Figure 10. Transect across plume axis, 625 m from dredge, 5 August 1999

ERDC TN-DOER-E7

April 2000

(4,000 cu yd). The three dredging operations occurred in the same general area, and similar material

was dredged in each case. The material dredged was 0.9 to 1.8 m (3 to 6 ft) of sandy silt.

Background monitoring consisted of four to six transects (approximately 190 m long) across the

harbor near the dredge, in an area that was upstream of the dredge during ebb flow. Data from these

transects were used to calculate the standard deviations of the acoustic backscatter variations for

each day. Immediately after these transects were completed and just prior to dredging, the

background transect was run from near the dredge downstream to the end of the area monitored.

Data from these transects were subtracted from acoustic backscatter measurements made while

monitoring the plumes. Due to operational difficulties, the background transect on 5 August could

not be run.

On each of the 3 days that a plume was monitored, transects were made perpendicular to the

downstream axis of the plume at various distances from the dredge. An example of one of these

transects is shown in Figure 10. These transects were run to delineate the cross-stream boundaries

of the plume, and extend beyond the edges of the plume into areas where no suspended sediment

from the dredging operation was detected. Data from these areas on 5 August were compared with

data from the axis transect made before dredging started on 6 August, at equivalent latitudes (the

downstream transects along the plume axis are primarily along a constant longitude), and at

equivalent depths. It was found that the differences are random, and that 94 percent of the

differences are less than two times the standard deviations of the background variations recorded

on 6 August. Only 1 percent of the differences are greater than 2.5 times the standard deviations of

the background variations. Therefore, the along-axis background transect made on August 6 appears

Figure 10. Transect across plume axis, 625 m from dredge, 5 August 1999