Reports
Studies of sediment plumes formed during open water placement of dredged sediments in Area G-West, March, 1994
1994, Halka, J.P., Rasmussen, R., and Ortt, R.A., Jr.
File Reports, Coastal and Estuarine Geology, File Report 1994-04
Introduction
The concentration of suspended sediments in the water column is commonly increased during the open-water placement of dredged sediments. Although most of the released sediment descends rapidly to the bottom, a small portion usually remains suspended in the water column as a turbidity plume. This is often referred to as a primary turbidity plume. These sediments settle to the bottom slowly or are diluted to near ambient levels over time and space. While suspended, the sediments are capable of being transported in the direction of the prevailing currents as they continue to settle through the water. The concentration in the water column and the extent of transport are dependent upon the particle characteristics of the dredged sediment, water column characteristics, the method of dredging and placement, and the current speed and direction.
The dredged sediments which are released can exhibit variations in grain size distribution, density, shape and degree of aggregation. Larger, denser, and rounder particles will have higher settling velocities than those with the opposite characteristics. However, sediments with a very fine grain size distribution tend to aggregate into groups with a resulting increase in settling velocity. Although particle aggregation in the estuarine environment is a natural process that commonly occurs, the degree of aggregation can be strongly influenced by the method of dredging and placement utilized. Hydraulic dredging results in a greater degree of disaggregation than does clamshell dredging and placement from bottom release scows. The density and viscosity of the receiving water, which are primarily controlled by temperature and salinity, effects the settling velocity of suspended particles. Pycnoclines, across which there is a density difference, may be present in the water column. The pycnocline depth may change over both shorter and longer term time scales as the tides ebb and flood, and the fluvial input of fresh water to the system changes. Turbulence, which develops in the water column as a result of relatively high current velocities, serves to retain particles in suspension by maintaining an upward flow component within the overall tidal flow. As a result of the spatial and temporal variations that are likely to occur in the above factors, both the magnitude of the increase in suspended sediment concentration and the transport direction and distance are likely to be highly variable depending on local conditions at the time of placement.
Primary turbidity plumes associated with the open-water placement of dredged sediments have been routinely observed in the Chesapeake Bay. In an early study, Biggs (1970) identified a plume of suspended sediment extending 3,600 to 5,500 meters from a hydraulic discharge point. In a number of subsequent studies of open-water placement operations (Cronin et al., 1981; Cronin, Taylor and Mallonee, 1981; Zoltan and Halka, 1984; Blakeslee and Halka, 1985; Panageotou and Halka, 1988, 1990; Halka et al., 1994), the longest plume identified was found to extend just over 3000 meters (Halka et al., 1994; Table I). The plume extent and direction in all studies was found to be directly related to the speed and direction of the prevailing tidal currents. In general, greater current speeds resulted in a longer plume with slack water producing a localized plume.
Most of the above studies concerned sediments hydraulically dredged and discharged from a pipeline which has been the most commonly utilized method of channel maintenance in the northern Chesapeake Bay. Plumes generated by placement from bottom release scows have been examined during only three dredging operations in this area. In a study which included tracking two plumes at the mouth of the Patapsco River, Zoltan and Halka (1984) found limited horizontal movement with placement in shallow water. One plume, tracked during slack water in Area G-South in 1984, extended a maximum distance of 200 meters from the placement location (Blakeslee and Halka, 1985). The most comprehensive study of plumes generated from bottom release scows was conducted in Areas G-Central and G-South in the winter of 1991-1992 (Halka et al., 1994). A total of 14 plumes were observed in that study. These extended an average distance of 560 meters from the placement location. The longest plume was generated from placement in G-South and extended 1300 meters (Halka et al., 1994; Table I).
Within the plumes the suspended sediment concentration is higher than in the surrounding ambient water; and, in general, decreases with distance from the discharge location. In the studies identified above, the highest concentrations of suspended sediment that resulted from hydraulic discharge operations were in the range of 1,000 to 4,000 mg/l within 150 meters of the discharge point. Concentrations in excess of 100 mg/l were located within 500 meters, while at the distal end of the plumes (> 500 meters) concentrations were largely in the range of 25-50 mg/I. During placement from bottom release scows, the extent of the plumes was much less than that observed from hydraulic dredging operations. This results from the fact that the scow releases the sediment over a very short time frame < 1 minute), while hydraulic discharge is continuous. Blakeslee and Halka (1985) identified concentrations in the range of 50-60 mg/l within 200 m of a bottom placement location. The concentration of suspended sediment returned to ambient levels in 20 to 40 minutes following the release from the scow. Michael and others (1991), in a study of water quality characteristics in the vicinity of four bottom placement operations, identified a concentration in excess of 1000 mg/l resulting from one operation, while at the others the maximum concentration was < 300 mg/I. The maximum concentration observed in the study of bottom placement in G-South in 1992 was 325 mg/l (Halka et al., 1994).
During the winter of 1994 maintenance dredging operations were conducted in the approach channel to the Chesapeake and Delaware Canal. Sediment was removed by clamshell dredge and placed in bottom release scows which were towed to Area G-West (Figure 1). The sediments were released in such a manner as to create a berm across the southern portion of the placement area (Maryland Environmental Service, n.d.). Placement operations occurred on January 14 and 15, and over the period from February 21 through March 26, 1994. The objective of the study effort reported herein was to document the location of elevated concentrations of suspended sediment in the water column that resulted from selected placement events and to determine the extent of transport under a variety of conditions.
