18 



SAIC 1987). If a hopper dredge is used, 

 slightly more sediment will be dispersed or 

 suspended. The 3 to 5% of dredged 

 material in suspension will eventually 

 settle or be transported by currents. At 

 90 m water depth, this sediment will settle 

 in at least four hours. 



The increased water entrainment for 

 the dredged material traveling through the 

 water column may also affect mound 

 height. Increased water content in the 

 dredged sediment, either through water 

 entrainment or dredging methods, may 

 alter the form of the dredged material 

 from a peaked mound to a flat deposit. 

 The lateral extent will be the same as a 

 more peaked mound, but the height will be 

 more uniform across the deposit. A 

 pancake-like mound can be more difficult 

 to detect acoustically if the overall height 

 of the mound is less than the resolution of 

 the fathometer. 



While disposal and capping of dredged 

 material in deeper water may require 

 tighter control during the disposal 

 operation, greater disposal depth has an 

 advantage for the stability of the mound. 

 The increased water depth can act as a 

 buffer from wave action. During major 

 storm events, such as Hurricanes David 

 and Gloria, some erosional effects were 

 noted at the Long Island Sound disposal 

 sites on recently completed caps in the 

 early stages of consolidation (Fredette et 

 al. 1992). At the depths found at MBDS, 

 there will be a minimal effect from storm 

 waves (SAIC 1987). 



Once the dredged material mound and 

 cap have been formed in deep water, the 



effectiveness of the cap has to be 

 monitored. As in shallow water, 

 monitoring of the capped mound should 

 verify the thickness and areal extent of the 

 cap and confirm that recolonization by 

 benthic infauna has occurred within 4 to 

 12 weeks after capping (Germano et al. 

 1994). When acoustic bathymetric surveys 

 are used to monitor the capped mound in 

 deeper water, a higher resolution 

 fathometer than is used in shallower water 

 is needed. Small changes in bathymetry, 

 indicating the presence of dredged material 

 or cap, may be missed using acoustic 

 bathymetry if they are smaller than the 

 resolution of the fathometer. The dredged 

 material deposits causing the small 

 bathymetric changes, generally less than 

 20 cm, usually can be detected during a 

 REMOTS® sediment profiling survey 

 where the distinctive character of the 

 dredged material will contrast with the 

 underlying ambient sediment. 



3.1 Deep Water Disposal Operations 



Distinct dredged material mounds have 

 been mapped at five dredged material 

 disposal sites where the water depth is 

 greater than 25 m: Massachusetts Bay 

 Disposal Site (MBDS; 90 m), Elliott Bay 

 (108 m), in Seattle, WA, Port Gardner 

 (132 m), in Everett, WA, Portland 

 Disposal Site (60 m), and Rockland 

 Disposal Site (65-80 m). MBDS, Elliott 

 Bay, and Port Gardner have been proposed 

 as possible locations for the capping of 

 contaminated dredged material. The 

 observation of well-defined dredged 

 material mounds at these locations 

 supports the feasibility of capping at sites 

 ranging as deep as 130 m (Figure 3-2). 



Deep Water Capping 



