Structure of turbulent flow in a meander bend of a lowland river

The effects of channel curvature on turbulent flow in meander bends have been the main focus of extensive experimental and theoretical research hitherto. This paper is motivated by growing evidence that riffle-pool morphology in bends can also have strong implications for the flow structure. Flow in a bend of a lowland river with a shallow riffle and a deep pool is examined in this study by using the results of field measurements. In the study, reach flow is quasi-uniform in the riffle. In the pool, two layers compose the vertical structure of flow with a quasi-uniform flow near the riverbed. The upper layer is affected by lateral advection of fluid with low momentum from the inner bank that results in reduced tangential velocities, submergence of velocity maximum, and almost zero shear stress. Bed shear stress is estimated from the local profiles of turbulent shear stress and is shown to comply with values computed with the modified length scale. The maximum bed shear stress is situated in the riffle while theoretical considerations based on the effect of curvature suggest the pool as the most probable location of the maximum. This fact supports the hypothesis that the effects of riffle-pool morphology are dominant for the study reach. The study contributes to the clarification of up-scaling results from laboratory experiments to natural rivers by providing a detailed analysis of turbulence profiles. It shows that parameters of turbulence profiles, scaled with the proper velocity and length scales, are similar to those reported by previous experimental studies for open-channel flow.