Three-Dimensional Flow Structures in Bend-Straightened Sections within Continuous Bend Channels

To clarify the complex hydrodynamics of bend-straightened reaches inside continuous meandering rivers, three dimensional simulations were performed with a Reynolds Stress Model (RSM) coupled to the Volume of Fluid (VOF) method. The numerical experiments examined water surface fluctuations, velocity structures, and turbulent kinetic energy (TKE) distributions under different channel curvature ratios K and Froude numbers Fr . Channel stability, ecological value, and navigation management strategies were also evaluated. The results show that bifurcation and confluence nodes are hydraulically intricate sections of the straightened reach. They display marked water surface fluctuations, strong secondary flows, and clear shear layers that create separation zones, recirculation cells, flow deflection regions, and core flow corridors. Water surface change, separation zone size, and secondary flow strength respond almost linearly to Fr , but they respond nonlinearly to K . Successive straightened units interact with one another; an upstream shift of the velocity core and its secondary circulation can persist downstream, lowering the flow share through bends, enlarging downstream separation zone, weakening secondary flows, and amplifying water surface fluctuations. From a functional viewpoint, rectilinear reaches carry higher and more uniform velocities. This favors navigation but increases the risk of bed scouring. Bends, on the other hand, have lower velocities and more complex flow structures. Their separation and slack water zones encourage sediment deposition and create refuges that benefit aquatic habitats. Therefore, sequences of bend straightening units can help balance navigation efficiency with ecological enhancement in river management.