36 



Particular reliance has been placed on 

 acoustic profiling techniques. Since the 

 beginning of the DAMOS Program in 

 1977, high resolution bathymetric 

 observations have been included in all 

 surveys. Survey systems and associated 

 high accuracy electronic navigational 

 techniques have evolved progressively, 

 providing increased accuracy in ship 

 positioning, depth measurements, and 

 associated sediment volume calculations. 



The bathymetric profiling system used 

 during the 1979 Stamford-New Haven 

 project provided resolution sufficient to 

 detail changes in water depth of 

 approximately 20 cm (Morton 1980b, 

 1983b). By 1987, incorporation of higher 

 frequency, narrower beam- width systems 

 and improved, computer-based data 

 processing procedures had improved 

 system resolution by about 10 cm. The 

 associated sediment volume estimates were 

 further improved by the inclusion of the 

 REMOTS® sediment-profiling camera 

 system (Rhoads and Germano 1982) in 

 many of the monitoring surveys conducted 

 after 1982 (Appendix C). This system, 

 with a vertical resolution of millimeters, 

 permitted accurate mapping of sediment 

 distributions along the flanks of the capped 

 mound, an area difficult to resolve using 

 conventional acoustic techniques. 



3.1.2 A Case Study: STNH 



The first NED capped mound projects, 

 Stamford-New Haven North and South 

 (STNH-N and STNH-S), were the most 

 intensively studied and documented. 

 Bathymetric surveys and SCUBA diver 



observations were used to document 

 capped mound coherence and stability. 

 The passage of Hurricane David within 6 

 months of disposal provided a natural 

 laboratory to test the long-term stability of 

 a capped mound. 



The acoustic data obtained during the 

 Stamford-New Haven project (STNH) 

 provided immediate confirmation that it 

 was possible to point-dump dredged 

 material to form a well-defined, coherent 

 deposit centered at a specified point 

 (NUSC 1979a-f, SAI 1980a). In the 

 spring of 1979, STNH-N and STNH-S 

 received 31,000 and 38,000 m 3 of 

 contaminated material, respectively, 

 according to disposal barge load estimates, 

 and formed discrete mounds approximately 

 100 m in diameter (Table 2-4 and Figure 

 3-1). 



Material transport and loss during the 

 disposal operation were estimated by 

 comparing dredged material volumes 

 shown in the NED disposal barge logs to 

 those calculated using the bathymetric 

 data. For example, the total volume of 

 Stamford material deposited at each mound 

 was calculated by comparing the pre- and 

 post-Stamford disposal bathymetry. At 

 STNH-S, the results of the volume 

 calculation for Stamford material were 

 34,000 m 3 , which accounted for 90% of 

 the barge load estimate. The estimate was 

 later revised by quantifying the volume of 

 material on the mound flanks, below the 

 resolution of the bathymetric methods at 

 the time, using SCUBA observations and 

 bottom sampling. An estimate of an 

 additional 1980 m 3 brought the total 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



