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Table 2-1. Existing Elkhead Dam/Reservoir Characteristics I 1 1 LIJ Inventory of Dams I.D. Number CO 00976 State of Colorado I.D. Number C-1339A Elevations/Capacities Crest 6,375 feet Service Spillway Crest/Normal Maximum W.S. 6,365 feet Primary Outlet, Intake Invert 6,333 feet Primary Outlet, Outlet Invert 6,316 feet Channel at Downstream End of Stilling Basin 6,295 feet Reservoir Capacity at Normal Maximum W.S. 13,700 acre-feet Service Spillway Capacity at Crest of Dam 17,000 cfs Primary Outlet Capacity at Crest of Dam 180 cfs Primary Features/Dimensions • 1,160-foot long homogeneous earthfill embankment with chimney drain • 315-foot long 36-inch RCP primary outlet pipe; operated by a hydraulically controlled sluice gate • 135-foot ogee "duckbill or bathtub" shaped service spillway crest with a 40-foot wide spillway chute • 40-foot wide USBR Type III Stilling Basin • 20-foot wide county road on crest 2.1.2 Hydrology The operational and flood hydrology of Elkhead Creek drainage area upstream has been extensively studied as part of the dam/reservoir original construction (ECI 1974); dam safety evaluations (USACOE 1980), (CDOW 1984), (HARZA 1991), and (Ayres 1996a and b); and enlargement and modification studies (MK 1985 a and b), (MK 1986), (MK 1987), (Hydrosphere 1993), and (Hydrosphere 1995). Annual operational hydrology is dominated by snowmelt runoff during April through June when approximately 90 percent of the annual 20,000-100,000 acre-feet of water occurs as inflow to the reservoir. Part of that annual yield is allocated to storage at Elkhead Reservoir including; 8,310 acre- feet to the Craig Power Plant for standby cooling water and 5,390 acre-feet (including 3,722 acre- feet of dead storage) to CDOW and the City of Craig for recreation and water supply. Rainfall also occurs during this period giving the hydrology a mixed event characteristic. Rainfall is potentially more dominant during the mid-summer through early fall in the form of rare, short duration intense thunderstorms. The theoretically greatest flood [probable maximum flood (PMF)] is projected to be associated with this late summer period. The original dam safety evaluation (USACOE 1980) identified several minor safety issues which needed to be addressed and a hydraulically inadequate spillway as a safety issue. Resolution of these safety issues has been considered integrally with the several dam/reservoir enlargement proposals previously referenced and that activity continues to the present time (Ayres 1996a and b). The PMF is not further addressed herein because of its non- A Control Structure Feasibility Evaluation 2-4 Miller Ecological Consultants, Inc., February 18, 1997