Dynamic meandering in response to upstream perturbations and floodplain formation

Abstract

10River 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 highsinuosity 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 chutecutoffs 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 barfloodplain conversion are required for sustained high-sinuosity meandering. Figure 1: Examples of a. High-sinuosity meandering river characterized by abundance of vegetation along the river, a lack of bars, and neck-cutoffs (Rio Purus, Brazil); b. Low-sinuosity meandering river with low vegetation density and chute-cutoffs (Allier, France); c. Meandering river with bars forced by channel curvature (Wabash River, USA); d. Meander bend with free bars, including mid-channel bars (Rio Parnaiba, Brazil); e. Low-sinuosity river with free alternate bars (Cross River, Nigeria); f. Asymmetrical shape of the alternate bars with bar-tail limbs at the downstream (Indus River, Pakistan). Flow in all examples is from left to right. Source: BingMaps (c, d, f) and GoogleEarth (a, b, e).

Effects of mud supply on large-scale estuary morphology and development over centuries to millennia

“…The bars in the models are also slightly bigger with marine mud supply rather than for fluvial mud supply. The braiding index is strongly related to estuary width as found for natural estuaries (Leuven et al, 2016) and in agreement with the relation between tendencies to form floodplains in rivers and the resulting relation between channel aspect ratio and bar pattern (Kleinhans, 2010;Kleinhans and Van den Berg, 2011;Schuurman et al, 2016).…”

“…Discharge is prescribed as a constant value through time of 100 m 3 s −1 over the inflow cross section. However, the partitioning of discharge between the upstream grid cells of the river at the boundary varies sinusoidally through time from one side to the other to simulate weak upstream "meandering" perturbations with a period of 500 years to trigger bars if the self-formed channel aspect ratio would allow bars (Schuurman et al, 2016).…”

“…In rivers, the formation of cohesive floodplains with mud and vegetation causes river channels to be narrower and deeper than in systems with only sand given otherwise equal conditions (Tal and Paola, 2007;Kleinhans, 2010;Van Dijk et al, 2013;Schuurman et al, 2016). This results from a dynamic balance between floodplain erosion by migration of channels and new floodplain formation by mud sedimentation and/or vegetation development.…”

Network response to disturbances in large sand-bed braided rivers

“…Furthermore, the opposite process of bank erosion, which is bar-floodplain conversion by, for example, vegetation encroachment, is not considered in Delft3D. The necessity of this bar-floodplain conversion for channel migration was demonstrated by Schuurman et al (2016), and the large effect of riparian vegetation on braiding river morphology has previously been demonstrated by, for example, Murray and Paola (2003) and Crosato and Saleh (2010). This missing mechanism must be included to fully understand the contribution of floodplain-channel interaction on the morphodynamics in braiding rivers.…”

“…Asymmetrical inflow stimulates bar and bend formation, which has been deployed in flume experiments to generate meander bends (e.g., Peakall et al, 2007;Van Dijk et al, 2012). Inflow asymmetry enhances the initiation of steady bars and subsequent channel bending that propagates over a distance of at least several meander lengths (Van Dijk et al, 2012), but the direct effect of the disturbance damps rapidly in the downstream direction (Struiksma et al, 1985;Schuurman et al, 2016). Linear theory also explained that a disturbance in a river with, among other things, a sufficient width-depth ratio amplifies in the downstream direction (Struiksma et al, 1985;Crosato and Mosselman, 2009;Kleinhans and Van den Berg, 2011).…”