POINTS OF CONTACT - doere70012

ERDC TN-DOER-E7

April 2000

Subtract these background values of acoustic backscatter for the equivalent depth and position

across the channel (Mobile), or along the plume axes (Boston), from the values measured

while monitoring, and divide the differences by the standard deviations of the naturally

occurring backscatter for the equivalent depths.

The results of this procedure are ABAB values. When they were plotted for plume cross-sections,

they produced qualitative visualizations of the location of the plume and the relative concentrations

of suspended sediment.

It was observed in both Mobile and Boston that the most significant difficulty with this procedure

was the large ABAB values produced by wakes from ships and boats. The effects of the wakes

persisted for a long period of time behind the vessels that produced them, and a method to distinguish

ABAB produced by these wakes from that produced by suspended sediment has not been identified.

At present, it is necessary to note the wake effects in the field when a vessel passes the area being

monitored, and to take the effects into account when interpreting the data.

It was demonstrated in Mobile that measurements from a fisheries hydroacoustic system could be

acquired simultaneously with the plume monitoring system and could be plotted on the plume

visualizations to show the locations of fish targets relative to the plumes. The BBADCP system

used had the additional capability of showing the details of the current flows relative to the plume

and fish locations.

The ability to use the acoustic data to visualize the location of the plumes and relative concentrations

within the plumes during the monitoring proved to be critical to the success in making other

measurements at known locations within the plumes. During the monitoring in Boston, in situ light

attenuation and optical backscatter measurements were made simultaneously with the Battelle

Ocean Sampling System. Water samples were also taken while the survey vessel was underway.

These samples were analyzed to determine TSS. Results from these measurements will provide

input to efforts to improve and validate models of dredging-related suspended-sediment plumes. In

addition, the information produced by the acoustic system on the spatial extent of the plumes will

be valuable input for these modeling efforts. The Boston monitoring also demonstrated the value

of acoustic data by itself, by showing qualitatively the relative degrees to which each bucket

produced suspended sediment in the water column.

POINTS OF CONTACT: For additional information, contact the authors, Mr. M. W. Tubman

(601-634-3009, tubmanm@wes.army.mil) or Mr. W. D. Corson (601-634-2189, corsonw@wes.

army.mil), or the Program Manager of the Dredging Operations and Environmental Research

Program, Dr. Robert M. Engler (601-634-3624, englerr@wes.army.mil). This technical note should

be cited as follows:

Tubman, M. W., and Corson, W. D. (2000). "Acoustic monitoring of dredging-related

suspended-sediment plumes," DOER Technical Notes Collection (ERDC TN-DOER-E7),

U.S. Army Engineer Research and Development Center, Vicksburg, MS.

www.wes.army.mil/el/dots/doer