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1 1 1 1 rd Reservoir Site Characterization There are four USGS stream gages located on Elkhead Creek. Three of these gages are located above the site of the existing reservoir: gage 9244500 (Elkhead Creek near Clark); gage 9245000 (Elkhead Creek near Elkhead) and gage 9245500 (North Fork Elkhead Creek near Elkhead). A fourth gage is located immediately above the confluence with the Yampa River (gage 9246500). Inflow estimates at the reservoir site was developed using linear regression on gages 9245000 and 9245500 which are both located above the reservoir. These gage records were filled in and extended using regression equations developed against flows recorded at the Maybell gage on the Yampa River. After the Elkhead Creek gages were extended to cover the entire study period, the resulting flow records were adjusted to reflect conditions at the site of the reservoir. The area-elevation-yield method was then used to translate the extended gage data to the site of the reservoir. Based on these data development procedures, the estimated annual inflow at the existing Elkhead Reservoir ranged from 18,800 of (in 1977) to 99,100 of (in 1978); the average annual inflow was estimated to be 64,500 af. The seasonal pattern and range of monthly inflows is shown in Figure 3-4 which indicates that roughly 90%a of the annual inflow occurs in the months of April, May, and June. Inflows in the fall and winter months are regularly less than 10 cfs. Flood Hydrology A concept for handling extreme hydrologic events at the enlarged Elkhead Dam/Reservoir was presented in the Phase I Study. That concept consists of a primary outlet sized to handle emergency reservoir drawdown, to deliver downstream flow demands, to carry water to produce hydroelectric power and to pass frequent floods up to the mean annual peak flood without the use of 'a service spillway. It is not functionally satisfactory or economically feasible to provide a primary outlet which can handle more infrequently occurring flood flows. A service spillway and an emergency spillway were therefore proposed to handle flows larger than the mean annual peak flood. The intent of this conceptual approach of flow handling is to provide a primary outlet and a service spillway which together control releases from the reservoir for all hydrologic events having a reasonable likelihood of occurrence during the 100-year design life of the dam/reservoir. In addition, in order to meet State of Colorado and standard dam safety criteria, the dam must be able to withstand, without failure by overtopping, a "safety evaluation flood" (SEF). In this case the SEF is the statistically indeterminate PMF. An emergency spillway, located in an adjacent topographic saddle, was therefore proposed to assist in the conveyance of all flows exceeding the 100-year event. Since this emergency spillway directs rare flood flows to an adjacent drainageway which in turn will carry the water back to Elkhead Creek one mile downstream, and since the drainageway must otherwise continue to function as it does now, the frequency and magnitude of water which it must handle under emergency conditions must be minimized. Therefore, a three-foot surcharge pool has been provided within the reservoir to function together with the service spillway (neglecting the primary outlet) to handle most floods without using the emergency spillway. This combination of flow capacity and hydrograph attenuation reduces the frequency of use of the emergency spillway to approximately once in five hundred years (based on rainfall flood frequency) but maintains a physical capability of handling extremely rare flows up to the PMF. 3-11