Revisiting slope influence in turbulent bedload transport: consequences for vertical flow structure and transport rate scaling

Abstract

Gravity-driven turbulent bedload transport has been extensively studied over the past century in regard to its importance for Earth surface processes such as natural riverbed morphological evolution. In the present contribution, the influence of the longitudinal channel inclination angle on gravity-driven turbulent bedload transport is studied in an idealised framework considering steady and uniform flow conditions. From an analytical analysis based on the two-phase continuous equations, it is shown that : (i) the classical slope correction of the critical Shields number is based on an erroneous formulation of the buoyancy force, (ii) the influence of the slope is not restricted to the critical Shields number but affects the whole transport formula and (iii) pressure-driven and gravity-driven turbulent bedload transport are not equivalent from the slope influence standpoint. Analysing further the granular flow driving mechanisms, the longitudinal slope is shown to not only influence the fluid bed shear stress and the resistance of the granular bed, but also to affect the fluid flow inside the granular bed - responsible for the transition from bedload transport to debris flow. The relative influence of these coupled mechanisms allows us to understand the evolution of the vertical structure of the granular flow and to predict the transport rate scaling law as a function of a rescaled Shields number. The theoretical analysis is validated with coupled fluid-discrete element simulations of idealised gravity-driven turbulent bedload transport, performed over a wide range of Shields number values, density ratios and channel inclination angles. In particular, all the data are shown to collapse onto a master curve when considering the sediment transport rate as a function of the proposed rescaled Shields number.

References in corpus (3)

• Numerical simulation of turbulent sediment transport, from bed load to saltation
• Dense granular flow rheology in turbulent bedload transport
• A minimal coupled fluid-discrete element model for bedload transport

Cited by in corpus (9)

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• Universal friction law at granular solid-gas transition explains scaling of sediment transport load with excess fluid shear stress
• Turbidity currents propagating down an inclined slope: particle auto-suspension
• Mobility of bidisperse mixtures during bedload transport
• Fluid-particle interaction regimes during the evolution of turbidity currents from a coupled LES/DEM model
• Bridging the gap between particle-scale forces and continuum modelling of size segregation: application to bedload transport
• Addressing bedload flux variability due to grain shape effects and experimental channel geometry