Phd position :Experimental and numerical analysis of sediment remobilization modes under unsteady flows

Thesis work: This thesis builds on the work of Florent Grattepanche, who showed that the instationarity of hydrographs can induce distinct remobilization modes directly linked to the temporal shape of the hydrographs. These results highlight the need to analyze the parameters describing non-stationarity, beyond maximum flow alone, in order to better understand the mechanisms governing the stability of localized sediment inputs. The main objective of this thesis is to conduct a fundamental analysis of the modes of sediment remobilization subjected to idealized and controlled hydrographs, based on clearly identified temporal parameters. Simplified hydrographs will be considered in order to separate the effects of maximum flow, characteristic rise and fall times, and associated accelerations. This choice is consistent with a modeling approach aimed at identifying the dominant mechanisms, as is typically sought in fundamental studies of unsteady hydraulics. The analysis may be extended to multi-peak hydrographs, allowing the effect of successive stresses on remobilization dynamics to be studied and phenomena already observed in unsteady transport contexts to be explored. The influence of the initial geometry of the sediment supply will also be studied. The shape of the deposit (height, characteristic length, upstream and downstream slopes, symmetry) determines the spatial distribution of hydrodynamic stresses and forces exerted on the sediments, and plays a decisive role in destabilization mechanisms. The methodological approach will be based on a close combination of laboratory experiments and numerical simulations. The experiments will be conducted in a hydraulic channel, with sediment inputs of controlled geometry subjected to precisely defined unsteady hydrographs. The velocity fields and their temporal evolution will be characterized using non-intrusive measurement techniques, such as particle image velocimetry (PIV), to assess acceleration effects and stress variations in the vicinity of the deposit. In parallel, numerical simulations of unsteady flows coupled with sediment transport will be developed to complement the experimental analysis. This work aims to identify the parameters of unsteadiness governing the remobilization of sediment inputs and to determine critical thresholds that depend jointly on the temporal dynamics of the flow and the geometry of the deposit. The expected results will contribute to a better fundamental understanding of the processes of erosion and sediment transport under unsteady flows, and to the improvement of existing models. Required profile/areas of competence: Master's degree in Fluid Mechanics, Environment, or Engineering Sciences. Writing skills in English would be appreciated. Duration: 36 months (from October 1, 2026, to October 1, 2029) Location: Institut Pprime, Département Fluides, Thermique et Combustion, Université de Poitiers, UPR 3346, 11 Boulevard Marie et Pierre Curie, TSA 51124 86073 Poitiers Cédex 9, France