"Proceedings, Federal Interagency Workshop,
"Sediment Technology for the 21'st Century,"
St. Petersburg, FL, February 17-19, 1998"

Measurement of Suspended Sediment Concentration in Unsteady Flows with an Acoustic Backscatter Profiler

By Marcelo García

Pre-Proposal

Issue/Problem

The measurement of suspended sediment concentration is difficult for any flow. The oldest and most trustworthy method for measuring concentration is to physically sample the flow. Although suction sampling has been used for measuring sediment concentration in unsteady flows, application of the method is not practical for rapidly changing, unsteady flows.

Acoustic profilers can be used to overcome the temporal problems associated with suction samplers and at the same time are less intrusive in the flow. The profiler transmits a sound pulse through the water column. The sound beam is reflected by particles in the water, and the strength of the return signal is a function of the concentration of sediment in the water. A large number of profiles must be gathered and ensemble averaged to obtain a mean concentration profile. This provides an efficient way to measure suspended load very quickly and non intrusively.

Beneficiaries

Federal agencies and other parties involved in the monitoring of suspended sediment will be the main beneficiaries of this technology. An ideal application would have been the interagency experiment conducted in the Colorado River, where a substantial amount of sediment was transported by a rapidly changing flow; thus making it very difficult to monitor suspended sediment concentration throughout the water column with conventional means. With this technology it would be possible to determine how much sediment is transported in suspension during floods.

Objective

To develop a measuring technique based on an acoustic backscatter profiler that can be used for field measurements of suspended sediment concentration in the water column, under both steady and unsteady flow conditions.

Approach

Laboratory experiments will be conducted in a large, tilting flume at the Hydrosystems Laboratory, University of Illinois. The flume is 160 ft long, 6 ft wide and 5 ft deep. Sand and water will be recirculated, and suspended sediment concentration profiles will be measured for different flow conditions. These experiment will concentrate on steady, fully developed flow conditions. To date, more than one hundred experiments have been completed with rapidly changing, unsteady flows, as part of research conducted to assess sediment resuspension due to navigation and its impact in the Upper Mississippi River Basin (funded by US Army Corps of Engineers Waterways Experiment Station). The acoustic profiler has been found to work quite well for 0.1 mm and 0.5 mm material. We would like to test the profiler in deeper flows and with other sediment sizes. Once the sensor has been extensively tested in the lab, its performance will be tested in the field.

Funding for Fiscal Year '99

This project can be completed in 1 year of work. The cost of the research is estimated to be around $70,000.

Personnel

The principal investigator will be Dr. Marcelo Garcia. An important resource for the project, will be Mr. David Admiraal, who will participate as a Research Associate. Mr Admiraal is completing his dissertation on sediment resuspension by unsteady flows under the supervision of the P.I., and has substantial experience with acoustic current profilers, flow measuring techniques, and data acquisition and analysis.

References

Crawford, A.M., and Hay, A.E., (1993) "Determining Suspended Sand Size and Concentration from Multifrequency Acoustic Backscatter", Journal of the Acoustical Society of America, 94(6), pp. 3312-3324.

Garcia, M.H., Admiraal, D., and Rodriguez, J., (1998) "Navigation Induced Sediment Resuspension", Report prepared for the US Army Corps of Engineers Waterways Experiment Station, Hydrosystems Laboratory, Department of Civil Engineering, University of Illinois.


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