Bedform Characteristics And Sand Transport In a Region Of Large Sand Waves, Lower Cook Inlet, Alaska
Authors: Bouma A.H. , Rappeport M.L. , Orlando R.C. , Cacchione D.A. , Drake D.E. , Garrison L.E. , Hampton M.A. 
Published: October 2008

The central part of lower Cook Inlet, Alaska, is covered by marine bedforms of varying size and type. Swift tidal currents sweep the area, and assessment of the effects of these currents on the sandy bottom materials is essential to the industrial development of lower Cook Inlet. As part of the summer 1978 field program, we selected a small area for detailed observations. In addition to high resolution seismic profiling and side-scan sonar, we conducted bottom television/camera observations and vertical current-velocity profiling. Time-series data on bottom currents, bottom pressure, near-bottom suspended matter, and bottom photography were obtained with the Geological Processes Bottom Environmental (GElJPROBE) system during August and September 1978 in the study area.

An anchor station was occupied for 26 hours over 9-m-high sand waves after the site was surveyed in detail. While the vessel was anchored, current-meter profiles and bottom television/camera observations were made. In addition suspended sediment samples were obtained during deployment of the GEOPROBEs.

Surface currents showed a rotary tidal-current pattern with peak velocities of about 100 cm/s. Near the bottom we observed a trimodal current-direction distribution and measured unexpectedly low velocities that were possibly due to obstruction by the bedforms. We observed sand transport along the bottom only during the last hour of both the ebb-tidal and flood-tidal cycles of the spring tide, when velocities exceeded 30 cm/s at 1 m above bottom. The complex bathymetry of the large sand waves, characterized by rapid variations in the elevation of crests, complicates comparison of the anchor station results.

Short-term deployments of GEOPROBEs in July 1978 showed that most of the transport of bottom material, with a mean grain size of fine to medium sand, occurs at threshold current velocities of at least 20 cm/s at 1 m above bottom. The highest transport rate in the study area occurred during ebb tide near the crest of a large sand wave. GEOPROBE bottom protographs show that ripples near the crests of large sand waves reverse orientation from ebb to the next flood. Suspended-sediment samples and light-beam transmission data also show active resuspension during peak tidal currents in both the crest and trough area. During neap tides, however, a negligible amount of bottom sediment moves in the sand-wave trough, despite bottom current velocities of more than 20 cm/so an admixture of silt and organic detritus may inhibit sediment response in the troughs.

INTRODUCTION

Cook Inlet is a large estuary in south-central Alaska that extends from Anchorage to the Pacific Ocean. Tidally dominated currents control the hydrographic regime and may have a significant influence on surficial bottom sediments. Response of the sediments to strong tidal currents was part of the environmental geologic studies conducted by the U.S. Geological Survey during 1976-78 on board the research vessel Sea Sounder. Surveys were conducted utilizing high-resolution seismic reflection profiling and side-scan sonar. Several stations were occupied for bottom sampling, underwater television and camera observations, vertical current-velocity profiles, and deployment of GEOPROBEs (Geological Processes Bottom Environmental system; Cacchione and Drake, 1979).