Figure 3. Transect 2, Mobile Bay monitoring

ERDC TN-DOER-E7

April 2000

in ABAB values. Each ABAB value was assigned a color based on an incremental scale of between

0 and 1, 1 to 1.5, 1.5 to 2, 2 to 2.5, 2.5 to 3, 3 to 3.5, 3.5 to 4, and greater than 4 times the standard

deviation above background variations. These are plotted for Transects 2, 6 and 8, in Figures 3, 4,

and 5, respectively.

Figure 3. Transect 2, Mobile Bay monitoring

To accomplish the objective of superimposing fishery hydroacoustic data on plume-monitoring

acoustic data acquired synoptically, a BioSonics DT 4000 Series digital transducer system was used

to make measurements in tandem with the BBADCP. The BioSonics DT is a 200-kHz high-

resolution, high-accuracy echo sounder. It is a computer-controlled and -calibrated system used to

perform acoustic surveys of aquatic and marine fishery resources. Figure 5 shows targets detected

by the system superimposed on ABAB data along Transect 8.

Interpretation of the images presented in Figures 3, 4, and 5 is influenced by the nature of the currents

during the monitoring. The monitoring was performed while the survey vessel followed the Wheeler,

which was dredging while going down the channel toward the Gulf of Mexico. Consequently, the

survey vessel needed to catch up to any suspended sediment plumes created by the dredging if the

currents were ebbing, but a plume could be expected to move northward into the path of the survey

vessel if the currents were flooding. Processing of the BBADCP current data revealed that the flows

were ebbing in the upper part of the water column and flooding in the bottom layer. The currents

measured across Transect 6, which are typical of those measured along all the transects, are shown

in Figure 6. The currents are contoured through points of equal magnitude with positive values

being ebb currents and negative values being flood currents. The passage of the Wheeler down the

channel, and probably its prop wash, have an effect on the current measurements. An asterisk in