Figure 1. Locations of study areas

ERDC TN-DOER-E7

April 2000

occur regardless of the amount of suspended sediment present; consequently their effects on the

measurements are removed in the first processing step (Tubman 1995).

One of the most important aspects of applying acoustic technology to monitoring suspended-

sediment plumes from dredging operations is to define the ambient acoustic background. It is

necessary to determine what conditions exist at the measurement location just before sediment is

put into suspension by the operation, and what naturally occurring variations can be expected over

the length of time the operation is monitored. Ideally, this is accomplished by making acoustic

measurements just before dredging starts, and by calculating statistics from those measurements.

In the monitoring reported herein, the naturally occurring variations in ambient acoustic backscatter

were determined from measurements made by the BBADCP fifth beam along transects across the

study area during times when there was no dredging. In each of the fifth-beam 25-cm bins, the

standard deviation of the acoustic backscatter was calculated for all measurements made along these

transects. Since the fifth beam points straight down, these values for each bin represent the standard

deviation of acoustic backscatter as a function of depth. Just prior to the start of the dredging

operations, a transect was made in the area where the plume from the dredging operation was

expected to be located. Acoustic backscatter values from this transect (referred to as the background

transect) were subtracted from the values obtained during monitoring of the plume, and the results

were divided by the standard deviations of the background variations. The result is numbers that

represent the observed acoustic backscatter above background (ABAB). The precise manner in

which this was done was somewhat different for Boston Harbor than it was for Mobile Bay. In

Mobile Bay, the hopper dredge conducted its dredging operation while traversing a confined

channel, producing a plume as it proceeded, whereas in Boston Harbor, the clamshell dredge stayed

in approximately the same location, in an open area of the harbor, and produced a plume that

extended downstream from the dredge. For the Mobile Bay study, the background transect was

made across the channel at the midpoint in the length of channel dredged. In Boston Harbor the

background transect was run from the dredge, parallel with the current, to a point approximately

600 m downstream from the dredge.