Electrical Power for Valdez and the Copper River Basin-1981

expected to Allison Creek below Allison Lake indicate the highest flowsoccur in June, July, and August. July and August are the most criticalmonths because of spawning salmon.Winter withdrawals from the lake wOUld be warmer than the natural streamtemperatures. Thermograph readings show water temperatures in the wintermonths to be near 0° C while lake temperatures are at 3.5 0 C. Sincewinter is the high power demand period, the project would probably berunning at maximum output part of the time. The maximum power releasehas been estimated at approximately 100 CUbic feet per second (cfs) witha predicted streamflow above the powerhouse Detween 1 to 5 cfs.Hydroelectric projects tend to even flows. Natural high summer flowswould be utilized to refill the lake, therefore reducing outflows fromthe lake. During the winter months when natural flows are low, the waterused for electrical generation would SUbstantially increase the flows.The entire stream would be impacted by the proposed project with bothsummer and winter flows decreased above the powerhouse location. Theproposed powerhouse site is located above the existing weir,approximately one quarter mile from Port Valdez. At this location, thewater would reenter Allison Creek.Tne natural flUShing process woula be affected by the hydroelectricproject, thereby reducing streambed scour and possibly increasingsedimentation of spawning gravels. Allison Creek has a stable streambed,consisting mainly of boulders and slaty cobbles with very small amountsof sands and gravels. Little fine grain material appears to be enteringthe system, consequently, sedimentation of the spawning bedS may notoccur. During high water years, the extra quantities of runoff mayprovide adequate flows for flUShing. The intertidal area supports themajority of spawning activity, and tidal fluctuation should keep thespawning area free from sedimentation.Winter is the tinle of high power consumption and also the time of lowflows into the lake. The increased flows through the powerhouse wouldcause the lake to De drawn down a maximum of 100 feet from the naturalwater 1 eve 1.The tailrace to the creeK would consist of a 5-foot diameter steel pipeat a 10 percent slope. The length would be 20 feet with a 10-foottransition to a 2.5-foot diameter steel pipe on the sanle slope. Anenergy dissipater Ivould be built on tne banks of the creek with very1 itt le instream construct ion. Tne impacts occurring with the constructionof the dissipater should be minimal; however, in order to assure noimpacts occur on either spawning salmon or their incubating eggs, constructioncould occur between early June and mid-July or during thewinter months when construction could be accomplished out of the streambecause of the reduction in flow. Approximately 5 cubic yards of riprapwould be placed in the creek just below the energy dissipater. Tneriprap would be approximately 1.5 feet thick and cover about 10 squareyards. The riprap WOUld assure that water from the powerhouse would notcause scour or erosion.EIS-9