4.0 DISCUSSION 



4.1 Cape Cod Canal Disposal Site 



The Cape Cod Canal serves as a major thoroughfare for recreational and 

 commercial vessels transiting Massachusetts waters. During the maintenance and 

 improvement dredging conducted in the spring of 1990, sediments were disposed at 

 CCDS and a near-shore site off Springhill Beach. The objective of the pre- and 

 postdisposal bathymetric surveys was to delineate the areal extent of these disposed 

 sediments. 



The designated disposal point within CCDS was located approximately 600 m 

 west of the CCDS center. Previous disposal records show that 228,735 and 4590 m 3 

 were disposed north of the CCDS center in 1980 and 1986, respectively. The 

 objectives of the 1990 REMOTS® sediment-profile photographic survey were to 

 assess the areal extent of dredged materials and the colonization status north of 

 CCDS center at the historical CCDS mound. 



The disposal logs indicated that 7,837 m 3 of sediments were deposited at 

 CCDS during the two days prior to the predisposal bathymetric survey. There was 

 no evidence of any obvious topographic anomalies due to this disposal activities 

 (Figure 3-1). A conspicuous shoaling was observed in the postdisposal contour plot 

 around the "CCD" buoy (Figure 3-2). The depth difference plot, reflecting changes in 

 depth attributable to dredged materials released after the 22 March survey, revealed 

 several individual mounds within this shoaled area with dredged material 

 thicknesses up to 0.7 m. 



The calculated volume of accumulated material for the CCDS was 21,823 m 3 

 with 95% confidence limits of 10,739 and 32,908 m 3 . The volume calculation was 

 based on changes in depth observed between lanes 16 and 35 of the bathymetric 

 survey grid, where there was evidence of the newly-deposited material. The disposal 

 log estimate of 15,296 m 3 fell within the calculated confidence limits; however, 

 unlike previous DAMOS surveys (e.g., Western and Central Long Island Sound 

 (WLIS, CLIS) and New London Disposal Sites (NLON); SAIC 1990c, 1990a, and 1990b, 

 respectively), this estimate was less than the calculated volume. Tavolaro (1984) 

 showed that "depth difference" volume estimates based on successive bathymetric 

 surveys wall be less than barge log estimates because of compaction of dredged 

 material on the seafloor following disposal. Significant consolidation of the CCDS 

 sediments may not have occurred prior to the 24 April survey because all 15,296 m 3 

 of sediment were deposited within one month of the survey. Volume difference 

 calculations for CLIS, WLIS, and NLON were based on postdisposal surveys 

 conducted up to six months after initial sediment disposal, allowing significantly 

 more time during which compaction could occur. Until a comprehensive mass 

 balance study can be performed and methods are developed to measure barge 

 volumes easily and accurately, it will be difficult to eliminate discrepancies between 



Monitoring Cruise at the Cape Cod Canal Disposal Site and Springhill Beach Site 



