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I I I I I I I I I I I I I I I I I , I I -24- As previously mentioned, a meeting was held with the SEO on October 31. 1986 to discuss results of the hydrology study and of the IDA. The design concepts for Options 1 and 2 were also discussed at the meeting, as noted in the memorandum dated October 31, 1986 given in Appendix C. In addition to the general discussion of the options, the overall design philosophy for both the spillway channel and the downstream toe area of the overflow section were discussed. It was agreed at this meeting that a design philosophy that permits some damage to either the spillway channel or the toe area of the overflow section would be appropriate for design discharge conditions. However, the overall integrity of the dam and spillway should be maintained. The design concepts illustrated on Figures 6 through 10 are consistent with this design philosophy. Some erosion of the spillway channel walls at 1.5H:1V side slopes in the lower portion of the channel would occur during large flood discharges. However, the flatter channel bottom slopes and corresponding lower channel flow velocities in the urper channel will result in a stable channel bottom and side slopes, even during the design flood event. The current spillway channel side slopes are at approximately 1H:IV to 1.5H:1V and have remained relatively stable during all discharges that have occurred since the spillway channel construction. Likewise, some erosion downstream of the overflow section apron may occur during design discharge. However, the tailwater control and flow training berms along the downstream edge of the apron should help to minimize the damage by controlling energy dissipation in the apron area. A 4-foot-deep soil-cement cutoff wall and site grading downstream of the apron have been included in our apron concept illustrated on Figure 10. A deeper cutoff may be indicated during final design. Excess riprap material processed from the excavation material may also be used in this area to prevent damage that threatens the integrity of the dam during design discharge. For these two options, we estimated total costs for construction including final design engineering and administrative costs, construction phase engineering costs, a construction cost contingency of 22 percent, pro- jected price increase for 1988 construction, interest during construction, and payback of feasibility study funds. Details of the cost estimates including quantities and unit cost derivations are given in Appendix C and discussed in Chapter V. The resulting estimated costs are as follows: Option 1 - Spillway Enlargement. $821,100 $600,000 Option 2 - Overtopping Protection In addition to being significantly lower in cost, Option 2 preserves the 'current level of reservoir storage. Therefore, Option 2 has been selected as the preferred modification scheme to provide PMF flood routing protection at Sheriff Reservoir. A detailed discussion of this option is given below. A discussion of the estimated costs for this option is given in Chapter V. Physical Plan - A general plan of the Option 2 modifications to Sheriff Reservoir is shown on Figure 9, and the main elements include: