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4.0 DISCUSSION 



The objectives of the combined REMOTS® and precision bathymetric surveys were to 

 delineate the extent and topography of the dredged material deposit resulting from disposal at 

 the MBDS since 1990. Also, with the passage of Hurricane Bob and the Halloween Storm, 

 there was concern regarding the effects these storms may have had on the containment of 

 dredged material. 



4.1 Bathymetry 



As predicted, bathymetric analysis showed an accumulation of up to more than 2 m of 

 dredged material near the disposal buoy since 1990 (Figure 3-3); the majority of material 

 was located west of the buoy. The dredged material deposit contour in the depth difference 

 plot for the 1990 and 1992 bathymetric surveys followed the northwestward trend seen in the 

 plot of barge release points for the 1990-1992 disposal seasons (Figure 4-1). 



A shallow mound (0.8 m) was formed during the 1988-1990 disposal seasons (Figure 

 4-2). The average diameter of the mound was 420 m, and most of the accumulation was 

 100 m east of the buoy location. Depth differences on the order of 20 cm (i.e., approaching 

 the limits of detection in this comparison of the 1990 and 1988 surveys) occurred within 

 400 m from the buoy, primarily to the west. A plot of the barge release points for the 1988- 

 1990 disposal season showed that the majority of barges released dredged sediment within a 

 100 m radius of the buoy location (Figure 4-3). 



A comparison was made of the depth matrices from 1988 to 1992 (Figure 4-4). 

 These results were similar to the 1990 to 1992 depth difference which showed a maximum 

 mound thickness of 2 m. Most of the matericd has accumulated withm a 500 x 500 m area 

 immediately west of the buoy. To the east and soudi of the buoy, dredged material 

 thicknesses ranged from 0.25 m to 1.0 m (Figure 4-4). These results reflect the patterns of 

 accumulation noted in mound formation from 1988 to 1992 (Figures 3-3 and 4-2). 



The successful formation of a mound from the 1988-1992 disposal activities 

 demonstrates that a distinct mound can be formed with dredged material at this site providing 

 that tight control is exercised over disposal operations (Wiley 1994). Any bottom 

 disturbance that may have occurred with the passage of Hurricane Bob and the Halloween 

 Storm did not measurably disturb the mound. 



Calculations of "depth difference" volumes based on successive bathymetric surveys 

 have been estimated to be as much as 41% less than estimates based on barge disposal log 

 volumes (Tavolaro 1980). This discrepancy has been attributed not only to the inaccuracies 

 inherent in estimating dredged material volumes in the barges, but also to the compaction of 

 the dredged material on die bottom following disposal and the significant volume of material 



Monitoring Cruise at the Massachusetts Bay Disposal Site, March 31 - April 4, 1992 



