.[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).

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