Significant bed elevation changes related to Gulf Stream dynamics on the South Carolina continental shelf

Photographs of the seabed taken from an instrumented bottom tripod located approximately 100 km east of Charleston, South Carolina, reveal bed elevation changes of over 20 cm between July and November 1978. The tripod was in 85 m of water and was equipped with two current meters at 38.7 and 100 cm from the bed, a pressure sensor, a transmissometer, which fouled early during the deployment, a temperature sensor and a camera. The sediment under the tripod was composed of poorly sorted sand, some shell debris and numerous small biological tubes. Bed roughness varied throughout the deployment from biologically-produced mounds (2-5 cm high and 5-20 cm diameter) to streaks to a smooth bed, depending upon the frequency and magnitude of the sediment transporting events. Even though these events were common, especially during the later part of the deployment, the bed was rarely rippled, and there was no evidence of large bedforms such as dunes or sand waves migrating through the field of view of the camera. Photographs did clearly show, however, a gradual net deposition of the bed of nearly 20 cm, followed by erosion of approximately 5 cm. The flow field near the bed was dominated by sub-tidal period currents. Hourly-averaged currents at 100 cm from the bed typically varied between 10 and 30 cm s-1 and occasionally were as high as 60 cm s-1. The large flow events were predominantly toward the southwest along the shelf in the opposite direction of the northeast flowing Gulf Stream. The cross-shore component of the flow near the bed was predominantly directed offshore due to a local topographic steering effect. Current, temperature and satellite data suggest that the largest flow events were associated with the advection of Gulf Stream filaments past the tripod. Erosion events, as seen from the photographs, were highly correlated with the passage of these Gulf Stream filaments past the tripod. Gradual deposition of sediment, which occurred during the first half of the deployment, appears to have been associated with the convergence of the near-bed sediment flux near the shelf break.