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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #230296
Title: Estimation of concentration and particle size distribution of natural sands suspended in water using acoustic backscatter

Author
item CHAMBERS, JAMES - UNIV OF MISSISSIPPI, NCPA
item SMITH, CHRIS - UNIV OF MISSISSIPPI, NCPA
item Wren, Daniel
item Kuhnle, Roger

Submitted to: National Sedimentaton Laboratory (NSL)- 50 Years of Soil & Water Research in a Changing Agricultural Environment
Publication Type: Proceedings
Publication Acceptance Date: 9/1/2008
Publication Date: 9/26/2008
Citation: Chambers, J.P., Smith, C., Wren, D.G., Kuhnle, R.A. 2008. Estimation of concentration and particle size distribution of natural sands suspended in water using acoustic backscatter. National Sedimentaton Laboratory (NSL)- 50 Years of Soil & Water Research in a Changing Agricultural Environment. p. 907-921.

Interpretive Summary: Utilizing acoustic instruments to study suspended sediments in water bodies as an alternative to traditional sampling methods is critical to providing reduced labor costs and better data resolution in both time and space. One main drawback to using acoustic devices is the need to know the size of the particles that are suspended in the water before accurate concentration measurements can be made. Accomplishing this is best done using multiple acoustic frequencies, allowing particle size to be measured along with concentration. Most work on this topic has been with limited size distributions present in near-shore marine environments. A main goal of the present work was to simplify the use of multiple acoustic frequencies by sending them through one acoustic transducer. A single transducer system reduces the amount of electronic hardware needed in the field. It was found that the method produced 0.1%-36% error in particles sizes in fine to medium sands and for very fine sands the error was 24%-160%. The large errors were attributed to the narrow range of frequencies that the single transducer was able to produce. Therefore, it was concluded that, for more accurate particle size estimates, a multiple transducer system is necessary. The applicability of acoustic techniques to the wider range of particle sizes present in streams and rivers was broadened by this work. The results shown here will help other researchers and those in action agencies who are using acoustic technology to measure suspended sediments in streams and rivers.

Technical Abstract: Utilizing acoustic instruments to investigate suspended sediments holds great promise as an alternative to traditional methods such as pump-sampling as acoustic devices are less labor intensive, provide better temporal and spatial resolution of data, and can operate in an autonomous mode. The measurement of suspended sediments using multiple, megahertz range acoustic frequencies has typically been focused on particles with radii of 50µm -150µm. The present study seeks to extend the applicability of the technique to particles with radii ranging from 50µm -425µm. A single acoustic transducer, transmitting a waveform with three peak frequencies, was used to measure backscatter from natural sand particles entrained in a turbulent jet. An approach similar to that taken by Sheng (1991) was used in the data processing, where ratios of backscattered signals at varying frequencies were used to evaluate particle sizes. It was found that the method produces 0.1%-36% error for particles with radii of 150µm - 425µm and 24%-160% error for particles with radii of 75µm -150µm, depending on the concentration of the suspended particles.