.[1] Braided rivers have complicated and dynamic bar patterns, which are challenging to fully understand and to predict both qualitatively and quantitatively. Linear theory ignores nonlinear processes that dominate fully developed bars, whereas natural river patterns are determined by the combined effects of boundary conditions, initial conditions such as planimetric forcing by fixed banks and the physical processes. Here we determine the capability of a state-of-the-art physics-based morphological model to reproduce morphology and dynamics characteristic of braided rivers and determine the model sensitivity to generally used constitutive relations for flow and sediment transport. We use the 2-D depth-averaged morphodynamic model Delft3D, which includes the necessary spiral flow and bed slope effects on morphology. We present idealized scenarios with the smallest possible number of enforced details in the planform and boundary conditions in order to allow free development of bars driven by the physical processes in the model. We analyze bar and channel shapes and dynamics quantified by a number of complementary metrics and compare these with imagery, field data captured in empirical relations, flume experiments, and predictions by linear analyses. The results show that the chosen set of boundary conditions and physics in the numerical model is sufficient to produce many morphological characteristics and dynamics of a braided river but insufficient for long-term modeling. Initially, braiding intensity with low-amplitude bars is high in agreement with linear analysis. In a second stage when bars merge, split, and increase amplitude up to the water surface, the shape, size, and dynamics of individual bars compare well to those in natural rivers. However, long-term modeling results in a reduction of bar and channel dynamics and formation of exaggerated bar height and length. This suggests that additional processes, such as physics-based bank erosion, or enforced fluctuations in boundary conditions, such as spatial-temporal discharge variation, are necessary for the simulation of a dynamic equilibrium river. The most important outcome is that the modeled pattern of bars and channels is highly sensitive to the constitutive relation for bed slope effects that is used in many morphological models. Regardless of this sensitivity and present model limitations of many models, this study shows that physics-based modeling of sand-bed braided improves our understanding and prediction of morphological patterns and dynamics in sand-bed braided rivers.Citation: Schuurman, F., W. A. Marra, and M. G. Kleinhans (2013), Physics-based modeling of large braided sand-bed rivers: Bar pattern formation, dynamics, and sensitivity,

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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).

Abstract.Morphodynamics in sand-bed braided rivers are associated to simultaneous evolution of mid-channel bars and channels on the braidplain. Bifurcations around mid-channel bars are key elements that divide discharge and sediment. This, in turn, may control the evolution of connected branches, with effects propagating to both upstream and downstream bifurcations. Recent works on bifurcation stability and development hypothesize major roles of secondary flow and gradient advantage. However, this has not been tested for channel networks within a fully developed dynamic braided river. A reason for this is a lack of detailed measurements with sufficient temporal and spatial length, covering multiple bifurcations. Therefore we used a physics-based numerical model to generate a dataset of bathymetry, flow and sediment transport of an 80-km river reach with self-formed braid bars and bifurcations. The study shows that bar dissection due to local transverse water surface gradients is the dominant bifurcation initiation mechanism, although conversion of unit bars into compound bars dominates in the initial stage of a braided river. Several bifurcation closure mechanisms are equally important. Furthermore, the study showed that nodal point relations for bifurcations are unable to predict short-term bifurcation evolution in a braided river. This is explained by occurrence of non-linear processes and non-uniformity within the branches, in particular migrating bars and larger-scale backwater-effects, which are not included in the nodal point relations. Planform morphology, on the other hand, has predictive capacity: bifurcation angle asymmetry and bar-tail limb shape are indicators for near-future bifurcation evolution. Remote sensing data has predictive value, for which we developed a conThis article is protected by copyright. All rights reserved.ceptual model for interactions between bars, bifurcations and channels in the network. We conducted a preliminary test of the conceptual model on satellite images of the Brahmaputra.

