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I I , I il I I I I I I I I I I I I I I II -20- of Sheriff Dam would significantly increase the flood stage and mean channel velocity at Knotts Ranch, over 7 miles downstream from Sheriff Dam. Specifically, during the full PMF, assuming that Sheriff Dam does not fail, the peak discharge at Knotts Ranch is 11,300 cfs with a corresponding water surface elevation of 8045.7 feet and a flow velocity of 7 ft/s. During the full PMF, and assuming dam failure, the peak discharge at Knotts Ranch is 44,500 cfs, with a corresponding stage of 8050 feet and a flow velocity of 10 ft/s. Figure 5 is a summary plot of these results including a plot of the cross section at Knotts Ranch and the location of the house. We estimate that dam failure under nonflood conditions (sunny day or clear night failure) would result in a peak discharge at Knotts Ranch of approximately 36,000 cfs, with a corresponding stage of approximately elevation 8049. The Knotts Ranch resi- dence has a surveyed first-floor elevation of 8044. The results indicate that the residence lies within the incremental flooding zone under all conditions during which dam failure might occur. Dam failure would cause the water sur- face to rise by approximately 5 feet at Knotts Ranch under PMF base flood con- ditions. Larger increases in flow depth would be anticipated under base flood conditions less than the PMF. Under all conditions, the increase in flood stage, significantly exceeds the SEO guidelines outlined in Section IV.4, and hence, the IDA does not provide justification for reducing flood routing pro- tection requirements for Sheriff Reservoir to less than the full PMF. A summary of DAMBRK computations from Sheriff Reservoir downstream to Knotts Ranch is given in Table 2. Required Flood Routing Protection for Sheriff Reservoir - Results of the IDA described in Section IV.4-3 have indicated that dam failure by overtopping during a flood would result in significant incremental impacts in developed areas downstream of the dam. These results were reviewed with the SEO on October 31, 1986. At this meeting and during the course of phone conver- sations over the following week, the SEO indicated that either of the following two alternatives for Sheriff Reservoir would be acceptable: 1. Full PMF spillway or overtopping protection. 2. A spillway capable of safely passing 50 percent of the PMF, combined with a reliable early warning system for evacuation of downstream residents. Alternative 2 above was judged acceptable because only one dwelling with two residents would require evacuation and because there is only a 1 percent chance of a flood event greater than 50 percent of the PMF (estimated 5000-year recurrence interval) occurring over the remaining useful life of the dam (50 years). It was agreed that the early warning system should provide a minimum of 1.5 hours response time to evacuate downstream residents. The minimum response time of 1.5 hours is considered necessary for effective evac- uation of downstream residents.2 2U.S. Bureau of Reclamation, ACER Technical Memorandum No.7, "Guidelines to Decision Analysis," 1986, p. C10.