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• <br />3.16 A grout curtain would be installed to control seepage <br />beneath the dam. A single line curtain on 10-foot centers is <br />assumed in the valley and a triple line curtain on 10-foot <br />centers is assumed on the abutments. The depth of grout curtain <br />holes would range from 50 feet in the valley bottom up to <br />150 feet on the abutments. Blanket grouting would be installed <br />on 10-foot centers throughout the bottom of the core trench to a <br />depth of 10 feet. <br />3.17 Pressure relief drains would be installed along the <br />downstream embankment toe to reduce any undesirable water <br />pressures in the stratified foundation rock. <br />3.18 The stream flows would be diverted through the low-level <br />tunnel outlet works during construction. An upstream cofferdam, <br />about 10 feet high, would divert flows into the -outlet works. <br />The outlet works and cofferdam would be capable of diverting the <br />10-year flood around the construction area. <br />3.19 The outlet works are designed to accommodate the functions <br />of diversion during construction, water supply, minimum stream <br />flow, emergency reservoir drawdown and possible future generation <br />of hydropower. The construction diversion criterion selected at <br />this stage of preliminary design is the 10-year flood peak, <br />estimated as 1945 c£s. Water release requirements are from <br />minimum needs of 50 cfs or less, to a maximum of 200 cfs. The <br />emergency evacuation criterion is to drawdown 90 percent of the <br />storage volume, from spillway crest elevation 6,932 feet msl <br />within 4 months considering maximum average 4 months' inflow <br />occurring at the same time. Both ends of the diversion conduit <br />would be permanently plugged with concrete at the end of the <br />• construction period when it is no longer needed. <br />3-7 <br />