Reconnaissance report on geology of Eklutna Lake dam site and conduit route near Anchorage, Alaska

Abstract

Summary and Recommendations 1. Eklutna Lake and Eklutna Creek lie in a wide, deep, glaciated, trough-like valley. Downstream from the lake, this valley is partially filled with unconsolidated glacial and alluvial deposits. 2. Eklutna Lake dam site, located about 400 feet below the lake outlet, is suitable for a low dam of flexible, earth-embankment type. Adequate control of the stream can be obtained by raising the lake level about 50 feet to altitude 910 feet, which will provide hold-over storage from wet years to dry years. Such a structure will have a crest length of about 1,950 feet. A. Bedrock is probably 200 to 475 feet below stream bed at the proposed axis. The dam will rest on glacial deposits of till, clay, sand and gravel, and on deposits of lake-shore and alluvial fan gravels. B. Geologic conditions in the area of the right abutment, as yet imperfectly known, may make necessary a long, deep cutoff extending for an unknown distance beyond the north end of the dam. C. A foundation exploration program is recommended that includes deepening test pit No. 1 and drill hole No. 2, and drilling 11 new holes. It is suggested that one drill hold near the center of the valley be taken to bedrock to give a complete picture of the fill materials underlying the foundation. 3. Delivery of water from the forebay of the reservoir to the powerhouse eight miles downvalley by means of a conduit is regarded as infeasible because: difficult terrain of the route will require earthwork more extensive than the volume of the dam; the route is subject to land slides, and will require expensive maintenance; it is more or less completely exposed to adverse winter conditions that may engender icing conditions; and it is easily subject to sabotage. It is recommended that the water be taken to the powerhouse through a rock tunnel.