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Preliminary Design of Dam and Spillways 10. The site is accessible has a reasonably long period of low inflow, has an adequate length normal construction season and otherwise does not have any special limitations requiring rapid construction or special flow handling. 11. A Roller-Compacted Concrete (RCC) spillway/dam raise at the site was found to be less economical than a left abutment spillway in a previous study (M-K, 7/85). 12. On-site aggregate is of a marginal quality for concrete and RCC. A commercial off-site source approximately 10 miles away may be the best source of this material, thereby increasing the cost of these types of structures at the site. 13. Geotechnical, flood handling and other evaluations conducted as a part of this feasibility study provide no new information which would indicate a need to study dam types other than an earthfill in detail. Considering these and other factors, no logical alternative to an earthfill dam exists that would warrant detailed cost effectiveness studies. As a part of the geotechnical investigations for this feasibility study various earthfill sections were considered. The most feasible of these dam raise options is described in more detail in the following section. DAM EMBANKMENT DESIGN Elkhead Dam was originally constructed as a 90-foot high homogeneous earthfill embankment. The crest of the dam spans approximately 1,140 feet at an elevation of 6,375 feet with a crest width of 20 feet. The upstream and downstream slopes of the dam embankment are 3:1 and 2.5:1, respectively. Elkhead Dam will be enlarged by constructing a raise on the downstream side of the dam embankment to a crest elevation of 6,418.5 feet. Integral with the dam will be a new service spillway and primary outlet located near the left abutment. A new emergency spillway will be provided away from the dam in the topographic saddle on the right abutment as shown in Figure 4-1. The height of the embankment raise was determined based upon achieving the maximum storage permitted by site topography and sizing of the spillways to route the Probable Maximum Flood (PMF) past the dam without overtopping. More detailed discussions of flood hydrology and spillway configuration were presented in Chapter 3 and are further discussed later in this chapter. The basis for the dam raise design is the composite geotechnical report (Woodward- Clyde, 1993). A controlling element of the dam geometry is seepage control. The dam currently experiences noticeable seepage through the right abutment and an unmeasured (though small) amount of seepage through the embankment. A total combined flow of approximately six gallons per minute (gpm) of seepage was measured from three weirs at the lower right abutment. It is expected that the larger hydraulic heads associated with the dam raise will increase this seepage flow by at least one order of magnitude to 10-50 gpm. A recommended dam raise design was defined for addressing this expected seepage, while appropriately tying into the existing embankment and making maximum utilization of available' construction materials. 4-2 1