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3.0 RESULTS 



3.1 Bathymetry 



All four bathymetric surveys at CSDS (August 1991, October 1991, December 1991, 

 and May 1992) show a seafloor that slopes from north to south, and a northeast/southwest 

 trending trough in the southern half of the survey area. All disposal locations were located 

 on the northern slope. 



The August 1991 bathymetric survey (Figure 3-1) was conducted prior to the 

 1991/1992 disposal season. There was no evidence in the bathymetric plot of any previous 

 disposal mound at the site. The next bathymetric survey was conducted on October 18 and 

 19, 1991 after about 53,800 m 3 of sandy/silty dredged material was released at buoy A 

 (Figure 3-2). Again there was no distinct mound on the contour plots, but the depth at the 

 disposal location decreased from approximately 54 m to 53.5 m. 



A depth-difference plot, showing relative changes between the August 1991 and the 

 October 1991 surveys showed up to 1 m of material accumulated over wide areas northwest 

 and south of A. Between these areas of accumulation there were losses of up to 1 m of 

 material (Figure 3-3). 



After the October 1991 bathymetric survey the disposal buoy was relocated to position 

 B. Fine-grained dredged material was removed from North Cove and released at buoy B 

 starting November 13. Between November 13 and December 10, 1991, 13,891 m 3 of 

 material was released at CSDS. When the survey was conducted (December 10), the 

 disposal buoy was located west of its initial location. However, positions noted on the barge 

 logs indicated that the barges continued to release material at the original (intended) B 

 location (Figure 3-4). The December 1991 bathymetric survey did not indicate any well- 

 defined accumulation of material at buoy location B (Figure 3-5). 



A depth-difference comparison of the December and October, 1991 bathymetric 

 surveys did not indicate any accumulation at buoy B (Figure 3-6). Instead, material 

 accumulated to the west of B, and approximately 100 m south of buoy A. A loss of material 

 appeared further south, the same area where material accumulated between August and 

 October (Figure 3-3). 



Between December and May, about 91,588 m 3 of fine-grained material was released 

 at buoy B. The May 1992 bathymetric survey shows the same north-to-south sloping 

 seafloor with a northeast/ southwest trending trough in the southern half of the study area 

 (Figure 3-7). In contrast to the 1991 surveys, the May data show a decrease in depth just 

 north of the B buoy where the contours bend to the south. A depth-difference comparison 



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



