Electrical Power for Valdez and the Copper River Basin-1981

INTRODUCTIONAllison Lake is located on the south side of Port Valdez, directly acrossfrom the city of Valdez, filling approximately one-half of a glacier formedhanging valley. The area is underlain by the Late Cretaceous Valdez Group, aseries of metagraywackes, shales and slates with occasional beds of pillowbasalt. This is indicative of a rapid, unsorted deposition in an oceanenvironment. The beds are highly disturbed and no regional structure has yetbeen defined.LOCAL LINEAR FEATURESAerial photographs and regional mapping show three strong lineaments nearAllison Lake. They are: Jack Bay, east-west leg on the north side of JackBay, and an east-west trend approximately 1 mile north of Jack Bay. Thesefeatures have been mapped as faults by the U.S.G.S. (Bulletin 989-E dated1954) .SEISMICITYAllison Lake is located in Seismic Zone 4, approximately 46 miles east ofthe epicenter of tne 27 March 1964 earthquake. The major source of seismicactivity in this region is the relative motion of the Pacific and NorthAmerican plates. The Pacific plate is plunging under the North American plateand it is active along the Benioff Zone which controls the tectonic setting ofthe area.Prior to 1964, there were approximately 70 magnitude 5, or greater,earthquakes recorded in the Valdez vicinity. Magnitude 5 earthquakes haveaveraged approximately one per year in recent years. Submarine landslideshave been associated with several of the larger events. Magnitude 8 orgreater events have occurred three times this century, therefore there is agood probability of a large earthquake occurring during the life of theproject. Maximum Credible Earthquake (MCE) should be tentatively set atmagnitude 8.5 at 40 miles.It is important to note that there has been no evidence of rupture in areabedrock. Most destruction in Port Valdez has been caused by tsunami andsubmarine mass movement of unconsolidated Lowe River delta deposits.LAKE TAPAllison Lake valley has the very typical "U" shape of glacier scouring.Sidewalls are extremely steep metagraywacke cliffs with extensive talusdeposits and fans at lake elevation. Lake bottom slope changes indicate talusslopes continuing to the bottom. DH-l, drilled near the original taplocation, went through 33 feet of pourous sandy boulders. Metagraywackebedrock is highly fractured to 70 feet and from 130 feet to 168 feet. A zoneof liqht to moderate fracture extends from 70 feet to 130 feet and from 168feet to the bottom of the hole at 176 feet. Artesian water was encountered atapproximately 110 feet. Shutoff pressure of 50 psi indicate approximatelyG-l