CONCLUSIONS - doere70011

ERDC TN-DOER-E7

April 2000

to be a reasonable representation of background conditions on 5 August, and it was used to process

the data collected on 5 August. In processing the data, background acoustic-backscatter measure-

ments from equivalent depths and distances from the dredge were subtracted from the plume

monitoring data, and the results were divided by the standard deviations for equivalent depths,

determined on the day of dredging. This resulted in the ABAB values shown in Figures 7 through

10. The ABAB values were assigned a color based on an incremental scale between 0 and 3, 3 to

5, 5 to 7, 7 to 9, 9 to 11, 11 to 13, 13 to 15, and greater than 15 times the standard deviation above

background variations. Since the increments are two standard deviations, using the background

transect from 6 August to analyze the data from 5 August may randomly move some ABAB values

for that day one increment up or down. A very small percentage of the data may move more than

one increment. This needs to be taken into account when analyzing the results presented in the

figures, but it does not affect the general conclusions drawn from them.

Figure 7 shows the results from a transect run on 5 August, down the axis of the plume, starting

near the dredge and running downstream (i.e., north to south). The distances along the horizontal

axis are distances from the end of the crane that was used to conduct the dredging. An interesting

feature of this transect is that near the dredge, the maximum ABAB values (and therefore the highest

concentrations of suspended sediments) are not on the bottom, but approximately 3 m above the

bottom. On this day, dredging was conducted with the Cable Arm Clamshell bucket. Examination

of the bucket revealed that less than half the seals on the top of the bucket were intact, and it is

believed that the higher ABAB values 3 m above the bottom are from sediment being squeezed out

the top of the bucket. Figure 8 shows a transect down the axis of the plume on 6 August, when

dredging was being conducted with the Great Lakes Closed Environmental Bucket. This figure

shows maximum suspended sediment concentrations near the bottom. The relatively high ABAB

values at the surface are believed to be from spillage over the side of the scow, resulting from the

placement of some sample dredged material on the side deck of the scow for sampling purposes.

Figure 9 shows the dredge monitoring results for the conventional bucket. Here maximum concen-

trations cover more than half the water column and extend all the way to the bottom. In a general

qualitative way, the conclusion drawn from these three figures is that for this operation, the Great

Lakes Closed Environmental Bucket created less suspended sediment than the Cable Arm Clam-

shell, and that they both produced less suspended sediment than the conventional bucket. Analysis

of other quantitative data (i.e., turbidity and suspended solids measurements) collected during the

study is currently under way and will be reported separately.

CONCLUSIONS: A procedure for making acoustic measurements and processing the data was

shown to make it possible to determine the extent and relative concentrations of suspended sediment

plumes from dredging operations. The procedure was successful in an area of high natural turbidity

(i.e., Mobile Bay) and in an area of relatively low natural turbidity (i.e., Boston Harbor). The steps

in the procedure are as follows:

Make four to six transects in the dredging area before or after dredging-related suspended

sediments are present to determine the naturally occurring variations in acoustic backscatter

as a function of depth.

Just prior to the start of the dredging, make acoustic background measurements along a

representative transect that will be used for monitoring the plumes during the dredging.