. (2014) 'Bifurcation instability and chute cuto development in meandering gravel-bed rivers. ', Geomorphology., Further information on publisher's website:http://dx.doi.org/10.1016/j.geomorph.2014.01.018Publisher's copyright statement: NOTICE: this is the author's version of a work that was accepted for publication in Geomorphology. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reected in this document. Changes may have been made to this work since it was submitted for publication. A denitive version was subsequently published in Geomorphology, 213, 15 May 2014, 10.1016/j.geomorph.2014.01.018. Additional information:Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractChute cutoffs reduce sinuosity of meandering rivers and potentially cause a transition from a single to a multiple channel river. The channel bifurcation of the main channel and the mouth of the incipient chute channel controls sediment and flow partitioning and development of the chute. Recent channel bifurcation models suggest that upstream bend radius, gradient advantage, inlet step, and upstream sediment supply at the bifurcation are important factors in the evolution of bifurcations. Our objective is to unravel the relative importance of these factors for chute cutoff success and development. We compare results from a morphodynamic three-dimensional (3D) model and a one-dimensional (1D) model with nodal-point relation with field observations of chute cutoffs in a meandering gravel-bed river. The balance between increased gradient advantage and flow curvature upstream of the chute channel bifurcation was systematically investigated with the 1D model. The 3D model runs and the field observations show the development of two types of chute cutoffs: a scroll-slough cutoff and a bend cutoff. The morphodynamic 3D model demonstrates that chutes are initiated when flow depth exceeds the floodplain elevation. Overbank flow and a significant gradient advantage result in a bend cutoff. The outcome of the 1D model shows that channel curvature at the bifurcation determines the success or failure of the chute cutoff when the chute channel is located at the inner bend, as in the case of scrollslough cutoffs. We conclude that chute initiation depends on floodplain characteristics, i.e., floodplain elevation, sediment composition, and the presence of vegetation. Chute cutoff success or failure is determined by the dynamics just upstream of the channel bi...

[1] Migrating alternate bars form in alluvial channels as a result of morphodynamic instability. Extensive literature can be found on their origin and short-term development, but their long-term evolution has been poorly studied so far. In particular, it is not clear whether migrating bars eventually reach a (dynamic) equilibrium, as in previous studies bars were observed to elongate with time. We studied the long-term evolution of alternate bars by performing two independent long-duration laboratory experiments and some numerical tests with a physics-based depth-averaged model. In a straight flume with constant water flow and sediment recirculation, migrating bars followed a cyclic variation. They became gradually longer and higher for a while, then quickly much shorter and lower. In one case, all migrating bars simultaneously vanished almost completely only to reform soon after. At the same time, steady bars, two to three times as long, progressively developed from upstream, gradually suppressing the migrating bars. We also observed simultaneous vanishing of migrating bars in an annular flume experiment, this time at intervals of 6-8 d. Numerical simulations of long alluvial channels with constant flow rate and fixed banks show periodic vanishing of a few migrating bars at a time, occurring at regular spacing. Under constant flow rates, migrating bars appear as a transition phenomenon of alluvial channels having a cyclic character. These observations, however, might hold only for certain morphodynamics conditions, which should be further investigated.

The braiding intensity and dynamics in large braiding rivers are well known to depend on peak discharges, but the response in braiding and channel–floodplain transformations to changes in discharge regime are poorly known. This modelling study addresses the morphodynamic effects of increasing annual peak discharges in braiding rivers. The study site is a braiding reach of the Upper Yellow River. We estimated the effects on the larger‐scale channel pattern, and on the smaller‐scale bars, channel branches and floodplains. Furthermore, we determined the sensitivity of the channel pattern to model input parameters. The results showed that the dominant effect of a higher peak discharge is the development of chute channels on the floodplains, formed by connecting head‐cut channels and avulsive channels. Widening of the main channel by bank erosion was found to be less dominant. In addition, sedimentation on the bars and floodplains increased with increasing peak discharge. The model results also showed that the modelled channel pattern is especially sensitive to parametrization of the bed slope effect, whereas the effect of median grain size was found to be relatively small. Copyright © 2018 John Wiley & Sons, Ltd.