Estimation of Concentration and Load of Suspended Bed Sediment in a Large River by Means of Acoustic Doppler Technology

Abstract

The standard experimental methods used for sampling suspended loads in large rivers are usually time consuming, unsafe, rather expensive, and have a limited spatial resolution. Acoustic Doppler current profilers (ADCPs), usually applied to measure the flow discharge, may also be used to assess the suspended sediment concentration by analyzing the backscattering acoustic strength. Though important efforts have been dedicated to test this method, results are not as reliable as engineering practices require, especially in large fluvial systems. In this paper, the correlation between the corrected backscatter from a 1,200 kHz ADCP and the suspended concentration from a depthintegrated sampler is presented and discussed. Despite the assumptions required to utilize this method (i.e., monosized grain and homogeneous vertical concentration), the results showed acceptable differences when they were compared with traditional methods. An evaluation of the backscatter and attenuation of sound produced by fine and coarse material is presented. Finally, the total suspended load of bed sediment is assessed using moving-boat ADCP measurements and compared with results from the corresponding standard method. Differences are at most 46%.

The Acoustic Properties of Suspended Sediment in Large Rivers: Consequences on ADCP Methods Applicability

“…xA ss y " 20log 10 peq¨xζ ss y¨M s (15) xA sv y " 20log 10 peq¨xζ sv y¨M s (16) using the International System the backscattering strength is given in dB and the attenuation coefficients in dB/m. The attenuation coefficients times the acoustic range doubled give the two-way round-trip attenuations (in dB) due to suspended sediments.…”

“…This occurrence was exploited in previous studies by using a single frequency ADCP working at 1200 and 600 kHz, which produced reliable correlations between sand concentrations and measured echo levels [15], and between mean grain sizes of suspended sand and the ratios of echoes for those two frequencies [13].…”

“…Aiming to guide ADCP users in classifying an actual case study by the acoustic method to be applied for the indirect measurement of sediment transport, our objective was to investigate the dependence of acoustic properties (i.e., backscatter and sound attenuation) on the variation of suspended particles concentration and PSD sampled in the water column at two large rivers. Although excellent studies already exist [19][20][21][22][23][24][25], which aim to clarify these relationships, practical applications are mainly focused on a specific investigated case study that bore out an ADCP specific method [13,15,18,22]. Therefore, our aim was to give an answer on the practical need of evaluating the applicability of different acoustic methods.…”

A sediment fluxes investigation for the 2-D modelling of large river morphodynamics

“…The sediment transport is commonly classified based on the following dominant mechanisms: the bed-load is referred to as the transport occurring near the riverbed in which the stream flow sweeps the bed particles over the underlying sand bedforms or the gravel bed and the suspended load in which the particles are transported fully suspended in the water column. The suspended-load, containing the finer particles, often represents the majority of the transported sediments reaching the sea; therefore, the measurement technologies have primarily focused on the flux of the suspended materials [1][2][3][4][5][6][7]. However, the morphology of large rivers is also influenced by sand fluxes close to the bottom.…”

“…New methodologies [18][19][20] have overcome the traditional limited measurement devices used in the past; consequently, the data taken by these new methods may assist the numerical models simulation for a better understanding of the riverbed dynamics. Innovative instrumentations employed in river surveys include Acoustic Doppler Current Profilers (ADCPs), which could be used to characterise the secondary flows and the vertically averaged velocity patterns [22,22], to evaluate the shear stress by fitting the logarithmic velocity profiles [16,23] and to study the suspended-load [2,7,24]. Concerning the sand-beds, acoustic technologies have also been applied to track the bedforms, eventually estimating the bed-load.…”