30 



elliptical shaped mound extending 500 m to the north and south, 400 m east, and 800 m west 

 of the disposal site center. 



Dredged material deposited on the flanks of the mound was deposited in layers too 

 thin to be detected reliably by precision bathymetry (the acoustical limit for this survey was 

 approximately 20 cm). Therefore, this material was not included in the bathymetric volume 

 calculation. It can, however, be measured with REMOTS® photography which can detect 

 dredged material layers in the range of 1 mm to 20 cm. When the area representing the 

 flanks of the mound was digitized and measured, it was found to occupy 661,000 m 2 and 

 increased the area of the seafloor affected by dredged material an additional 83 % beyond that 

 detected by bathymetry. 



A conservative estimate for the average thickness of the fresh dredged material layers 

 in this area was 10 cm. This was based on the actual thickness of dredged material layers 

 measured in photographs obtained from several flank REMOTS® stations. This estimated 

 depth, applied over the entire 661,000 m 2 area, resulted in an estimated volume of 66,100 m 3 

 of dredged material on the mound flanks not included in the bathymetric volume calculation 

 (78,075 m 3 ). When combined, the two surveys total 144,175 m 3 (Table 4-1). 



The measurements of dredged material thickness were underestimated due to limited 

 camera penetration. It is likely that the layers of dredged material as determined by 

 REMOTS® are deeper than the average camera penetration depth for this survey (10-12 cm). 

 A more reasonable estimate for the depth of dredged material layers on the mound flanks in 

 this case would be 20 cm, the maximum penetration depth of the REMOTS® camera. The 

 volume of dredged material on the mound flanks based on an average depth of 20 cm is 

 132,200 m 3 .- This volume, combined with the bathymetric volume calculation, accounts for a 

 total volume of dredged material at the "MDA" buoy of 210,275 m 3 (Table 4-1). 



Since the dredged material volume calculated from REMOTS® includes material 

 occurring in thin (10-20 cm) layers, comparisons were not made with the barge log volume 

 corrected to Tavolaro's 41% factor, but with barge log volumes corrected to 15.4%. 

 Tavolaro's 15.4% factor (1980) accounts for loss of interstitial pore water during disposal 

 and initial self-compaction of the disposed material. When the combined volumes for 

 bathymetry and REMOTS® at 10 cm (144,175 m 3 ) and 20 cm (210,275 m 3 ) depths are 

 compared to 15.4% of the barge log volume (220,214 m 3 ; Table 4-1), they account for 

 65.5% and 95.5% of the corrected volume, respectively. 



The presence of dredged material in a large number of REMOTS® stations away from 

 the disposal buoy is primarily related to vessel positioning at the time of disposal. The 

 expected radius of an individual disposal event (2,000 m 3 at a 90 m depth) as calculated by 

 the DAMOS capping model for MBDS indicates that material would spread a distance of 300 

 meters (Figure 4-2) from the point of impact. A plot of the barge release points (Figure 4-1) 



Monitoring Cruise at the Massachusetts Bay Disposal Site, August 1990 



