21 and 22 May, Table 3-1) . The distribution of suspended 

 sediment immediately after disposal would probably be less for a 

 single scow load of the same volume because more material would 

 make up the single descending plume and a smaller percentage of 

 material would experience the entrainment of water at the water 

 column/plume interface. 



Results from the current meter data showed that the 

 dominant flow was to the N-NE and that the maximum current 

 velocities occurred on the flood tide. For the Rockland area, 

 NOAA tidal current tables estimate that slack water before flood 

 tide occurs approximately 1 hour before low tide and that maximum 

 flood tide current velocities occur about 3 hours after low tide. 

 This suggests that the sediment transport out of the disposal 

 site during the 24 May plume study was caused by the peak tidal 

 velocities occurring at that time. This period of maximum 

 transport appears to be relatively short. The survey of 21 May 

 occurred one hour later in relation to the stage of the tide, and 

 no material was detected outside the disposal site. Current 

 velocities greater than 24 cm/sec occurred only 13% of the time, 

 or approximately 1.6 hours during each tidal cycle. 



Examination of Figures 2-9 to 2-12 indicates that the 

 thermocline usually occurred at or near the 10 m depth where the 

 current meter was deployed. Therefore, the results of the 

 current meter analysis would reflect the potential for movement 

 of sediment that may be suspended at that depth. However, a 

 closer look at the results of the acoustic records obtained 

 during the plume tracking (Figures 3-3, 3-5, and 3-7) did not 

 reveal a significant accumulation of suspended sediment at the 

 thermocline. 



In order to estimate the long-term effect of disposal 

 at this site based on conditions described above, a calculation 

 was made to determine the percent of material that could be 

 expected to leave the disposal site during a prolonged disposal 

 project. If we assume that after disposal approximately 6% of 

 the material will be in the water column and available for 

 transport by current velocities greater than 24 cm/sec, then only 

 0.8% (or 6% times 13%) of the material disposed during the entire 

 dredging project is available for transport outside the site 

 boundaries. As mentioned earlier, this material would be so 

 widely dispersed that detection of any accumulation would be 

 almost impossible. 



4 . CONCLUSIONS 



The results of the precision bathymetric survey 

 conducted on 20 May 1985 did not detect any significant 

 development of a mound of dredged material from disposal 

 operations occurring since September 1984. The side scan survey 

 revealed the presence of dredged material near the disposal buoy, 



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