LAUSR

Bulle-Effect is a phenomenon in which a disproportionately higher amount of near-bed sediment load at a fluvial diversion moves into the diverted channel, even for cases in which the proportion of water (with respect to the main flow) entering the diversion channel is relatively small. This phenomenon has wide ranging implications for both engineered and natural systems; from efficient design of channels to redirect water and sediment for reclaiming sinking deltas, designing navigational channels that do not need frequent dredging, to morphological evolution of river bifurcations. The first ever, and one of the most extensive set of experiments conducted to explore this phenomenon, were conducted by Bulle in 1926. In the current study the experiments conducted by Bulle have been simulated using an open-source, free-surface finite-element based hydrodynamic solver. Main objectives were to explore to what extent the complex phenomenon of Bulle-Effect at the scale of a laboratory experiment can be simulated accurately using Reynolds-Averaged Navier-Stokes (RANS) based hydrodynamic solver, and to understand the details of the hydrodynamics that Bulle could not analyze through his experiments. The hydrodynamics captured by the simulations were found to match the observations made by Bulle through his experiments, and the distributions of sediment at the diversion predicted by the numerical simulations were found to match the general trend observed in the laboratory experiments. The results from the numerical simulations were also compared with existing one-dimensional models for sediment distribution at bifurcations, and the three-dimensional numerical model was found to perform appreciably better. This is expected due to the complex flow features at the diversion, which can only be captured satisfactorily using a three-dimensional hydrodynamic model.