Figure 2. Wheeler monitoring transects

ERDC TN-DOER-E7

April 2000

RESULTS

Mobile Bay, Alabama. In January 1999, the U.S. Army Corps of Engineers hopper dredge

Wheeler dredged a portion of the upper reach of the main navigation channel in Mobile Bay (Fig-

ure 1). The dredging operation took approximately 30 min. Figure 2 shows the transects over which

measurements were made along the main Mobile Bay navigation channel, and the track the Wheeler

took while dredging. Transects 1 and 2 occurred before the Wheeler began dredging, while

Transect 8 occurred after the Wheeler had finished dredging. In addition to these transects, four

additional transects were run after the Wheeler had cleared the area, at approximately the same

location as Transect 1. Data from Transect 1 were used when the acoustic background was

subtracted from the measurements made while monitoring the plume, and the four transects run

after the Wheeler left the area were used to determine the background acoustic backscatter variations

unrelated to the dredging. Data from Transect 1 were subtracted from measurements made along

Transects 2 through 8 by using data from equivalent depths (i.e., from the same bin), and for the

same longitude plus or minus 0.0005 min of longitude. This means that each pair of values (i.e.,

monitoring measurement and background measurement) is from the same position across the

channel within approximately 2 m. The results were then divided by the standard deviation for the

equivalent depth, determined from the four transects made after the Wheeler left the area, resulting

Figure 2. Wheeler monitoring transects