Coarse sediment transported by steep mountain tributaries during channelized flash flood events with intense bed load transport poses a significant threat to life, property, and infrastructure. Intense bedload transport occurring in tributary channels and insufficient transport capacity of the main channel flow can provoke flooding at the confluence. Deposition in the confluence can lead to overbank flooding and sedimentation into adjacent settlement areas. Extensive research has been carried out investigating lowland river confluences, where it has been determined that the confluence angle and discharge ratio have the most significant influence on channel morphology and hydraulics. However, there is a lack of information concerning confluences with low width and discharge ratios, high sediment concentrations, and gradients, typically found in steep mountain channels. This study presents results from large‐scale laboratory experiments coupled with numerical modelling with a standardized river confluence geometry. Bedload transport capacities, the shape, and volume of the deposition zone in the confluence, bedload dispersion characteristics, hydraulic and morphological dynamics, and spatial boundaries were analysed for various discharges and sediment concentrations. The confluence angle was 90°, sediment concentration was 5%, 7.5%, and 10%, and the discharge ratio was 0.1. The model was designed to accommodate scale factors of 20–40. With this configuration, a set of experiments based on steady‐state hydraulic conditions was accomplished. Results show that when the discharge ratio and confluence angle are constant, different morphologies occur, indicating that in addition to the confluence angle and the discharge ratio, the sediment concentration, flow velocity, and unit stream power significantly impact both hydraulic and morphologic zones in the confluences of mountain rivers. Additionally, backwater effects upstream of the confluence, and sedimentation in the tributary channel increases with increasing sediment concentration, which not only influences confluence morphodynamics and hydraulics but also the potential for overbanking of both channels.
Discharge behavior at side weirs is significantly influenced by the water surface profile along the weir crest. In the past century, different approaches were developed to describe this profile and the associated discharge coefficients. However, the application of these methods to practical problems poses a particular challenge, as a complex three-dimensional funnel is formed due to the discharge reduction, leading to significant uncertainties in determining the relevant flow depth. For this reason, a new approach for the determination of the discharge coefficient of side weirs was developed that refers to the undisturbed normal flow depth in the main channel. Based on a comprehensive parametric study utilizing 3D-numerical simulations, the influence of the weir and channel characteristics on the discharge behavior at the side weir was analyzed. A revised formula for estimating the discharge coefficient for side weirs with multiple weir fields was derived using multiple regression analyses. Validation of the numerical simulations was carried out by applying a physical scale model, showing good agreement between the results.
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