of distribution suggests that some scows began depositing the 

 material before and/or after reaching the disposal buoy. 

 However, a large percentage of the material has apparently 

 accumulated southeast of the buoy. 



A rough estimate of the amount and thickness of dredged 

 material can be calculated using the approximate area of seafloor 

 indicated by the side scan sonar survey to be covered with 

 dredged material. Examination of Figure 2-3 shows an area 

 roughly 700 by 700m square of intermediate acoustic reflectance, 

 usually indicating dredged material. Assuming that approximately 

 275,400 m-^ of material (estimated from scow logs) was evenly 

 spread over this area, a layer of dredged material of 

 approximately 0.5 m thickness could be expected. When 

 considering that both the loss of interstitial water during 

 descent and compaction after impact with the bottom would reduce 

 the actual volume of material expected to be seen on the bottom 

 by as much as 40% (Tavalaro, 1983) , the thickness of the dredged 

 material may actually be approximately 0.3 m. It would be 

 difficult to confirm the presence of this layer with bathymetry 

 in a depth of 70 m due to the limitations of available fathometer 

 systems and a combination of errors associated with the speed of 

 sound, tide, and navigation. More detailed study of this area 

 with precision bathymetric surveys at a smaller lane spacing (25 

 m) would detect any significant topographic features caused by 

 dredged material disposal while REMOTS® sediment profiling would 

 be needed to accurately measure the thickness of the dredged 

 material layer and determine its areal limits. 



3.0 SEDIMENT PLUME TRANSPORT STUDY 



3 . 1 Introduction 



In order to assess the potential impact of dredged 

 material disposal on the surrounding environment, a plume study 

 was conducted to track suspended material in the water column 

 after a disposal event. Three studies were performed during the 

 period of 21 May to 2 4 May 1985. Although attempts were made to 

 track plumes on both the flood and ebb tides, coordination of 

 scow arrival and dumping, weather, and tide prevented studies 

 occurring during ebb tide. However, because the dominant current 

 feature is the flood tide (with maximum peak velocities and long 

 durations) , emphasis on the flood tide was warranted due to its 

 greater potential for transport. 



Multi-frequency acoustic profiling has been under 

 investigation since 1975 as a means of measuring concentrations 

 of suspended matter in the water column. Much of the work has 

 been carried out by the NOAA Atlantic Oceanographic and 

 Meteorological Laboratory in Miami. The work has included study 

 of diffusion properties and acoustic measurements, as well as 

 development of a model to describe the relationship between 



