Three‐dimensional numerical modeling of the Bulle effect: the nonlinear distribution of near‐bed sediment at fluvial diversions

Abstract

The 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 . In the current study the experiments conducted by Bulle have been simulated using an open‐source, free‐surface finite‐element‐based hydrodynamic solver. The main objectives were to explore to what extent the complex phenomenon of the 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. Copyright © 2017 John Wiley & Sons, Ltd.

Morphologic adjustments of actively evolving highly curved neck cutoffs

“…The effect was quantified on diversions where one channel laterally branches off at a given angle from the main channel, which continues on a straight path. Research on these diversions has shown that a disproportionate amount of bedload sediment enters the diversion channel compared to the water discharge ratio between the two channels (Bulle, 1926;Dutta et al, 2017). The amount of bedload entering the diversion channel increases as the diversion angle increases from~10 to 120° (Bulle, 1926;Dutta et al, 2017).…”

“…Research on these diversions has shown that a disproportionate amount of bedload sediment enters the diversion channel compared to the water discharge ratio between the two channels (Bulle, 1926;Dutta et al, 2017). The amount of bedload entering the diversion channel increases as the diversion angle increases from~10 to 120° (Bulle, 1926;Dutta et al, 2017). This phenomenon is associated with near-bed flow being diverted into the diversion channel, which is responsible for transporting bedload into the channel, and near-surface flow primarily continuing into the main channel (Bulle, 1926;Dutta et al, 2017;Dutta and Garcia, 2018).…”

Morphodynamic assessment of side channel systems using a simple one‐dimensional bifurcation model and a comparison with aerial images

“…; Dutta et al. ). If flow separation occurs, the influence of the Bulle effect on the sediment partitioning seems small compared to the influence of the flow separation zone (De Heer and Mosselman, ; Van der Mark and Mosselman, ).…”

“…), sediment diversion due to the bifurcation angle (Bulle, ; Van der Mark and Mosselman, ; Dutta et al. ), partitioning of suspended load (Slingerland and Smith, ; Edmonds and Slingerland, ; Gaweesh and Meselhe, ), mixed sediment processes (Sloff et al. ; Frings and Kleinhans, ; Sloff and Mosselman, ; Kästner et al.…”

“…It is expected that the Bulle effect increases with an increasing bifurcation angle (De Heer and Mosselman, ; Van der Mark and Mosselman, ; Dutta et al. ). However, for large bifurcation angles a flow separation zone may develop (Figure ) (Bulle, ; Riad, ; Constantine et al.…”