Sediment transport measurements on the mid-continental shelf in Onslow Bay, NC

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Title: Sediment transport measurements on the mid-continental shelf in Onslow Bay, NC
Author: Wren, Patricia Ansley
Advisors: Dr. Elana Leithold, Committee Co-Chair
Dr. Lynn A. Leonard, Committee Co-Chair
Abstract: Long-term in situ measurements have been made on the mid-continental shelf in Onslow Bay, NC to determine the frequency and direction of bottom sediment movement and the processes responsible for bottom sediment motion. As part of the Coastal Ocean Research and Monitoring Program (CORMP) at the University of North Carolina at Wilmington, a quadrapod frame with a downward looking Pulse-Coherent Acoustic Doppler Profiler (PC-ADP) and an upward looking Acoustic Doppler Current Profiler (ADCP) has been maintained on the continental shelf at approximately 30 m depth since May of 2000. The instruments are moored 27 nautical miles off the coast of Wilmington, North Carolina in Onslow Bay, adjacent to a productive marine hardbottom. Simultaneous measurements of flow velocities from the surface to the seabed, along with acoustic backscatter measurements, have been obtained. Measurements of seabed elevation, temperature, conductivity, and pressure were also collected at the site. Bed stresses due to wave-current interactions are calculated using a bottom boundary layer model (Styles and Glenn, 2002). Wave-current interactions resulted in shear stresses at the sediment-water interface that exceeded the critical threshold for sediment movement over 50% of the time during a climatological average year. Sediment transport was been shown to occur during four different types of events at the site: 1) small to moderate northerly wind events 2) Gulf Stream Intrusion events 3) strong southerly wind events associated with the passage of frontal systems 4) the passage of tropical storm systems. Over the course of a year the total net suspended sediment flux at 1 mab was in the positive along-shelf direction (southwest) and in the negative across-shelf direction (onshore), where there was several times more net transport in the onshore direction than the along-shelf direction. Three mechanisms leading to significant sediment transport on the mid-continental shelf have been indentified: 1) wave-current interactions 2) subtidal currents associated with sustained wind-driven flows and the intrusion of Gulf Stream water on the shelf 3) infragravity waves. Subtidal currents played a key role in the sediment transport during all types of events. Wind driven subtidal currents were important in determining the magnitude and direction of sediment transport during storm events, while subtidal currents associated with a Gulf Stream intrusion event combined with fair-weather swell resulted in accretion at the site of 3 cm over a three week period. The results show that a moderate northeasterly wind event with sufficient duration to generate wind driven subtidal flows resulted in an order of magnitude more sediment transport (20,237 g cm⁻²) than a similar northerly wind event and the southerly wind event without developed wind–driven flows. These two events resulted in similar amounts of sediment transport of 3,600 g cm⁻² and 3,061 g cm⁻², respectively. The passage of a hurricane resulted in an order of magnitude more sediment transport than the moderate northeasterly wind event, on the order of 240,000 g cm⁻². Shear velocities during the passage of Hurricane Isabel resulted in the largest bed shear stresses (~300 dynes cm⁻²) on record. Field measurements in the bottom boundary layer were compared with the bblm generated current profiles during moderate wind events and Hurricane Isabel. In addition, the suspended sediment concentration profiles from the model were compared with the ABS profile measurements to verify shape and magnitude as the storms increased and waned. In general, there was good agreement between the measured and model derived current profiles, and between suspended sediment measurements and the model concentration profiles for both large and small-scale events that occurred at the site.
Date: 2004-03-15
Degree: PhD
Discipline: Marine, Earth and Atmospheric Sciences

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