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

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. 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. 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. 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. (Abstract shortened by UMI.)...