Modeling dynamic equilibrium in a braided river using a fully coupled two-dimensional hydro-morphodynamics framework

ABSTRACT Sediment transport governs the dynamic exchange of sediment and flow that underpins channel–floodplain interactions and is a primary controller of fluvial morphology in large alluvial braided rivers. Quantitative analysis of sediment flux is essential to determine whether the river bed is in a dynamic equilibrium or undergoing sedimentation and/or erosion under varying flow conditions. This work depicts the representation and understanding of the physical processes governing river morphodynamics by developing a fully coupled hydro-morphodynamic model for a braided reach of the Brahmaputra River at Nimatighat, India. The Van Rijn sediment transport model is coupled with the 2D Shallow Water Equations (SWEs) to calculate the hydraulic parameters and the bedload flux. For validation, a few benchmark case studies were taken from published literature. A good agreement with R2 = 0.99, RMSE = 0.002 m, and Pbias = 1.067% was found between the simulated results and experimental data across different scenarios. The framework was applied to the Brahmaputra River to investigate the equilibrium geomorphic response to anthropogenic interventions, including mid-channel sandbar dredging. The study further evaluated the model's ability to mitigate bank erosion in the vulnerable reach upstream of a bull-head structure along the outer bank. We found that dredging had minimal effect in controlling the erosion in the eroded bank due to the immediate sedimentation of the dredged bed to attain equilibrium.