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Développement d’indicateurs de la dynamique spatio-temporelle sédimentaire d’un cours d’eau mesurés par acoustique passive

Abstract : This thesis deals with theoretical and experimental concepts of passive acoustic monitoring of sediment transport in rivers. Hence, the motivation is the use of hydrophones to sense the sound pressure generated by impacts between the sediments that are transported on the bed river. The technique is very cheap and easy to deploy on the field but it lacks of knowledge on the nature of the river soundscape’s acoustic sources. In order to separate the various types of noise sources composing the soundscape, a spectral analysis is frequently used to detect the bedload noise passband. The bedload spectral information is used in this thesis to estimate the transported, or bedload, grain size distribution. The study is based on the physical evidence of the existence of a dependency between the size of impacting particles and the frequency of the impacts. Therefore, the spatial and temporal dynamics of the bedload grain size distributions in gravel rivers will be assessed by developing acoustic spectra indexes.Firstly, the analytic solutions of hertzian impact between two rigid spheres and between a sphere and a slab are studied. The spectrum’s center and peak frequencies are most sensible on the grain size and then on the impact velocity. The analytic solutions and grain size distributions are used to model bedload acoustic spectra. Such model is sensible on the grain size distribution shape followed by the impact velocities of sediments. Its definition does not include non-linear transmission losses, i.e. attenuation with frequency due to scattering and absorption effects, and also the impact velocity is constant no matter the dimension of particles.Secondly, the bedload acoustic model is used for implementing inversion methods to estimate the grain size distributions. Such a method is defined in a least square algebraically framework, named the Non-Negative Least Square method, and uses analytical solutions of hertzian impacts to inverse the acoustic spectra. Field measurements on two large gravel rivers like Isère River, in Grenoble, France, and Drau River, at Dellach, in Austria, revealed coherent results as validated by physical sampling trials of bedload transport. It was observed a spatial variability in the estimated grain size distribution across the Isère River whereas a temporal variability was observed from the inversion of Drau River’s spectra.The previous bedload spectral model is enhanced by including concepts from the physics of fractional transport rate in gravel rivers, of particle saltation model and acoustic models of propagation. One can model acoustic maps of bedload noise from spatializing the impact rates at the reach scale. Here, the model is tests to localize the bedload noise in the Isère River’s cross-section by matching the measured spectra to the modeled ones. The acoustic maps obtained from this model are successfully predicted as validated by the measured maps in the Isère River in Grenoble.
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Teodor Petrut. Développement d’indicateurs de la dynamique spatio-temporelle sédimentaire d’un cours d’eau mesurés par acoustique passive. Hydrologie. Université Grenoble Alpes, 2017. Français. ⟨NNT : 2017GREAU028⟩. ⟨tel-01697869⟩

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