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Mesure acoustique des sédiments en suspension dans les rivières

Abstract : With this PhD, we have tried to develop new measurement methods, based on acoustic backscattering, to estimate the mass concentration of suspended sediments in rivers. These sediments often show a bimodal grain-size distribution, i.e. composed of a mixture of fine and sand particles. The main advantage of hydroacoustic methods is their ability to provide measurements with a much better spatial and temporal resolution than conventional sampling techniques. The ultimate goal is to improve the sediment load estimation in rivers. Multifrequency acoustic measurements, combined with physical sampling, were carried out in the field and in an experimental laboratory tank. Acoustic inversion methods were tested and developed on a homogeneous suspension of fine sediments in the laboratory. A new method was implemented, combining the analysis of acoustic backscatter and attenuation, and led to retrieve the sediment mass concentration with a precision in the order of ± 20%. In rivers, a semi-empirical method combining calibration data and acoustic inversion at two frequencies has been developed, allowing, under certain conditions, to estimate the concentration of fine and sand sediments throughout the entire river cross-section. This result confirms the ability of hydroacoustic technology to provide spatial information on the suspension. Significant differences were frequently observed between the theoretical acoustic response, computed from concentration and particle size data, and the acoustic measurements in rivers. It seems that these differences could be due to the presence of other scatterers in rivers, probably flocs and/or air micro-bubbles. This work calls for the development of a more efficient theoretical framework suitable for river suspensions.
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Adrien Vergne. Mesure acoustique des sédiments en suspension dans les rivières. Autre. Université Grenoble Alpes, 2018. Français. ⟨NNT : 2018GREAU046⟩. ⟨tel-02115348⟩

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