Reports
Resuspension studies conducted on dredged sediments placed in Area G-South
1994, Halka, J.P., Sanford, L.P., and Ortt, R.A., Jr.
File Reports, Coastal and Estuarine Geology, File Report 1994-03
Introduction
Comprehensive studies of the placement of dredged sediment at the Pooles Island overboard disposal sites were necessary to ensure that dredging operations maintained and potentially improved habitat features while assuring no occurrence of adverse impacts elsewhere in the Bay. Given a better understanding of the effects of overboard placement of dredged sediments, alternatives which may be considered for the future disposal needs can be better evaluated. This report documents the results of resuspension studies which estimated erosion and suspended sediment transport at disposal Area G-South (Figure I). The sediment was derived from the dredging of the Tolchester Channel by the Great Lakes Dredge and Dock Company under contract to the U.S. Army, Corps of Engineers, Baltimore District. The sediment was removed from the channel by clamshell bucket dredge, transported to, and placed in the designated areas from bottom release scows over a period between November 27, 1991 and March 27, 1992. A portion of the sediment was placed in designated Area G-Central with the majority placed in G-South (Panageotou and Halka, 1994). The resuspension studies were limited to the area in and around G-South.
Resuspension is significant for two reasons when attempting to understand the effects of dredged sediment disposal. First, resuspension is the most effective mechanism for dispersal of the sediments following deposition. Secondly, resuspension injects any particulate associated contaminants and dissolved contaminants which may be present back into the water column, directly affecting water quality. Depending upon the current speed and direction in the overlying water column and the settling characteristics of the resuspended material, the sediments may be transported varying distances from the original site of deposition. Thus resuspension is likely to result in an increase in the spatial extent of the dredged sediments within the area, and may result in transport away from the disposal sites into surrounding areas.
The potential for resuspension depends upon both the erosive stress exerted by tidal and wind-wave generated currents, as described in the next section, and the resistance of the surficial sediments to erosion. Resistance to erosion depends upon, among other things, the size of the individual sediment grains and the water content of the bed. Newly deposited sediments should exhibit the least resistance to erosion, but the resistance should increase over time as the sediments consolidate and the water content decreases. The rate at which resistance to erosion increases in deposited dredged sediments under natural conditions is, at present, unknown.
The objectives of this project were to document changes in resuspension of deposited sediments both immediately after disposal and over time as the sediment consolidated, under varying fair and storm weather conditions, and to document differences between resuspension of the deposited sediments and resuspension of the naturally occurring bottom sediments in the region, and the resultant suspended sediment levels.
