2.0 OPEN WATER DISPOSAL AND 

 CAPPING OF DREDGED 

 MATERIAL 



The increased use of open water sites 

 for confined aquatic disposal, or capping, 

 of contaminated dredged material is due to 

 a decrease in the availability of upland or 

 wetland areas for the disposal of dredged 

 material, associated costs, and concerns 

 over disposal of contaminated material 

 near freshwater aquifers. Successful 

 disposal of contaminated dredged material 

 at open water sites requires formation of a 

 distinct dredged material mound, careful 

 placement of capping materials, and 

 concurrent bathymetric and environmental 

 monitoring to ensure that the operation is 

 successful initially and effective over the 

 long term. This has been accomplished at 

 several locations within the CLIS Disposal 

 Site (e.g., Morton 1979, Morton and 

 Miller 1980, SAIC 1984b, Murray et al. 

 1992). 



The formation of a dredged material 

 mound requires good navigational control 

 during disposal operations and a disposal 

 method that contributes to the formation of 

 a mound. There is a wealth of experience 

 in the DAMOS Program to demonstrate 

 that point dumping of the dredged material 

 using LORAN-C coordinates and a taut- 

 wired buoy will provide accurate 

 placement of the dredged material (e.g., 

 SAIC 1990a, 1991, Murray et al. 1992). 

 Point dumping requires that the disposal 

 barge pull up close to the buoy and slow 

 or stop before opening the barge doors 

 rather than opening the doors underway. 

 The taut-wired buoy design incorporates a 

 hang weight between the anchor and the 



surface buoy. This hang weight keeps the 

 wire vertical between it and the surface, 

 reducing the watch circle of the buoy at 

 the surface. When accurate navigation and 

 a taut-wired buoy are used, the onboard 

 inspection/control must also guarantee that 

 the instructions are followed. 



Dredging and disposal of subtidal 

 sediments are accomplished with either a 

 hopper dredge for dredging and disposal or 

 a clamshell dredge with barge disposal. 

 The majority of dredging projects in New 

 England are accomplished with a clamshell 

 dredge. During dredging, a hopper dredge 

 entrains water and breaks down the 

 cohesiveness of the sediments. If 

 sediments are dredged with a clamshell 

 dredge, however, the sediments will 

 maintain most of their cohesiveness. 

 Therefore, the combination of clamshell 

 dredging and a split-hull or pocket barge is 

 the most efficient method to form a 

 mound. This method keeps the dredged 

 sediment's water content at a minimum 

 and helps control the dispersion of dredged 

 material following release. 



Field data has indicated that 80% of 

 the dredged material released from a 

 stationary barge and detectable by acoustic 

 methods should be deposited within a 30 m 

 radius of the release point in water depths 

 < 50 m. A total of 90% of the material 

 detectable by acoustic methods will settle 

 within a 120 m radius under most 

 conditions (Bokuniewicz et al. 1975). 



Once a stable dredged material mound 

 has formed in deep water, it can be 

 capped. To isolate contaminated dredged 

 material, a sediment cap must be of 



Deep Water Capping 



