Abstract
A sustained dynamic inflow perturbation and bar–floodplain conversion are considered crucial to dynamic meandering. Past experiments, one‐dimensional modelling and linear theory have demonstrated that the initiation and persistence of dynamic meandering require a periodic transverse motion of the inflow. However, it remains unknown whether the period of the inflow perturbation affects self‐formed meander dynamics. Here, we numerically study the effect of the inflow perturbation period on the development and meander dynamics of a chute‐cutoff‐dominated river, which requires two‐dimensional modelling with vegetation forming floodplain on bars. We extended the morphodynamic model Nays2D with growth and mortality rules of vegetation to allow for meandering. We tested the effect of a transversely migrating inflow boundary by varying the perturbation period between runs over an order of magnitude around typical modelled meander periods. Following the cutoff cascade after initial meander formation from a straight channel, all runs with sufficient vegetation show series of growing meanders terminated by chute cutoffs. This generates an intricate channel belt topography with point bar complexes truncated by chutes, oxbow lakes, and scroll‐bar‐related vegetation age patterns. The sinuosity, braiding index and meander period, which emerge from the inherent biomorphological feedback loops, are unrelated to the inflow perturbation period, although the spin‐up to dynamic equilibrium takes a longer time and distance for weak and absent inflow perturbations. This explains why, in previous experimental studies, dynamic meandering was only accomplished with a sustained upstream perturbation in flumes that were short relative to the meander wavelength. Our modelling of self‐formed meander patterns is evidence that scroll‐bar‐dominated and chute‐cutoff‐dominated meanders develop from downstream convecting instabilities. This insight extends to many more fluvial, estuarine and coastal systems in morphological models and experiments, which require sustained dynamic perturbations to form complex patterns and develop natural dynamics. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.
Highlights
A meandering river forms an elaborate floodplain topography through meander initiation, expansion and cutoff (Hickin and Nanson, 1975; Hooke, 2004)
Our findings show that morphology in state-of-the-art physics-based models requires continuous perturbation to maintain dynamics over the full length of the domain of limited length, which is in agreement with theory and experiments
The most striking dynamics that emerge with varying forcing are commonly observed in nature, i.e. the merging and splitting of bars leading to a change in the number or rip currents. No such patterns were reproduced by the model without continued perturbations enforced at one of the boundaries (Castelle and Ruessink, 2011). These findings strongly suggest that a dynamic perturbation on the boundary conditions is necessary in all physical and numerical models for fluvial, estuarine and coastal morphodynamics
Summary
A meandering river forms an elaborate floodplain topography through meander initiation, expansion and cutoff (Hickin and Nanson, 1975; Hooke, 2004). Hickin and Nanson, 1975; Furbish, 1988; Crosato, 2009) While this approach focuses on individual bends, it is evident, for example from the River Otofuke in Japan (Figure 1) and the River Allier in France (Kleinhans and Van den Berg, 2011), that the migration rate of one bend affects that of its downstream neighbours. The implication of the downstream propagation of bend instabilities is that perturbations need to be imposed on the upstream boundary of all numerical models and experiments to maintain dynamic meandering (Lanzoni and Seminara, 2006; Van Dijk et al, 2012; Weiss, 2016) It remains unclear what the period of such perturbations would need to be and whether that affects the modelled meander migration rate and chute cutoff frequency
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