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Home > News > Ph.D. Thesis > Ph.D. Thesis 2014

Jeudi 13 novembre 2014, soutenance de thèse de Thibault REVIL-BAUDARD - 14h00, Amphithéâtre K118, site Bergès ENSE3

Experimental and modelling study of the sheet-flow regime of sediment transport.

Thesis supervisor

- M Eric Barthélémy,
Professor, Grenoble-INP, Co-supervisor
- M Julien Chauchat,
Associate Professor, Grenoble INP, Co-supervisor
- M David Hurther,
Research Officer, CNRS, Collaborator

Abstract

Sediment transport controls river morphological evolution, coastal erosion and ecosystem equilibrium. It represents a risk factor for populations and infrastructures. The sheet-flow, or intense bed-load, is a regime of sediment transport occurring during river floods or in the coastal wave breaking region above sandy beaches. The large amount of sediment transported in this regime is the main source for morphological evolution in our
natural systemswater bodies. A good understanding of the underlying physical processes is a pre-requisite for accurate morphodynamic predictions. However, particle-particle interactions and turbulent flow interactions, which are the main driving mechanisms in this problem, constitute the scientific bottlenecks for sheet-flow modelling. This deficiency is mainly caused by the lack of high resolution experimental data. Based on this observation,
the objective of the present thesis is to propose a novel two-phase model and to generate a new set of high resolution experiment data to improve process based sheet-flow modelling.

First, the two-phase flow model is presented and the obtained results are compared with data from the literature. The result analysis has shown that the dense granular flow rheology (μ(I)/φ(I)) combined with a turbulent mixing length concept predicts the main sheet flow characteristics over a wide range of flow and sediment properties. Secondly, the experimental set up providing high-rate measurements of velocity and concentration under a uniform sheet-flow regime is presented. Third, the measured mean flow quan-
tities are analysed to describe the vertical structure of the flow. The obtained results show that a mixing length formulation and a Rouse profile allow to describe the turbulent stress and the concentration profiles in the turbulent suspension layer, provided that the von Karman parameter and the Schmidt number are modified (κ ≈ 0.2 and σs = 0.44). The frictional rheology (μ(I)/φ(I)) and the kinetic theory of granular flows predict qualitatively the observed behaviour but fail to reproduce measurements quantitatively. The
observed link between the turbulent coherent structures and the bed dynamic illustrates the importance of flow fluctuations and intermittency. This coupling could be responsible for the discrepancy found between the predictions from the intergranular stresses models and the measurements. Finally, the comparison between the statistical analysis performed for a sheet-flow regime and for a clear water flow over a rough fixed bed demonstrates
that the turbulent kinetic energy is weakly affected by the presence of sediments whereas the turbulent correlation level between horizontal and vertical fluctuations is significantly reduced, leading to a decrease of both the mixing length and the turbulent eddy viscosity. An increase of the equivalent roughness height induced by the moving bed is also observed.

Keywords

Sediment transport, sheet-flow, two-phase flow, granular rheology, turbulence, acoustic profilers, ACVP, von Karman constant, Schmidt number.