A Description of Summer and Winter Environmental Conditions within Cook Inlet, Alaska

Abstract

Abstract In spite of severe environmental conditions, successful oil exploration, production and shipping have occurred in Cook Inlet for over 30 years. This paper combines the results of an extensive literature search as well as the results of a continuing study of the winter ice conditions in Cook Inlet to describe the summer and winter environment of Cook Inlet. Cook Inlet, located in south-central Alaska, is unique in that it has a very high suspended sediment load, high tides (10 m), a moderate snow fall and an air freezing index of 1400 C days per year. These conditions combine to cause low visibility, extreme tidal currents (8 kts) and unique modes of ice generation within the inlet. The major mode of formation of floating ice is identified as slush ice formation. The slush then coalesces in the turbulent waters to form various ice features. Ice features with depths greater than two meters that formed in less than seven days are described. Ice buildup on shore based structures results in significant downward and upward vertical forces as well as potential horizontal forces. Information summarized in this paper will be useful when analyzing tidal currents and ice forces on offshore and nearshore structures. Information regarding tidal currents and ice conditions within the inlet will also be beneficial in the avoidance of or cleanup of product spills in the inlet. General Description of Cook Inlet Cook Inlet is a large tidal estuary located in Southcentral Alaska at 610 north latitude and 1510 longitude. Cook Inlet extends from the North Pacific Ocean approximately 350 km into Southcentral Alaska. The inlet varies from more than 130 km wide at its southern mouth to less than 15 km at the northern extremity of Knik Arm. The Aleutian and Alaska Mountain Ranges border the inlet to the west and the northwest, the Talkeetna and Chugach Mountains to the northeast, and the Kenai Mountains to the east and southeast. This estuary is divided into three subareas, the head region, upper inlet, and lower inlet. The head region includes Knik and Turnagain arms which are relatively shallow and each approximately 60 km long. The major constricting point, about 25% of the way down the inlet from the north and known as the Forelands, is the boundary between the upper and lower inlet. Water depths in the upper inlet reach 30 to 40 m with a narrow 100 m deep channel through the Forelands. Water depths in the lower inlet reach 150 m. Four major islands exist in Cook Inlet. The largest two are Kalgin Island, located in the northern part of the lower inlet, and Augustine Island, a volcanic edifice in the southwestern corner of lower Cook Inlet. The other two islands, Chisik in the lower inlet on the west, and Fire Island, in the upper inlet, are largely extensions of the mainland. P. 381