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I Executive Summary PRELIMINARY DESIGN Dam Embankment The embankment of existing Elkhead Dam would be raised 43.5 feet, using conventional earthfill techniques, to a crest elevation of 6418.5 feet. This would produce a reservoir volume of 44,900 acre-feet, an increase of 31,200 of over the present volume. The raise would be accomplished on the downstream face of the existing embankment and would thus use most of the existing structure. A new core and drain system is also proposed. The new embankment would contain slightly more than one million cubic yards of fill material. 1 i Outlets and Spillways The primary outlet tower of the enlarged reservoir would contain three slide gates for temperature control; this will permit management of both downstream water quality and fish escapement. The outlet will include a 5-foot diameter penstock emptying into the service spillway stilling basin through a Howell-Bunger valve. Stubouts would be provided in the valve house to allow future installation of a 1 megawatt hydropower generation facility. The service spillway would be on the left abutment of the dam and would be designed to pass the 100-year flood inflow; the spillway chute would end in a USBR Type II stilling basin. The emergency spillway would be constructed in a natural topographic saddle near the right abutment of the dam and would empty into an adjacent drainageway which rejoins Elkhead Creek roughly a mile below the dam. A plan view of the enlarged dam area is provided in Figure S-2. fi 1 1 1 1 DEVELOPMENT PLAN Permits and Acquisitions A variety of federal, state, and local permits will be required before construction of the dam enlargement can begin. In addition, it is likely that an Environmental Impact Statement will be required. It is recommended that these permit application and environmental reporting processes be closely coordinated to avoid duplication of effort and unnecessary delay. It was assumed that all lands and facilities below an elevation of 6420 (roughly the high water level during the PMF event) would need to be acquired and/or relocated. Based on this assumption, a total of about 200 acres of private land and 600 acres of public land (mostly owned by the Colorado Division of Wildlife) will need to be acquired for the enlargement. In addition, roughly 14,400 feet of county and private road, two bridges, and miscellaneous telephone and electric lines will need to be relocated. S-7