Lagrangian simulation of bedload‐sized particle trajectories at a 90° river confluence

Abstract Whilst confluence hydrodynamics are now very well known, their morphodynamics are less so. This is particularly true for bedload transport, including the trajectories followed by bedload supplied to a confluence by the main channel or the tributary, and their interactions. Field measurement of this phenomenon is currently difficult. Laboratory measurement allows study of gross effects, but not the detailed physics of the process. This paper applies a Lagrangian particle tracking model to a 90° discordant confluence, i.e. where the tributary enters the main channel at a height above the mainstream bed. The model was set up to represent the confluence of the Rhône and Avançon Rivers, Switzerland. The model is based upon a three‐dimensional solution of the incompressible Navier‐Stokes equations in the open‐source toolbox OpenFOAM. It applies a Delayed Detached Eddy Simulation. Particle movement allows for coupled interactions between particles as well as between particles and the stream bed. Results show that as bedload arrives at the tributary mouth, there is size segregation. Some particles from the tributary fall rapidly to the mainstream bed and then travel along the main channel margin to the zone where bank‐attached point bars commonly form. Others travel further into the main channel, notably if interactions between particles are allowed. The zone of maximum shear between the joining flows tends to have lower densities of particle tracks. There is sediment sorting at the junction with the very coarsest particles, due to interactions and momentum effects, and the very finest particles, due to greater ease of turbulent suspension, extending further into the main channel.