Response of a Small-Scale Bottom-Attached Estuarine Plume to Wind and Tidal Dissipation

Abstract

Ebb tide advects low salinity discharges onto the continental shelf where they mix with ambient shelf water. Only a portion of the mixture returns on the flood tide into the estuary. The remainder often forms a low salinity zone detached from the parent estuarine plume. Shipboard and aerial surveys conducted in autumn were used to determine the fate of the estuarine discharge from Charleston Harbor, South Carolina. We used a simple momentum balance to estimate the wind generated alongshelf current. We found that the plume deflection was predicted by the vector addition of the wind generated current and the tidal current operating at the observation times. Thus, the plume is easily deflected downwind since wind stress in shallow water is transferred directly to the bottom. A sequence of cross-plume sections was obtained during a portion of the ebb and flood tidal cycle. Ebb flow carried low salinity estuarine water seaward where it turned abruptly southward as it encountered the southward current that prevails along the shelf in autumn. The front separating estuarine discharge from shelf water was significantly stronger on the upstream side than it was on the downstream side, as indicated by salinity and suspended matter content that sharply distinguished the two water masses. During the flood stage, the upstream front weakened significantly.