Contaminated sediments are common in river networks. The flow convergence and particular flow structures in confluences, such as the flow separation zone, may result in greater accumulation of contaminated sediments than in other river locations, but this issue is rarely studied. In addition, the contaminated sediment transport is driven by the particular morphodynamics occurring at confluences. This article describes a novel confluence flume experiment on both morphodynamics and deposition patterns of contaminated sediments as a function of geometric and flow conditions. The initial equilibrium bed geometry was developed from a mobile bed and then fixed, allowing for subsequent flow velocimetry and sediment feeding. Colored sediments of fine gradation, which mimic contaminated sediments, were then fed to the tributary channel. The results suggest that the junction angle primarily determines the confluence bed morphology and sediment transport pattern while the discharge ratio is a secondary factor. It was also observed that most introduced sediments tended to deposit immediately after the cessation of feeding at the stoss of the bar in the flow separation zone. The time history of the transport of the contaminated sediments was also investigated. Sediment that initially deposited at the stoss of the bar eventually moved to the lee of the bar and deposited around the bar downstream of the confluence, demonstrating that the sedimentation pattern evolved to a state similar to the equilibrium bed morphology.

The fluid dynamics of channel confluences are highly complex due to flow separation and secondary currents. Although numerous studies in the past few decades have focused on the numerical simulation of confluence flow, deformed beds were rarely used. This study attempts to address this issue through numerical simulation of the flow behavior in an open-channel confluence flume with an equilibrium degraded bed in OpenFOAM (version 6.0) to compare the results with a flatbed. In the present study, different turbulence models, including Reynolds-Averaged Navier–Stokes (RANS), large-eddy simulation (LES), and detached eddy simulation (DES) models were performed using rigid-lid and volume-of-fluid (VoF) methods. The accuracy of the models was statistically analyzed by comparing them with observation data. The results demonstrated that the LES model had the best performance, with a minimum average normalized root-mean-square error (NRMSE) of 3% under the VoF assumption. The investigation also further illuminated the intricate interplay of vortical structures within the confluence zone. Notably, the number and behavior of vortices were found to be influenced by channel geometry and size, as well as interactions between separated shear layers. Circulation within the separation zone near the inner bank differed in rotation between the degraded and flatbed cases.

Despite decades of research on hydro‐morphodynamic processes at open‐channel confluences, significant discrepancies persist between flume experiments and field observations. This study investigated the underlying causes by compiling and comparing geometric and hydraulic parameters from both natural confluences and laboratory setups. The analysis suggested that these discrepancies largely stem from unrealistic boundary conditions commonly used in experimental designs. To address this issue, this study developed an improved physical model of concordant confluences that more accurately replicated the morpho‐hydraulic characteristics of natural confluences. Key features included a smooth downstream junction, a large post‐confluence width‐to‐depth ratio, downstream channel widening, representative junction angle and discharge ratio. This experiment avoided the unrealistic large separation zone and scour holes near the downstream junction corner caused by sharp‐angled junction. Large Reynolds stresses and turbulent kinetic energy within the shear layer primarily drove scour hole formation, while streamwise‐oriented vortical cells offered additional contributions. In contrast, flow acceleration along the main channel promoted scour step development through low‐intensity sediment redistribution. This study presented a more realistic and representative physical model for simulating hydro‐morphodynamics at confluences with concordant beds and helped bridge the gap between laboratory findings and field‐scale dynamics.

Due to the lack of numerical modeling and continuous in-situ monitoring of flow and sediment transport data, the mechanism of local-scale morphodynamic processes during very large floods are not fully understood. Thus, this study employs Delft3D to develop a two-dimensional (2-D) morphodynamic model to simulate and analyze the morphodynamic process of a gravel-bed river during an 80-year flood event in Calgary, Canada. The model was calibrated using velocimetry data and validated against the measured post-flood bed elevation data. The coefficient of determination (R2) and ratio of the root-mean-square error to the standard deviation (RSR) between the modeled and measured bed elevation was 90% and 0.33, respectively, which demonstrates the reliability of the model. The modeled flow velocity, bedload transport rate, surface sediment sizes, and corresponding morphological changes at different flood stages are presented and analyzed. Results show that bed incision and the mid-channel bar continuously developed throughout the flood while bank erosion and the growth of bank-attached bar mainly happened during the rising and peak periods. We found that the timing and duration of major morphological changes during a flood event varies from site to site within a reach, but is similar for similar morphological units. We also found that the spatial variation of channel planform is the dominant determinant of major morphological changes during floods, while flood events trigger the sediment motion and result in actual deposition or erosion. Improvements are needed in terms of the modeling of bedload transport and bed stratigraphy.