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II I I I I I I I II I I I I II I I I I I I -2- CONCLUSIONS AND RECOMMENDATIONS The following conclusions and recommendations were determined as part of the engineering and economic evaluations completed for this feasibility study: 1. The probable maximum flood (PMF) resulting from a probable maximum precipitation (PMP) event occurring in the catchment above Sheriff Reservoir has a peak inflow to the reservoir of 20,000 cubic feet per second (cfs) and a total inflow volume of 2500 acre-feet. 2. An Incremental Damage Assessment (IDA) of Trout Creek below Sheriff Reservoir indicates that significant impacts to downstream areas would occur in the event of a dam failure by overtopping during flood events in excess of the current spillway capacity. Consequently, the SEO has indicated that PMF flood routing protection is required for Sheriff Reservoir. The SEO has indicated that flood routing protec- tion for 50 percent of the PMF, in conjunction with an early warning system, would also be acceptable. provided that the system would be effective in evacuating downstream residents and provided that signi- ficant cost savings over PMF protection would be realized. However, the SEO prefers PMF flood routing protection for Sheriff Reservoir. 3. The flood routing capacity of the spillway at need to be 17,000 cfs to safely pass the PMF. capGcity is approximately 3,000 cfs. Sheriff Reservoir would The existing spillway 4. Four feasible alternative schemes for modifying Sheriff Reservoir to provide the required level of flood routing protection were iden- tified and evaluated. Two of these schemes were clearly identified as preferred alternatives, considering cost and overall long-term effectiveness and performance. The two preferred schemes are as follows: Option 1 - Spillway enlargement to provide the capacity to safely pass the full PMF event Option 2 - Overtopping protection (using soil-cement) to safely pass the full PMF event 5. Feasibility level cost estimates of these options indicated the following probable total costs for construction, including engi- neering and administrative costs, a contingency of 20 percent, the cost of this feasibility study, and other miscellaneous expenses: Option 1 Option 2 . . . . $821,100 $600,000 6. Based on the engineering evaluations and cost estimates, the overall preferred alternative for modifying Sheriff Reservoir to provide the required flood routing protection is Option 2 - overtopping protec~ tion using soil-cement.