28 



4.0 DISCUSSION 



The bathymetric analysis showed significant accumulations of dredged material in the 

 vicinity of the disposal buoy since 1988, which contradicted the prediction that changes in 

 mound height and diameter would not be detectable with bathymetry. Plots of barge release 

 points over the 1988-1990 disposal seasons (Figure 4-1) indicated mat the majority of barges 

 released near the designated location. Barge release points that are further than average from 

 the buoy location may be due to fluctuations in the LORAN readings or weather conditions. 

 The successful formation of a mound from these disposal activities demonstrated that a 

 distinct mound can be formed with dredged material at this site providing that tight control is 

 exercised over disposal operations (Wiley 1991). 



Barge log estimates indicated that 260,300 m 3 of dredged material was deposited 

 within 400 m of the "MDA" buoy at MBDS from November 1988 to August 1990. 

 Tavolaro (1980) showed that volume estimates based on barge logs overestimate considerably 

 the amount of dredged material because of the significant amount of interstitial water 

 associated with the material in the barges. He calculated "depth difference" volume 

 estimates based on successive bathymetric surveys to be as much as 41 % less than the barge 

 log volume estimates. The discrepancy was attributed not only to the barge log inaccuracies, 

 but also to the compaction of the dredged material on the bottom following disposal and the 

 significant volume of material deposited at the flanks of the mounds in layers too thin to be 

 detected acoustically. Applying Tavolaro 's maximum 41% correction factor to the barge log 

 estimate of 260,300 m 3 resulted in a corrected volume of 153,600 m 3 . The volume 

 calculation from the comparison of the 1988 and 1990 bathymetric surveys was 78,100 m 3 , 

 or 50.9% of the corrected volume of released material. Consolidation of underlying 

 sediments (disposal sediments from 1985 to 1988, and base material) may have contributed to 

 the apparent "loss" of material. As these sediments consolidated, the elevation measured in 

 1988 (which was used as a reference plane) was reduced. For every 1 cm of consolidation 

 over a 400 m diameter mound, an apparent loss of 1256 m 3 can occur. Inaccurate 

 positioning of some barges at the time of disposal may have also contributed to the apparent 

 loss of material; dredged material disposed on the flanks of the mound would have been 

 undetected by bathymetry. 



REMOTS® photographs confirmed the existence of dredged material layers beyond 

 the boundaries determined by bathymetry, a result which is consistent with results at other 

 disposal sites. The precise boundaries of the new mound were difficult to determine at some 

 stations (particularly 500E and 250NW) due to the presence of "fresh" and "relic" dredged 

 material layers. This difficulty in distinguishing between "fresh" and "relic" dredged 

 material was also found with REMOTS® results from January 1989. In the previous survey, 

 the radius of "fresh" material was determined to be approximately 300 to 350 m while relic 

 material extended the radius to approximately 500 m. The current survey indicated an 



Monitoring Cruise at the Massachusetts Bay Disposal Site, August 1990 



