the lowest inlet (6,333.5 feet). The 760-foot long penstock is located 200 feet west of the service spillway along the dam crest. Inside the tower a 10-foot high bell-mouthed entrance will transition into the 5-foot diameter penstock. From the tower, the penstock will angle down toward the stilling basin located at the' end of the spillway. The pipe will bifurcate before reaching the basin. The first branch will be a 15" diameter gated low-flow bypass. The second branch is to a butterfly valve followed by a Howell-Bunger valve. The purpose of the Howell-Bunger valve is to dissipate the water energy before it enters the stilling basin as well as to throttle the flow and bypass water from the hydroelectric facility. The 5' diameter penstock continues to the future turbine location. Until the hydroelectric facility is installed, an end cap will be used to keep flow from passing through this branch of the penstock. 4-6