10 



sufficient thickness and density to contain 

 the contaminants effectively. In order to 

 remain stable, the cap material should be 

 denser than the underlying contaminated 

 material (Shields and Montgomery 1984). 

 The cap must be thick enough to isolate 

 the contaminated sediments from the 

 water column, biota, and erosive forces. 

 In general, the thickness required for a 

 biological seal is greater than for a 

 chemical seal in the absence of biological 

 activity. Results from lab experiments on 

 contaminated dredged material from Long 

 Island Sound have been used to calculate a 

 minimum cap thickness on the order of 

 50 cm (Gunnison et al. 1987, Brannon et 

 al. 1987). To accommodate irregularities 

 in the placement of cap material and in the 

 topography of the dredged material 

 mound, the COE/NED generally 

 recommends a minimum cap thickness of 

 50 cm (T. Fredette pers. comm.). 



can result in uneven coverage when used 

 to place the cap. Hopper dredge 

 pumpdown, sand spray systems, and 

 submerged difmsers are some of the ways 

 proposed in various projects to ensure 

 adequate cap coverage (Shields and 

 Montgomery 1984, Palermo 1991, Sumeri 

 1989). However, these methods are more 

 expensive due to the cost for new 

 equipment and increased time for disposal 

 operations. Therefore, cap placement is 

 also likely to be by disposal barge. 

 Choosing multiple LORAN-C locations for 

 the disposal of cap material on a mound is 

 cost-effective and has been successfully 

 used for several previous projects. 

 Surveying the capped mound by acoustic 

 bathymetry after the cap material has been 

 deposited is critical to monitoring the 

 actual location of the cap material and to 

 verify that management objectives have 

 been achieved. 



Placement control for the capped 

 material will be as important as control for 

 the placement of the contaminated dredged 

 material. To contain contaminated 

 sediments successfully, both the 

 contaminated dredged material and the cap 

 must be placed without excess dispersion 

 and spread. The placement procedures of 

 cap material must insure that the 

 contaminated material mound is covered 

 completely. The DAMOS capping model 

 is used to predict the thickness and lateral 

 extent of the cap based on the amount of 

 material and a random distribution pattern 

 of disposal locations within a predefined 

 radius of operations (Wiley 1994). For 

 the placement of contaminated dredged 

 material, point dumping will maintain 

 control over the mound formation, but it 



Long-term monitoring of capped 

 dredged material mounds within the 

 DAMOS Program has helped to verify the 

 long-term stability of the mounds and the 

 ability of the cap to contain contaminants 

 effectively (SAIC 1989a, Murray et al. 

 1992). Survey techniques that have been 

 used to investigate long-term stability of 

 the cap include: acoustic bathymetry, 

 subbottom profiling, side-scan sonar, and 

 REMOTS® sediment-profile photography. 

 These techniques have been used to 

 document the presence of the cap either 

 through changes in mound height, differing 

 acoustic densities between the mound and 

 the cap, or photographs of the cap 

 material. Comparison of these surveys 

 over time has been used to document any 

 changes in the dimensions of the cap. The 



Deep Water Capping 



