Effect of Banks Morphology on Curved Open-Channel Bends Hydrodynamics

Predicting the flow field and the shear stresses in open-channel bends is vital for understanding natural river dynamics and for informing river engineering strategies. The flow in river bends is characterized by three dimensional patterns that are primarily induced by the interaction between centrifugal forces and inward transverse pressure gradients due to the elevated water level at the outer bend. The bend topography and the shape and roughness of the banks dictate the interaction between the local forcings and may lead to the development of flow features, such as flow separation and recirculation zones. Although many studies have focused on flows in open-channel bends, there are still some open questions regarding the influence of the bend characteristics in the flow, especially in sharply curved bends. This study investigated how key geometric factors, such as the shape and size of the inner point bar, the bend sharpness, and the arc length affect the hydrodynamics of open-channel bends using numerical modelling. The used models ranged from isotropic to non-isotropic Reynolds-averaged models to eddy-resolving models, to also determine the model complexity that is needed to capture the key flow features. For most of the tested cases, all models captured the mean velocities reasonably well but there were differences in the stresses and the turbulent kinetic energy. The Reynolds-averaged anisotropic model did not provide much better results than the isotropic model, highlighting the necessity for eddy-resolving models, especially when flow separation occurs. A parametric analysis clarified the individual roles of bend radius and width, while maintaining a constant ratio between the two. A decrease in bend radius increased peak bed shear stresses, whereas an increase in width reduced the shear stresses substantially. Shortening the arc length altered the local imbalance between gravitational and centrifugal forces, which led to increased velocities and shear stresses at the bend exit. Understanding how the modification of the bend characteristics influences the hydrodynamics can be particularly relevant in renaturalization of rivers that were straightened or fixed in their location, which involves remeandering and letting meandering rivers wander freely in their floodplains, respectively.