Electrical Power for Valdez and the Copper River Basin-1981

100 psi pressure at depth. The source is located in or above the cliffsbordering the lake. Precisely locating the tap will require underwaterphotography and possible use of diver inspection. Additional drilling will berequired during the General Design Memorandum phase to determine exactoverburden thickness and rock characteristics. Artesian conditionsencountered in the vicinity could pose a problem during construction and willrequire further investigation.POWER TUNNELAs noted, Allison Lake sits in a glacier scoured valley. The lake outletis in what appears to be deep till deposits. DH-2, located approximately4,000 feet downstream of the lake outlet, did not penetrate bedrock at a depthof 64 feet. Assuming a relatively flat rock line, bedrock may be at anelevation between 1,150 and 1,200, 170 to 220 feet below lake elevation. Thisis based on the elevation of the falls downstream of the outlet. DH-2 wasdrilled from an elevation of 1,350 feet, 1,200 feet east of metagraywackecliffs. This reinforces the suspicion of deep tills in the valley.Plans call for a power tunnel invert elevation of 1,050 feet. The tunnelhas been alined to be under the metagraywacke cliffs to assure that it waslocated in rock. Additional postauthorization explorations and studies maylocate suitable rock in the valley floor which would permit a shorter tunnelalinement. The cost estimate for the proposed tunnel was based on an 8-footunlined horseshoe tunnel for 50 percent of the length and a 6-foot diameterconcrete lined circular tunnel for 50 percent of the length. Duringpostauthorization studies, additional drilling will be done to determine theextent of overburden and condition of underlying rock.POWERHOUSEOne hole DH-3, has been drilled in the lower powerhouse area, penetrating69 feet of Allison Creek delta deposits before encountering moderately tohighly fractured metagraywacke. While the delta deposits would make goodfoundation material for a small surface powerp1ant, the area may be subject totsunami below an elevation of +85 MLLW. During slide induced sea waves, runupat Anderson bay, 7 miles west of Allison Creek, was measured at 70 feet.Measurements at Shoup Bay (Cliff Mine) on the north side of the arm show arunup of 170 feet. No runup measurements were made at Jackson Point orFort Liscum, but the cannery at Jackson Point floated away.Maximum high tide in Port Valdez is approximately +14 MLLW; therefore, toinclude the 70-foot runup at Anderson Bay the powerhouse elevation should beat least +85 MLLW. For this reason the upper powerhouse, located atapproximately +100 MLLW was selected. Additional reconnaissance andexplorations will be required for final powerhouse location duringpostauthorization studies.G-2