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

Abstract

Braided rivers have complicated and dynamic bar patterns, which are challenging tofully understand and to predict both qualitatively and quantitatively. Linear theoryignores nonlinear processes that dominate fully developed bars, whereas natural riverpatterns are determined by the combined effects of boundary conditions, initial conditionssuch as planimetric forcing by fixed banks and the physical processes. Here we determinethe capability of a state-of-the-art physics-based morphological model to r eproducemorphology and dynamics characteristic of braided rivers and determine the modelsensitivity to generally used constitutive relations for flow and sediment transport. We usethe 2-D depth-averaged morphodynamic model Delft3D, which includes the necessaryspiral flow and bed slope effects on morphology. We present idealized scenarios with thesmallest possible number of enforced details in the planform and boundary conditions inorder 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 ofcomplementary metrics and compare these with imagery, field data captured in empiricalrelations, flume experiments, and predictions by linear analyses. The results show that thechosen set of boundary conditions and physics in the numerical model is sufficient toproduce many morphological characteristics and dynamics of a braided river butinsufficient for long-term modeling. Initially, braiding intensity with low-amplitude barsis high in agreement with linear analysis. In a second stage when bars merge, split, andincrease amplitude up to the water surface, the shape, size, and dynamics of individualbars compare well to those in natural rivers. However, long-term modeling results in areduction of bar and channel dynamics and formation of exaggerated bar height andlength. This suggests that additional processes, such as physics-based bank erosion, orenforced fluctuations in boundary conditions, such as spatial-temporal dischargevariation, are necessary for the simulation of a dynamic equilibrium river. The mostimportant outcome is that the modeled pattern of bars and channels is highly sensitive tothe 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 studyshows that physics-based modeling of sand-bed braided improves our understanding andprediction of morphological patterns and dynamics in sand-bed braided rivers.