Calculation of in-situ acoustic sediment attenuation using off-the-shelf ADCPs in low concentration settings

Resource type
Authors/Contributors
Author: Dan Haught
Author: Scott Wright
Abstract
The use of ‘off-the-shelf’ acoustic Doppler velocity profilers (ADCPs) to estimate suspended sediment concentration and grain size in rivers requires robust methods to estimate sound attenuation by suspended sediment. Theoretical estimates of sediment attenuation require a priori knowledge of the concentration and grain size distribution (GSD), making the method impractical to apply in routine monitoring programs. In-situ methods use acoustic backscatter profile slope to estimate sediment attenuation, and are a more attractive option. However, the performance of in-situ sediment attenuation methods has not been extensively compared to theoretical methods. We used three collocated horizontally mounted ADCPs in the Fraser River at Mission, British Columbia and 298 observations of concentration and GSD along the acoustic beams to calculate theoretical and in-situ sediment attenuation. Conversion of acoustic intensity from counts to decibels is influenced by the instrument noise floor, which affects the backscatter profile shape and therefore in-situ attenuation. We develop a method that converts counts to decibels to maximize profile length, which is useful in rivers where cross-channel acoustic profile penetration is a fraction of total channel width. Nevertheless, the agreement between theoretical and in-situ attenuation is poor at low concentrations because cross-stream gradients in concentration, sediment size or GSD can develop, which affect the backscatter profiles. So we establish threshold concentrations below which in-situ attenuation is unreliable in Fraser River. Results call for careful examination of cross-stream changes in suspended sediment characteristics and acoustic profiles across a range of flows before in-situ attenuation methods are applied in river monitoring programs.
Published as
Haught, D., J. G. Venditti, and S.A. Wright, (submitted), Calculation of in-situ acoustic attenuation using off-the-shelf ADCPs. Water Resour. Res.
Copyright statement
Copyright is held by the author(s).
Permissions
You are free to copy, distribute and transmit this work under the following conditions: You must give attribution to the work (but not in any way that suggests that the author endorses you or your use of the work); You may not use this work for commercial purposes.
Scholarly level
Peer reviewed?
Yes
Language
Member of collection