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4.0 DISCUSSION 



Open water disposal of contaminated 

 dredged material followed by "capping" 

 with cleaner dredged materials has been 

 employed successfully in water depths 

 ranging from approximately 20 to 60 m. 

 Proposals to extend the depth of capped 

 disposal operations up to about 150 m have 

 raised several concerns, although there is 

 no evidence that capping cannot be 

 accomplished at these depths. In fact, all 

 the available theory and empirical evidence 

 supports its feasibility. Successful capping 

 of contaminated dredged material requires 

 the disposal of dredged material in a 

 discrete mound without extensive spreading 

 or dispersal into the water column. The 

 cap material must then be placed accurately 

 onto the mound without disturbing the 

 contaminants. There is concern that the 

 increased water depth will contribute to 

 wider dispersal and spreading of 

 contaminated material and poor control 

 over cap placement. These concerns focus 

 on the apparent lack of knowledge of 

 dredged material behavior in deeper water 

 and the 1982 attempt to form a dredged 

 material mound at MBDS which failed to 

 produce an acoustically discernable mound. 



The behavior of dredged material as it 

 descends through the water column was 

 discussed earlier. For the capped mounds 

 in Long Island Sound, a barge load of 

 dredged material reaches the seafloor while 

 in the convective descent phase and then 

 undergoes dynamic collapse and passive 

 dispersion. The effect of increasing water 

 depth on the descent of the dredged 

 material was investigated by modeling the 

 behavior of a 4000 m 3 barge load of 



dredged material as it descended through 

 water depths ranging from 377 m to 914 m 

 (Stoddard et al. 1985). The model results 

 indicated that dredged material should 

 reach neutral buoyancy and go from 

 convective descent to dynamic collapse 

 between 340 and 390 m. Therefore, 

 dredged material deposited at MBDS 

 (90 m), Elliott Bay (132 m), and Port 

 Gardner (108 m) should behave the same as 

 dredged material deposited in Long Island 

 Sound, reaching the seafloor during 

 convective descent without achieving 

 neutral buoyancy. 



The height and lateral extent of a 

 mound that would be formed by dredged 

 material disposal was modeled for MBDS 

 and for the Port Gardner Disposal Site. 

 For the approximately 836,148 m 3 of silty 

 dredged material deposited at MBDS from 

 1987 to 1992, the DAMOS capping model 

 predicted a mound height of 4.22 m and a 

 radius of 600 m. At Port Gardner, the 

 model incorporated a 10 cm-s" 1 NW/SE 

 bottom current and predicted a dredged 

 material footprint of 2000 m radius and 

 3.19 m mound height for the 762,000 m 3 

 of dredged material released within a 

 183 m radius target zone. 



Investigations of dredged material 

 disposal at MBDS, Port Gardner, and 

 Elliott Bay by bathymetry and/or 

 REMOTS® surveys determined that the 

 dredged material at these sites had 

 mounding characteristics very similar to 

 those predicted by the model. Where both 

 REMOTS® and bathymetric surveys were 

 conducted, the REMOTS® survey, due to 

 its finer resolution, mapped a larger areal 

 extent for the deposit. At MBDS, the 



Deep Water Capping 



