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<br />I <br />i I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I,J <br /> <br />-" <br /> <br />I <br /> <br />After assessing the existing condition of the dam and evaluating the <br />geotechnical information available, a preliminary design was prepared based on <br />the storage need resulting from the water utilization study and the <br />requi rements of the SED in regard to safely routi ng the PMF. Conserv3ti on <br />storage was increased 2,400 acre-feet by raising the existing service spillway <br />ogee five feet to elevation 6370. The auxiliary spillway channel invert was <br />set at elevation 6375 in order to operate only during flood events with a <br />recurrence interval greater than 100 years. A flood routing was performed <br />using the HEC-l computer model util izing the PMF developed by the CDOW, as <br />approved previously by the SED. The width of the auxiliary channel was sized <br />so that the excavation quantity would balance the embankment volume necessary <br />to raise the dam to an elevation sufficient to provide one foot of freeboal'd <br />during the routed PMF. <br /> <br />Three alternative appraisal-level designs and cost estimates were prepared for <br />the purpose of developin:;) a storage-cost relationship. The alternatives and <br />estimated costs are summarized as follows: <br /> <br />Alternative No.1 <br />This alternative is the minimum modification necessary to meet the SED <br />requi rement to pass the PMF, It i nvol ved excavati ng an auxil i ary spilll~ay <br />channel 110 feet wide and raising the dam crest five feet with roller <br />compacted concrete. Estimated project cost was $425,000. <br /> <br />Alternative No.2 <br />The dam could be raised 15 feet and the reservoir level 10 feet, to provide <br />4,800 acre-feet of additional storage without repl aci ng the exi sti ng servi ce <br />spillway, Rebuilding the service spillway would represent a significant jump <br />in unit cost per acre-foot of storage. The alternative consisted of <br />excavating an auxiliary spillway channel 220 feet wide through the left <br />abutment and utilizing the excavated material to raise the dam. The concrete <br />spillway ogee would be raised 10 feet. The estimated cost was $1,800,000. <br /> <br />Alternative No.3 <br />This alternative was identical to Alternative No.2, except it only increases <br /> <br />E-4 <br />