In an attempt to estimate the amount of material that 

 could be transported out of the disposal area, preliminary- 

 calculations were made from the results of the plume studies. A 

 representative range of values for bulk density of the dredged 

 material of 1.4 to 1.6 g/cm-^ was used to calculate the dry mass 

 of material in the scow. The estimated scow volumes were 

 obtained from NED. From each plume study, estimates of the 

 depths where the suspended sediment concentration occurred were 

 used in the calculations. Table 3-1 presents the estimates for 

 the percent of material present in the water column after the 

 disposal event. These calculations included determining the mass 

 of material (mass^) in the suspended sediment cloud as: 



mass^, = A X D X S 



where A = the area of the sediment cloud (m^), 



D = the height of the water column (m) with suspended 



sediment, and 

 S = the suspended sediment concentration (mg/1 or 



g/m^), 



and determining the mass of material (mass^) in the scow as: 



massg = scow volume (m-^) X bulk density (g/cm-^) X 10^ 



and, finally: 



% material = (mass^^ / massg) X 100. 



The estimates for the percent of disposed material 

 still in the water column vary widely for the three plume 

 studies. This is due to the estimated values for area and depth 

 of the suspended sediment concentrations determined in Figures 3- 

 2, 3-4 and 3-6. Although the acoustic measuring system can 

 detect suspended sediment vertically from the surface to the 

 bottom and along the ship's track (horizontally), another pass of 

 the ship through the suspended material is needed to delineate 

 the spatial area. The interval of time required for this allows 

 the suspended material cloud to settle or spread. Qualitative 

 judgments were made in order to graphically illustrate the best 

 approximation of the suspended sediment clouds. Despite the 

 variation in estimates of material in the water column, an 

 important feature common to all three plume studies is that 

 within two hours, 93% or more of the material was on the bottom 

 and suspended sediment concentrations were similar to background 

 levels. For the plume studies conducted on 24 May, tandem scow 

 loads were deposited at the disposal site. Doubling the volume 

 of disposed dredged material increased the initial sediment 

 concentrations (1420 mg/1 versus 950 or 1100 mg/1 on 21 or 22 

 May, Table 3-1) due to more material being available for 

 suspension and increased the distance from the disposal point 

 that the suspended material was tracked (1700 m versus 500 m on 



11 



