Transport modes and suspension and deposition velocities of solid particles in rivers

International audienceThis presentation concerns the modelling of the transport mode and the suspension and deposition velocities of solids particles in rivers. The comparison between the transport modes typically defined according to the Rouse number and the movement’s thresholds of the Shields parameter revealed inconsistencies. They are assumed to be a consequence of the differences between the application domain of the Shields parameter, based on the friction of the water flow on the bed and the Rouse number based on the conditions in the turbulent area. It is proposed to reduce these inconsistencies by proposing a new regard on the erosion process which allows assessing the transport mode of particles according to their transfers from the river bed to the turbulent layers of the water flow. The presented model is inspired from the resistance approaches applied in the domain of aerosols with the assumption that the vertical flux of particles is constant whatever the depth and that the erosion process can be considered through two successive steps which are the abrasion of bottom sediments and the suspension of abraded particles. With these assumptions, the suspension and deposition velocities are expressed as a function of the velocities (of settling and Brownian and turbulent diffusion and the transport modes are determined according to the ratio between the suspension and deposition velocities of the unstructured particles. Because the thickness of the diffusive exchange layer is determined as a function of the particles sizes, the water flow conditions and the bed roughness, the model allows finally assessing the transport mode of particles as a function of their size, the water flow and the bed roughness. The comparisons between these transport modes and the movement’s thresholds of the Shields parameter shows a significant reduction of the inconsistencies previously observed with the Rouse number that highlights the important role, while often neglected, of the diffusive exchange layer located between the bed and the turbulent flow