Large Eddy simulation (LES) of flow and bedload transport at an idealized 90-degree diversion: Insight into Bulle-effect

Bifurcations are an integral part of river systems, thus better understanding of the dynamics flow and sediment transport is important for accurate prediction of the long-term geomorphological evolution of these systems. A class of bifurcations in which one of the bifurcating channels continues along the direction of the main channel, and the other comes out laterally, is also referred to as diversion. In 1926, Bulle conducted the first study that quantified the hydrodynamics and bedload transport at a diversion, and to this day his study remains one of the most extensive studies on the topic. Bulle put forth the phenomena of preferential movement of near-bed sediment at a diversion towards the lateral channel, thus this phenomenon is often referred to as the Bulle-Effect. In the current study, the governing mechanism of Bulle-Effect has been explored by conducting LES of the flow and bedload transport at an idealized 90-degree diversion. The scale of the simulated diversion and the bulk Reynolds number are similar to the experiments conducted by Bulle. The simulation was conducted using the open-source spectral element based Navier-Stokes solver Nek5000. Bed load transport was modeled using the Lagrangian particle tracking method. The simulation results clearly portray the tendency of the near bed currents at the bifurcation to move into the side-channel, consequently taking most of the near-bed sediment along with it. The results also affirm the presence of vortices in the both the channels after the diversion. The Dynamics of bedload transport at the bifurcation was captured successfully, with the percentage of total sediment entering the side-channel matching Bulle’s experimental observation. Apart from taking a detailed look at the mechanism behind Bulle-Effect, the current study is also one of the first to use highresolution LES and Lagrangian particle dynamics to study the hydrodynamics and bed-load transport dynamics at an experimental scale bifurcation.