The implications of laser‐diffraction measurements of sediment size distributions in a river to the potential use of acoustic backscatter for sediment measurements

Abstract

We construct vertical profiles of the acoustic attenuation and back‐scattering properties of a river column from measured particle concentration and size distribution profiles. The particle size and concentration data were collected in situ in the Cowlitz River in Washington, U.S., using a laser diffraction‐instrument LISST‐SL. The particle size distribution was bimodal, comprising a vertically well‐mixed washload, and a suspended load that was similar to Rouse profiles. We then explore how well the results of converting these synthetic profiles to recover an acoustic equivalent sediment concentration and acoustic equivalent size compare with laser data in this bimodal environment. The acoustic scattering and attenuation properties are computed for four distinct frequencies: 0.5, 1, 3, and 5 MHz. It is seen that at the lowest frequency, 500 KHz, the acoustic attenuation throughout the water column is nearly constant and determined primarily by particles of size smaller than ∼30 microns, i.e., the washload. At the next higher frequency, 1 MHz, the suspended load also contributed to attenuation, but even so, attenuation remained nearly constant over the vertical profile. Thus, at the two lower frequencies, attenuation was decoupled from scattering, making the inverse problem explicit for inversion. In contrast, at the two highest frequencies, scattering of sound by the suspended mode became the dominant contributor to attenuation, and attenuation varied by an order of magnitude over river depth. As for backscatter, the computed acoustic backscatter strength was determined by a combination of the washload and suspended sediment mode at all four frequencies. A fairly narrow monotonic relationship was found between total sediment concentration and locally computed backscatter, effectively providing a calibration between local backscatter signal strength and suspended sediment concentration. Such a relationship existed throughout the water column, for every frequency. The sediment concentration derived from backscatter at a pair of frequencies was within ∼50% of the value measured by laser diffraction; however, the acoustic equivalent diameter exceeded the laser volume mean diameter by up to an order of magnitude. The robustness of these results for application to other flow regimes or rivers remains to be investigated.

Response of suspended sediment particle size distributions to changes in water chemistry at an Andean mountain stream confluence receiving arsenic rich acid drainage

“…Although benchtop instruments may be able to measure over a wider size range, the LISST 100X was chosen for its in‐situ deployment capability. This instrument has been evaluated and used extensively for freshwater systems (Agrawal & Hanes, ; Czuba, Straub, Curran, Landers, & Domanski, ; Filippa et al, ; Guo & He, ; Williams, Walling, & Leeks, ).…”

Applying ADCPs for Long‐Term Monitoring of SSC in Rivers

“…In addition, the continuous monitoring of SSC in a fixed position is hardly achievable by means of direct sampling because of the time and effort required for field operations and subsequent laboratory analyses. To overcome this, in the last years, a variety of indirect methods have been developed, where light and sound scattering and attenuation were used as a surrogate measurement of SSC (Agrawal & Hanes, ; Guerrero et al, ).…”

Estimating sand concentrations using ADCP‐based acoustic inversion in a large fluvial system characterized by bi‐modal suspended‐sediment distributions

“…Research has focused (using down-, side-or up-looking ADCP deployment) on the relation of suspended sediment and corrected backscatter (Thorne et al, 1993(Thorne et al, , 1998Reichel and Nachtnebel, 1994;Holdaway et al, 1999;Thorne and Hanes, 2002;Gartner, 2004;Wall et al, 2006;Topping et al, 2007;Deines, 1999;Szupiany et al, 2009;Hanes, 2012;Guerrero et al, 2013Guerrero et al, , 2016Latosinski et al, 2014;Thorne and Hurther, 2014;Landers et al, 2016;Manaster et al, 2016;Venditti et al, 2016;Topping and Wright, 2016;Mullison, 2017;Hackney et al, 2018), acoustic attenuation and scattering properties (Thorne and Meral, 2008;Wright, et al, 2010;Sassi et al, 2012;Moate and Thorne, 2013;Moore et al, 2013;Agrawal and Hanes, 2015;Hanes, 2016;Topping and Wright, 2016;Haught et al, 2017), and acoustic scattering by suspended flocculating sediments (Thorne and Hurther, 2014;Vincent and McDonald, 2015;Thomas et al, 2017).…”

“…Moreover, both wash load and sand fractions may vary by site and hydrologic condition, altering the characteristics of sound propagation into sediment-water mixture and resulting measurements of echo intensity level (acoustic backscatter) and attenuation (Wright et al, 2010;Agrawal and Hanes, 2015;Hanes, 2016). At most river sites, fine washload fractions of sediment are homogeneously distributed across a river cross-section, but sand-sized fractions of sediment can greatly vary with depth and width.…”

“…At most river sites, fine washload fractions of sediment are homogeneously distributed across a river cross-section, but sand-sized fractions of sediment can greatly vary with depth and width. Moreover, both wash load and sand fractions may vary by site and hydrologic condition, altering the characteristics of sound propagation into sediment-water mixture and resulting measurements of echo intensity level (acoustic backscatter) and attenuation (Wright et al, 2010;Agrawal and Hanes, 2015;Hanes, 2016). The importance of acoustic backscatter and attenuation of suspended sediment has received a fair amount of attention during the past few years; nonetheless, many theoretical formulations and simplifications remain invalid or untested in field environments when sediment characteristics substantially vary within an acoustic measurement volume, which hinders the development of a more general methodology to be applied by using down-looking ADCPs.…”