Dynamic meandering in response to upstream perturbations and floodplain formation

- Bar-floodplain conversion creates 'inner bank push' that creates high-sinuosity meanders
- Low bar-floodplain conversion rates result in chute cutoffs and low-sinuosity meanders
- Lateral inflow perturbations enhance only high-sinuosity meandering

River meandering results from spatially alternating bank erosion and bar growth. Recent flume experiments and theory suggest that a continuous inflow perturbation is a requirement for sustained meandering. Furthermore, flume experiments suggest that bar-floodplain conversion is an additional requirement. Here, we tested the effects of continuous inflow perturbation and bar-floodplain conversion on meander migration using three numerical morphodynamic models: a 1D-model, and two 2D-models with one of them using adaptive moving grid. We focused on the interaction between bars and bends that leads to meander initiation, and the effect of different methods to model bank erosion and floodplain accretion processes on meander migration. The results showed that inflow perturbations have large effects on meander dynamics of high-sinuosity channels, with strong excitation when the inflow is periodically perturbed. In contrast, inflow perturbations have rather small effect in low-sinuosity channels. Steady alternate bars alone are insufficient to cause high-sinuosity meandering. For high-sinuosity meandering, bar-floodplain conversion is required that prevents chute-cutoffs and enhances flow asymmetry, whilst meandering with chute-cutoffs requires merely weak floodplain formation, and braiding occurs without floodplain formation. Thus, this study demonstrated that both dynamic upstream inflow perturbation and bar-floodplain conversion are required for sustained high-sinuosity meandering.