Physics-based modeling of large braided sand-bed rivers: Bar pattern formation, dynamics, and sensitivity

Abstract

.[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,

Network response to disturbances in large sand-bed braided rivers

“…Erodible floodplains in the simulations of Nicholas (2013) resulted in major local braidplain widening. In contrast, Schuurman et al (2013) reported a relatively small difference in bar pattern statistics between a braided channel with erodible floodplains and non-erodible walls. However, that model failed to produce sustained bar and branch dynamics that would cause bank erosion, because the grid resolution was too low to produce cross-bar channels and thus new bifurcations.…”

“…This approach is similar to our earlier work in Schuurman et al (2013) and Schuurman and Kleinhans (2015). The hydrodynamics were computed in three dimensions by applying conservation of momentum (Eqs.…”

“…Hence, a single disturbance in a large braiding river could affect an entire reach, beyond the extent of individual engineering projects. The bar and branch dynamics of braided rivers have been studied by means of flume experiments (e.g., Fujita, 1989;Ashmore, 1991a), numerical modeling (e.g., Nicholas, 2013;Schuurman et al, 2013;Yang et al, 2014), and field observations (e.g., Bristow, 1987;Klaassen and Masselink, 1992;Klaassen et al, 1993;Ashworth et al, 2000;Best et al, 2003). However, in these studies, any artificial constraints and disturbances such as non-erodible (flume) walls, engineering works (Fig.…”

“…Application of numerical models for decision making is common in, for example, the highly regulated river Rhine in the Netherlands. Furthermore, the natural behavior and general bar dynamics in large braiding rivers were successfully modeled by Nicholas (2013), Schuurman et al (2013), andYang et al (2014). However, the application of numerical models in morphologically dynamic rivers for decision making, especially in large braiding rivers, is still in its infancy.…”

“…Discharge variation is attenuated by human-made hydropower dams and water storages, and affects the downstream morphodynamics (Brandt, 2000). Many river modeling studies have applied constant discharge, assuming a morphologically dominant or representative discharge exists that gives similar yearly morphodynamics to the "real" hydrograph (e.g., Nicholas, 2010;Schuurman et al, 2013). However, other studies have shown that discharge variation has a large effect on river morphology (e.g., Kiss and Sipos, 2007;Crosato and Saleh, 2010) due, among other things, to vegetation colonization on exposed bar sections (Gordon and Meentemeyer, 2006;Tealdi et al, 2011).…”