46 



An estimated volume of 556,000 m 3 of capping dredged material (CDM), originating 

 from the areas of the Thames River channel that had been classified as suitable for 

 unconfmed open-water disposal (Maguire Group 1995), was dredged and placed over the 

 UDM (Appendix A). The resulting ratio of CDM to UDM was 1.82:1. CDM placed at the 

 NDA 95 buoy was not included in this ratio (8,280 m 3 disposed after the precap survey and 

 another 4,900 m 3 after the postcap survey), but this material provided additional cap 

 coverage. Gahagan and Bryant, Inc. conducted a postcap survey in late February 1996 

 (Figure 3-4). The overall depth difference between the baseline and postcap survey data was 

 calculated to show the distribution and thickness of the entire deposit placed during the 

 Seawolf project. The resulting Seawolf Mound was a flat semi-circular deposit with a 

 diameter of approximately 600 m, with initial peak heights of 3-4 m above the pre-existing 

 seafloor (Figure 3-5). The deposit was elongated down slope (to the southwest) and extended 

 onto the margin of the NL-RELIC Mound to the east (Figure 3-5). 



The overall apparent thickness of CDM was determined by calculating a depth 

 difference between the precap (December 1995) and postcap (February 1996) surveys. The 

 resulting contour plot indicates that, in the center of the mound above the thickest areas of 

 UDM, the total mound height did not change, which would indicate substantial consolidation 

 of the underlying dredged material due to the placement of CDM (Figure 3-6). This central 

 area of apparent consolidation was analyzed further in the REMOTS® and core data collected 

 in that area (Sections 3.1.2 and 3.1.3 and Appendix D). Outside of these areas of 

 consolidation, apparent cap thickness over the UDM deposit reached up to 3 m or more, 

 assuming some consolidation of UDM everywhere. There was an isolated area of UDM in the 

 farthest eastern edge of the mound, which did not show coverage with CDM detectable by 

 acoustic bathymetry. However, REMOTS® photos did show presence of cap materials 

 (REMOTS® Station 300E), which is discussed in Section 3.1.2.1. 



The September 1997 bathymetric survey, conducted 18 months following cap 

 placement showed that the mound had a broad, flat plateau ranging from 16 to 18 m water 

 depth, with two small peaks at the apex of the mound to the west of the Navy buoy (Figure 

 3-7). The Seawolf Mound was a few meters lower than the NL-Relic Mound to the east. In 

 the September 1997 (18 months post cap) versus October 1995 (baseline) depth difference 

 plot, the overall footprint of the Seawolf Mound was similar to that observed the previous 

 year (February 1996 versus October 1995 depth difference; Figure 3-8). However, there 

 were some changes in mound topography in the intervening 1.5 years, shown by a close 

 comparison of Figures 3-5 and 3-8. In February 1996, there were two distinct peaks located 

 just west of the Navy buoy, with the taller peak having a height above baseline of 4.25 m 

 (Figure 3-5). In 1997, the two peaks were less distinct and the maximum height above 

 baseline was 3.5 m (Figure 3-8). Two peaks located further to the west also were no longer 

 as prominent in 1997 (Figures 3-5 and 3-8). A depth difference plot between the 



Monitoring Cruise at the New London Disposal Site, Seawolf Mound 1995 - 1998 



