Flow structure in a local meander bend of a gravel-bed river

Abstract

This study investigates the spatial variability of flow structure in a natural meandering bend of the Kheir- Abad River, a gravel-bed mountain stream in southwestern Iran, with emphasis on velocity distribution, boundary shear stress, and secondary circulation. Field measurements were conducted along a 270 m reach (arc angle 113°) across nineteen cross-sections, revealing a highly heterogeneous flow field characterized by a central high-velocity core and reduced velocities near the banks. Comparison with HEC-RAS simulations shows that the model captures the general spatial organization of velocity and shear stress, but produces a smoothed representation and underestimates localized variations, particularly in downstream sections where field data indicate pronounced acceleration and peak shear stress values (up to 84.1 Pa). The observed shear stress distribution deviates from classical outer-bank dominance, with maximum values frequently occurring in the central channel and downstream regions, reflecting the influence of coarse bed roughness, complex secondary flow structures, and thalweg-controlled momentum concentration. Analysis of the secondary flow coefficient ( C ) supports this interpretation, with peak values coinciding with zones of increased velocity and shear stress, indicating intensified momentum exchange and enhanced near-bed transport capacity. Overall, the results demonstrate that flow in gravel-bed meanders is governed by strongly non-uniform and three-dimensional processes that cannot be fully captured by simplified hydraulic assumptions, and that integrating field measurements with numerical modeling provides a robust framework for interpreting flow dynamics and their morphodynamic implications.