A method of using two acoustic Doppler current profilers operating at different frequencies and employed at the same measuring vertical to sample a profile of suspended sediment concentration has been previously applied in the Parana River (Argentina) but has not been validated by direct sediment samples. The present work fills this gap by reporting new field data and comparing them with acoustically inferred sediment concentrations. The agreement between directly measured sediment concentrations and grain sizes with corresponding estimates from an employed backscatter model was found to be good (squared correlation coefficients are 0.9 and 0.8, and mean deviations are 14 and 6%, respectively). The interrelations between flow velocity and suspended sediment concentration at fixed locations and in a moving mode along a river cross-section have been also investigated. Observed events of bed sediment re-suspension were found to be highly correlated with fluctuations of the vertical flow velocity, with a 100-150s quasi-periodicity. The size of re-suspension plumes was increasing from the channel thalweg to the low-submerged bar areas.

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%.

Abstract:The use of echo-levels from Acoustic Doppler Current Profiler (ADCP) recordings has become more and more common for estimating suspended bed-material and wash loads in rivers over the last decade. Empirical, semi-empirical and physical-based acoustic methods have been applied in different case studies, which provided relationships between scattering particles features derived from samples (i.e., concentration and grain size) and corresponding backscattering strength and sound attenuation. These methods entail different assumptions regarding sediment heterogeneity in the ensonified volume (e.g., particle size distribution (PSD) and spatial concentration gradient). Our work was to compare acoustic backscatter and attenuation properties of suspended sediments, sampled in the rivers Parana and Danube that represented rather different hydro-sedimentological conditions during the surveys. The Parana represents a large sandy river, characterized through a huge watershed and the typical bimodal PSD of sediment in suspension, while the Danube represents in the investigated reach an exposed sand-gravel bed and clay-silt particles transported in the water column in suspension. Sand and clay-silt concentrations clearly dominate the analyzed backscattering strength in the rivers Parana and Danube, respectively, with an effect of PSD level of sorting in the latter case. This comparison clarifies the extent of assumptions made, eventually advising on the actual possibility of applying certain ADCP methods, depending on the expected concentration gradients and PSD of suspended sediment to be investigated.

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