Bedload transport (transport of particles by a flowing fluid along the bed by rolling, sliding and/or saltating) has major consequences for public safety, water resources and environmental sustainabilty. In mountains, steep slopes drive an intense transport of a wide range of grain sizes implying size sorting or segregation largely responsible for our limited ability to predict sediment flux and river morphology. Size segregation can lead to very complex and varied morphologies of bed surface and subsurface, including armouring, and can drastically modify the fluvial morphology equilibrium. In this work, the transport rate of an armoured bed, made of large particles on top of a small particles bed, is studied. In order to gain understanding of this process, bedload transport numerical experiments of two-size particle mixtures were carried out, using a coupled Eulerian-Lagrangian fluid-discrete element model validated with experiments (Maurin et al. 2015, 2016). It is composed of a 3D discrete element model (based on the open source code Yade), describing each individual particle, coupled with a one dimensional Reynolds Average Navier Stokes model (Chauchat 2017). A 3D 10% steep domain (angle of 5.71°) is considered. Three different configurations are compared: 2 layers or 4 layers of 6mm particles deposited on top of a bed composed of 3mm particles, and a monodisperse case with only 6mm large particles. The bed is then submitted to a turbulent, hydraulically rough and supercritical water flow until steady transport rate. Shields numbers ranging from 0.1 to 0.5 are considered. The numerical experiments show that in all three configurations, the transport law, relating the dimensionless transport rate to the shields number, is a power law. In addition, it is observed that for the same Shields number, the transport rate is higher in the bidisperse cases than in the monodisperse case. This result can be explained by the rheological properties of bidisperse granular media. Finally, we show that the particles at the interface between large and small particles should be in motion in order to have an increase of particle mobility.

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• Maurin R, Chauchat J, Chareyre B, Frey P. 2015. A minimal coupled fluid-discrete element model for bedload transport. Physics of Fluids 27(11): 113302. hal-01399572
• Maurin R, Chauchat J, Frey P. 2016. Dense granular flow rheology in turbulent bedload transport. Journal of Fluid Mechanics 804: 490-512. hal-01399574