"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.
Workshop Contributions
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