THE EFFECTS OF HARDBOTTOM GEOMETRY ON SEDIMENT TRANSPORT PROCESSES ON THE MID-CONTINENTAL SHELF IN ONSLOW BAY, NORTH CAROLINA

Onslow Bay is a high-energy, sediment-starved shelf characterized by extensive areas of exposed hardgrounds varying in lithology and relief. These hardgrounds have been shown to be of economic importance due to the productive benthic habitats in which they support. Annual variations in sediment thickness and distribution on the hardbottom surfaces have been shown to have profound effects on these ecological communities. The present study compares variability of mid-shelf sediment dynamics around a productive marine hardbottom with vertical relief of ~1 m to a nearby area characterized by a broad flat bottom and consisting mainly of fine to coarse grained surficial sands. The effects of the reef geometry on the sediment transport processes around this hardbottom area are examined by utilizing two long-term data sets containing current velocity profiles from the sea surface to the seabed, acoustic backscatter profiles, and seabed elevation data at the two sites. Current velocity profiles measured above and below the elevation of the reef ledge were compared at both sites during sediment transport events that exhibited similar current magnitude and direction in order to elucidate any effects that the hardbottom reef may exert on the hydrodynamics. In addition, shear velocities were calculated using a 1-D bottom boundary layer model (Styles and Glenn, 2000) and sediment transport direction and magnitude of the fine sands in the vicinity of the hardbottom were determined at each site for each event. Preliminary analyses indicate that a “shadow effect” does occur at the site closest to the hardbottom ledge when currents are from the north, or passing over the hardbottom surface.