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United States Department of Agriculture

Agricultural Research Service

Research Project: Technologies for Managing Water and Sediment Movement in Agricultural Watersheds

Location: Watershed Physical Processes Research Unit

Title: Observations on the use of acoustic Doppler velocimeters over rough beds with suspended sediment

Authors
item Wren, Daniel
item Langendoen, Eddy
item Kuhnle, Roger
item Pellachini, Corrado -

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: August 13, 2012
Publication Date: August 13, 2012
Citation: Wren, D.G., Langendoen, E.J., Kuhnle, R.A., Pellachini, C. 2012. Observations on the use of acoustic Doppler velocimeters over rough beds with suspended sediment. Hydraulic Measurements and Experimental Methods Conference, Snowbird, Utah. 7 pp. CD-ROM.

Interpretive Summary: The interaction of a coarse stream bed with flow and sediment is complex, and the controlling factors, such as bed roughness, slope, and availability of fine sediments, are difficult to measure. It is essential that the behavior of flow in these conditions is well understood, if efforts to improve sediment transport modeling are to be fruitful. Acoustic Doppler velocimeters provide a means for measuring velocities and turbulence in challenging circumstances, such as in flows with suspended particles, which are difficult or impossible with laser-based techniques. The relatively non-intrusive measurement resulting from the offset sampling volume also presents an advantage relative to electromagnetic current measurement devices. Recent experiments at the USDA-ARS-National Sedimentation Laboratory have provided an opportunity to observe the operating characteristics of two different acoustic Doppler velocimeters in flows over rough beds with sand (median size 0.3 mm) in suspension. The results of the testing show that the strength of the acoustic signal did not have a significant effect on the measurements, and that the newer instrument yielded similar turbulence measurements to the older one, except with higher spatial resolution.

Technical Abstract: Acoustic Doppler velocimeters provide a means for measuring velocities and turbulence in challenging circumstances, such as in flows with suspended particles, which are difficult or impossible with laser-based techniques. The relatively non-intrusive measurement resulting from the offset sampling volume also presents an advantage relative to electromagnetic current measurement devices. Recent experiments at the USDA-ARS-National Sedimentation Laboratory have provided an opportunity to observe the operating characteristics of two acoustic Doppler velocimeters in flows over rough beds with sand (median size 0.3 mm) in suspension. It was found that there is a weak relationship between backscatter amplitude and Reynolds stress measurements for the Vectrino+, for flows with and without sand in suspension. The Vectrino II yielded increasing Reynolds stresses with increasing backscatter amplitude; however, <7% of the data were in the affected amplitude range. The same experiments also allowed a comparison between the Vectrino-II and Vectrino+. The two instruments measured similar Reynolds stresses throughout the flow depth, but the Vectrino-II provided much higher data density which resulted in an improved picture of flow quantities near and in the pores of the gravel and cobble beds.

Last Modified: 10/22/2014
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