Hydrogeomorphology of Asymmetric Meandering Channels: Experiments and Field Evidence

Meandering channels display complex planform configurations with upstream (US)- and downstream (DS)-skewed bends. Bend orientation is linked to hydrodynamics, bed morphodynamic regime, bank characteristics, riparian vegetation, and geological environment, which are the modulating factors that act specially in high-amplitude and high-sinuosity conditions. Based on the interaction between hydrodynamics and morphodynamics, previous studies have suggested that sub- (beta < beta(R)) and super-resonant (beta > beta(R)) morphodynamic regimes (where beta is the half width-to-depth ratio of the channel, and beta(R) is the resonance condition) may trigger a particular bend orientation (upstream- and downstream-skewed, respectively). However, natural rivers exhibit both US-skewed and DS-skewed bend patterns along the same reach, independently of the morphodynamic regime. Little is known about the hydrogeomorphology (forced and free morphodynamic patterns) under these bend orientations. Herein, using the asymmetric Kinoshita laboratory channel, experiments under sub- and super-resonant conditions (with presence or absence of free bars) for upstream-and downstream-skewed conditions are performed. The experiments with migrating bars (beta = 10, 15) are compared with previous experiments where only migrating dunes (beta = 2) under sub-resonant condition were observed. Additional, detailed field measurements at US- and DS-skewed bends of different skewness along the Tigre River in Peru are also presented. Conditions at field scale filter out the influence of the morphodynamic regime in high-sinuosity and high amplitude bends, where nonlinear processes (width variation, bedform dynamics) can directly affect the development of the three-dimensional flow structure, and consequently the erosional and depositional patterns, and the lateral migration patterns.