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Caractérisation de la morphologie des dunes dans des écoulements unidirectionnels et alternatifs

Abstract : This thesis deals with the morphodynamics of subaqueous dunes under a turbulent flow. The equilibrium State of the dunes under stationary flow, and then the return to equilibrium state after a change in the flow direction are characterized. The variation of the shape parameters (height, length, aspect ratio) and the migration speed of dunes are examined as a function of the mass m0 of sediment and the flow rate. This study is conducted experimentally into a narrow, closed flume and the results are compared to predictions of a 2D dune model that takes into account the transport relaxation process. For steady dunes, this model predicts a "small dune" and a "large dune" régime where the dune height respectively increases as m0 and √m0. These regimes are separated by a transitional régime where the dune height is proportional to m0 with an exponent ranging from 0.5 to 1. These régimes are identical to those predicted by the model of Kroy et al. (2002) for æolian barchans.Experimentally, the steady dunes range from the “transition” to the "large dune" régime as the mass increases. This agreement allows to assess the saturation length of the transport. The result is consistent with the deposition length proposed by Lajeunesse et al. (2010). Concerning the return to the steady-state shape after changing in the flow direction, two transient scenarios are identified. The first one is a contraction followed by an elongation of the dune shape. In the second one, there is an additional contraction phase associated with an elongation of the dune beyond its initial equilibrium length. For either scenarios, the return-to equilibrium time is not proportional to the mass of the dune.
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Serge Kiki Sandoungout. Caractérisation de la morphologie des dunes dans des écoulements unidirectionnels et alternatifs. Sciences de la Terre. Université de Bretagne occidentale - Brest, 2019. Français. ⟨NNT : 2019BRES0045⟩. ⟨tel-02453140⟩



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