Experiments in a high-amplitude Kinoshita meandering channel: 1. Implications of bend orientation on mean and turbulent flow structure

Meandering rivers exhibit complex planform patterns with both upstream and downstream valley oriented meander bends. In order to describe the effects of bend orientation on long-term river evolution, it is of great importance to be able to describe bend orientation (curvature) effects on the hydrodynamics of the flow as a first approximation. Mean flow and turbulence characteristics were investigated experimentally in a periodic, asymmetric, meandering channel herein called "the Kinoshita channel". The channel planform configuration retains high-order harmonic modes. Upstream and downstream valley oriented meander bends can be studied by reversing the flow. A flat, smooth bed (without sediment) condition has been considered to avoid further complexity. Spatial distributions of mean flow (e.g., velocities) and turbulence parameters (Reynolds stresses, turbulent kinetic energy) were observed at several cross sections along the meander wavelength. Measurements show that at the bend apex, the core of maximum velocity is found near the inner bank for both planform orientations. At the same cross section, observations show that when bends are oriented upstream valley, the secondary flow is not as well developed as in the case where bends are oriented downstream valley. Furthermore, for the upstream condition the energy gradient is smaller than that for the downstream condition, suggesting that the friction (i.e., flow resistance) due to curvature is higher for the downstream-skewed condition. Implications about having upstream and downstream bends in the meandering river migration framework are also discussed herein.