Reports
Turbidity plumes formed during fossil oyster shell dredging in the northern Chesapeake Bay, August 1998
1999, Wikel, G., Panageotou, W., and Halka, J.P.
File Reports, Coastal and Estuarine Geology, File Report 1999-01_Wikel
Introduction
Increased concentrations of suspended sediment have been reported to occur in the vicinity of relict oyster shell dredging in the northern Chesapeake Bay. Areas in which these elevated concentrations exist are called turbidity plumes. Previous studies, primarily investigating turbidity plumes formed during dredged sediment placement operations, found that the majority of sediment suspended in the water column descends quickly to the bottom. The remaining suspended sediment is capable of being transported in the direction of the prevailing currents as it continues to settle through the water. Suspended sediment concentrations are diluted to near ambient levels over variable distance and time. The extent of transport and magnitude of suspended sediment concentrations have not been well documented for plumes produced under various tidal conditions during oyster shell dredging activity.
The extent of transport and concentrations of suspended sediment within turbidity plumes are dependent upon a variety of factors, including sediment particle characteristics (e.g. size, shape, density, degree of aggregation), water column characteristics (e.g. turbulence, density, vertical structure), and current speed and direction. Consequently, the magnitudes of increase in transport distance and suspended sediment concentration are likely to be highly variable, depending on temporally and spatially variable sediment and estuarine characteristics.
The importance of shell dredging to the oyster replenishment program and overall preservation of critical habitat in the northern Chesapeake Bay necessitates this study. A sound understanding of plume characteristics can lead to informed management decisions concerning future dredge activity.
In August of 1998, C.J. Langenfelder & Son, Inc. hydraulically dredged relict oyster shell bars in the region east of Pooles Island and the Chesapeake & Delaware (C&D) Canal approach channel (Figure 1). The purpose of dredging was to collect cultch material for setting of oyster spat. Local suspended sediment concentrations increased due to 1) near-bottom sediment and shell agitation, cutting, and excavation and 2) near-surface hydraulic discharge of sediment separated from shell material. The objective of this study was to determine the degree of horizontal and vertical transport under various tidal conditions and document the location and magnitude of enhanced concentrations of suspended sediment in the water column.
