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4.0 DISCUSSION 



The series of bathymetric surveys conducted at CSDS between August 1991 and May 

 1992, the current and suspended sediment information from bottom and mid- water moorings, 

 and the REMOTS® survey, all provided circumstantial evidence for active bed transport at 

 the site. The depth-difference comparisons between consecutive bathymetric surveys 

 provided some indication of shifts in sediment accumulation due to both the release of 

 dredged material at different locations as well as transport by bottom currents. 



Between the August and October surveys, barges released dredged material around 

 buoy A (Figure 3-4). A depth difference plot of these two surveys showed some 

 accumulation up to 200 m east and south of the disposal buoy (Figure 3-3). This is 

 consistent with the barge disposal pattern. Large areas of accumulation were also seen more 

 than 200 m south of A and north and west of A. Each of these latter areas extended 400 m 

 or more east/ west. The source of this accumulation could be the material released by the 

 barges. Background levels of suspended sediments increased during dredged material 

 disposal as the material accumulated on the seafloor and was subjected to tidal resuspension 

 and winnowing. The increased background levels of suspended sediment during September, 

 if they indicated that the material has been resuspended and is then being moved (active bed 

 transport), could explain the accumulation pattern seen in Figure 3-3. Variations in the 

 predominant current flow between BTM-A and BTM-B (Figure 3-10) may be due to 

 variations in local topography (Bohlen et al. 1992). Similar modifications to the bottom 

 current by local topography may cause the slight southwest/northeast trend in the 

 accumulation pattern south of buoy A. 



From October to December, only 13,891 m 3 of fine-grained material from the 

 dredging of North Cove was released at CSDS. The depth-difference between the October 

 and December surveys shows no accumulation at the disposal location (Figure 3-6). It does, 

 however, show a loss of material at the southern edge of the survey, and an accumulation 

 running east/ west just south of A. The apparent displacement of this material along the east- 

 west axis is again consistent with the dominance of westerly currents. 



Despite evidence for active sediment transport at CSDS, material did accumulate at 

 the B disposal location between October and May (Figure 3-9). The accumulation of 

 dredged material at buoy B may be due to the amount and type of dredged material released 

 at CSDS. More than twice as much material was disposed at location B (103,375 m 3 ) than at 

 location A (50,803 m 3 ). The dredged materials released at B were fine-grained, cohesive 

 sediments that had been mechanically dredged. Cohesive sediments dredged in this fashion 

 are likely to be deposited at the disposal site as blocks or clumps of material that will reach 

 the seafloor rapidly with little or no dispersion (Bokuniewicz 1989). By contrast, the sandy 

 material dredged from the Connecticut River Bars was hydraulically dredged, which resulted 

 in noncohesive material that was easily resuspended. Suspended sediment measurements, 



Synthesis of Monitoring Surveys at the Cornfield Shoals Disposal Site, July 1991 to May 1992 



