Movement Of Sand Waves In Lower Cook Inlet, Alaska

Abstract

ABSTRACT Geologic environmental studies in lower Cook Inlet Alaska, indicate that in addition to earthquake accelerations, nuees ardentes from volcanic eruptions, and shallow gas, the only other significant potential geologic hazard is that associated with sand motion and migration of bed forms found in the central part of this area. Four major size groups of bed forms have been observed:small. asymmetric ripples normally less than 30 cm long with wave heights of about 5 to 7 cm,small sand waves with wave lengths less than a few tens of meters and wave heights less than a few meters,medium-size sand waves and small dunes with wave lengths ranging from 50 to 150 m and wave heights less than 5 m, andlarge sand waves, dunes, and sand ridges with wave lengths ranging from 350 to 1000 m and wave heights from 5 to 10 m. Several size classes typically are superimposed on each other. Sand grains either rollover the bottom or form sheet flows, the latter moving as fast as 30 cm/s over the crests of the larger bed forms. High-resolution seismic profiling and side-scan sonar with accurate Mini-Ranger positioning were used to map two areas in detail. Underwater television, bottom photography, and surficial sediment sampling gave additional information. Maintaining station position is often difficult due to the high tidal currents with peak velocities ranging from 3 to 5 nots. Scanning electron microscopic examinations of surface textures of quartz grains imply that there is no substantive net transport of grains. Drill cuttings, released during a period of 3 months, were incorporated to a depth of at least l3.5 cm. However, concentrations at 5.5 and 13 cm depth suggest transport by small ripples over small sand waves. Although more transport may occur during heavy runoff and storm periods, the present data-indicate substantial transport of sand over large bed forms at most times and possible additional seasonal mass movement over smaller ones. Understanding stability and migration of bed forms is a high priority for scientific and industrial interests. INTRODUCTION Cook Inlet is a large tidal inlet located in south central Alaska (Fig. 1), extending from Anchorage to the Pacific Ocean. Tidal currents are the main agents of mass water movement up and down the inlet. The influence of the currents on the bottom was one subject of an environmental geology study conducted during the summers of 1976 and 1977 on board the U.S. Geological Survey R/V SEA SOUNDER. High-resolution seismic profiling, side-scan sonar surveys, surficial sediment sampling, underwater television, and bottom photographs were made in lower Cook Inlet (Bouma and Hampton, 1976; Hampton and Bouma, 1976; Bouma and others, 1977a, 1978). The central part of lower Cook Inlet is characterized by a variety of bed form types. Our studies concentrated on movements of unconsolidated noncohesive grains as individual particles and on the influence such motion has on migration of bedforms. Two smaller areas were selected for detailed studies, one near the D/V OCEAN RANGER, and the other off Anchor Point (Fig. 2).