20 



material which was placed between the 

 two periods of cap deposition. 



2.2.2 STNH Operations: Conclusions 



Several observations and 

 recommendations were made as a result of 

 the STNH project (Morton 1980a): 



• The cohesion of the mound material 

 was an important factor in reducing 

 the spatial distribution of the 

 mound sediments. 



• Point-dumping, using a taut- wired 

 buoy, was an effective method of 

 dredged material placement. 



• Capping should take place as soon 

 as possible after mound deposition. 



• In addition to cap disposal at the 

 buoy, a portion (at least 1/3) of the 

 capping material should be disposed 

 along the radius of the mound 

 material to ensure complete 

 coverage. 



• Bathymetric monitoring during 

 mound and cap disposal was an 

 effective tool for modifying 

 program design during disposal 

 operations. 



2.3 Norwalk 



Following the operational success of 

 the Stamford-New Haven project, NED 

 next used capping as part of maintenance 

 dredging of Norwalk Harbor, Connecticut, 

 in 1980-81 (Figure 2-4). The proposed 



project called for clamshell dredging of 

 approximately 230,000 m 3 of sediment 

 from the navigation channel, with open- 

 water disposal planned for the CLIS 

 Disposal Site (US ACE 1979). 



Sampling surveys showed the material 

 from Norwalk Harbor to be primarily fine- 

 grained silts and sands. Sediments from 

 the harbor entrance to Fitch Point Light 

 (Figure 2-4) were classified as 

 predominantly low to moderately 

 contaminated material based on their 

 physical characteristics (Tables 2-2 and 2- 

 3). These sediments contained low 

 concentrations of all the NERBC 

 contaminants with the exception of Hg, 

 which was found in elevated 

 concentrations even at the more southerly 

 sampling stations. 



North of Fitch Point Light, 

 contaminant concentrations increased, 

 particularly for Hg and Pb. The bulk of 

 sediments to be dredged from this area 

 were moderately contaminated material 

 (Tables 2-1 and 2-2). In addition to the 

 NERBC range of contaminants, 

 concentrations of nitrobenzene and 

 naphthalene (EPA 1977) sufficient to make 

 the associated sediments unsuitable for 

 open-water disposal without further testing 

 were found in a small area along the 

 western edge of the channel near the 1-95 

 roadway bridge. 



To accommodate the range and 

 character of contamination found in 

 Norwalk Harbor, NED proposed a 

 multifaceted dredging program. Dredging 

 would be phased so that the highly 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



