Use of ADCPs for suspended sediment transport monitoring: An empirical approach

Abstract

A horizontally mounted 300 kHz acoustic Doppler current profiler was deployed in Fraser River at Mission, British Columbia, to test its capability to detect size-classified concentration of suspended sediment. Bottle samples in-beam provide a direct calibration of the hADCP signals. We also deployed a 600 kHz vertically mounted ADCP from a boat in combination with bottle samples. Fraser River at Mission is 525 m wide with moderate suspended sediment concentration (up to 350 mg L 21 in our measurements, mostly silt), and a modest sand load only at high flows. We use an entirely empirical approach to calculate the sediment load using ADCPs to test the reliability of acoustic methods when assumptions embedded in the sonar equation about the relation between suspended sediment size and concentration, and acoustic signals are violated. vADCP calibration using matched individual bottle samples and acoustic backscatter departed from the expected theoretical relation. Calibration using depth-averaged sediment concentration and acoustic backscatter more closely matched theoretical expectations, but varied through the season. hADCP calibrations conformed with theoretical expectations and did not exhibit seasonal variation. Silt and sand were successfully discriminated; however, silt dominates the correlations. We found no coherent relation between acoustic attenuation and silt concentration. In-beam results are extended by correlation to estimate mean sediment concentration and total suspended flux in the entire channel: this auxiliary correlation cancels any calibration bias and permits monitoring of size-classified suspended sediment in absence of detailed information of sediment grain-size distribution.

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

“…Empirical or semiempirical methods [cf. Woods and Teasdale , 2013; Venditti et al ., ], can be shown to be equivalent to the acoustic inversion when relating the sonar equation to measurements in a homogenous ensonified volume [ Thorne and Buckingham , ].…”

“…Use of hydroacoustics to calculate sediment concentration and particle size is based on the sonar equation, which models the response of a unique acoustic frequency to suspended particulates in an ensonified volume in the water column [Urick, 1975;Hay, 1991]. This is accomplished by finding the relation between acoustic signal metrics (e.g., fluid attenuation corrected backscatter, sediment attenuation corrected backscatter, sediment attenuation) and metrics of the suspended sediment concentration and grain-size distribution (GSD), typically the median or fraction in a particular grain-size range [Gartner, 2004;Hoitink and Hoekstra, 2005;Wright et al, 2010;Venditti, et al, 2016;Topping and Wright, 2016]. Acoustic backscatter is the sound energy reflected back toward an acoustic transducer in an ensonified volume.…”

“…In routine suspended sediment flux monitoring programs in rivers, application of the theoretical method is challenging because GSDs and, often, sediment composition change through hydrographs at time scales ranging from synoptic scale floods to annual flow cycles affected by sediment supply. Furthermore, the relation between flow, concentration, and GSD is known to Water Resources Research 10.1002/2016WR019695 be hysteretic in many rivers [e.g., McLean et al, 1999;Landers and Sturm, 2013;Attard et al, 2014;Venditti et al, 2016]. The information requirements for the theoretical attenuation method require direct and continuous measurement of GSDs, which largely negates the benefits of using hydroacoustics as a suspended sediment surrogate [Venditti et al, 2016].…”

“…Furthermore, the relation between flow, concentration, and GSD is known to Water Resources Research 10.1002/2016WR019695 be hysteretic in many rivers [e.g., McLean et al, 1999;Landers and Sturm, 2013;Attard et al, 2014;Venditti et al, 2016]. The information requirements for the theoretical attenuation method require direct and continuous measurement of GSDs, which largely negates the benefits of using hydroacoustics as a suspended sediment surrogate [Venditti et al, 2016]. This does not prohibit the application of the theoretical method, but assumptions must be made about how GSDs respond to flood flows to successfully apply it.…”

Using a Down‐Looking Multifrequency ABS for Measuring Suspended Sediments in Rivers

“…In that perspective, a vertical down-looking deployment 10.1029/2019WR024877 of the instrument would be more suitable. As far as the authors know, the studies conducted in rivers in this deployment configuration were mainly done with uncalibrated single-frequency ADCPs, usually using empirical approaches to retrieve SSC from backscatter (e.g., Reichel & Nachtnebel, 1994;Baranya & Józsa, 2013;Boldt, 2015;Defendi et al, 2010;Holdaway et al, 1999;Latosinski et al, 2014;Sassi et al, 2012Sassi et al, , 2013Venditti et al, 2016).…”