Mobility of bi-disperse sediment beds in bedload transport

Bedload transport has major consequences for public safety, water resources and environmental sustainability. 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 mobility of bidisperse beds is studied with coupled fluid-Discrete Element Method (DEM) simulations of bedload transport. Initially, a large particle layer is deposited over a 10% slope bed made of small particles. A gravity-driven water free surface flow induces a downslope shear-driven granular flow of the erodible bed. It is observed that, for the same water flow conditions, the bedload transport rate is higher in the bidisperse configuration than in the monodisperse one. Depending on the Shields number and on the depth of the interface between small and large particles, different transport phenomenologies are observed, ranging from no influence of the small particles to small particles reaching the bed surface due to diffusive remixing. In cases where the small particles hardly mix with the overlying large particles and for the range of studied size ratios (r < 4), it is shown that the increase of mobility of the sediment bed is a granular effect, which can be explained within the mu(I) rheology framework. The buried small particles are more mobile than larger particles and play the role of a “conveyor belt” for the large particles at the surface. Based on rheological arguments, a simple predictive model is proposed for the additional transport in the bidisperse case. It reproduces quantitatively the DEM results for a large range of Shields numbers and for size ratios smaller than 4.

Finally, a phenomenological map is proposed. It presents the different transport regimes of bidisperse mixtures, depending on the mechanism responsible for the mobility of the small particles.