Anurag Handique

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.

Confluences of rivers are classified as one of the most intricate hydrodynamic environments. The convergence of incoming flows at confluences generated by two rivers or subchannels in large braided rivers creates complex fluid motion patterns, including the growth of large-scale turbulence formations. A study on the dynamics of flow mixing and propagation from the adjoining channels could be tricky as the channels consisting of the confluence usually acts as a flow barrier to each other. The flow in the dominant channel would initially act as a virtual barrier to the incoming channel, intercepting its free movement to the main channel. Post this stagnation, the less dominant flow gain energy because of a temporary rise in its level due to flow accumulation and eventually merge into the main stream in due course of time. However, when the confluence involves two channels of relatively similar strengths, primarily observed in large complex braided rivers, the complete or partial flow stagnation is observed in both the sub-channels alternatively in subsequent times. This distinctive flow phenomenon was first observed by our research team during 2004 while doing a hydrographic survey for modelling purpose in the Brahmaputra River of Assam, India, and is popularly called “Hamol” by the riverine community. Owing to this peculiar occurrence, there can be several implications on the river, such as changes in sediment dynamics, influencing the aquatic biota and habitat alterations, etc. For numerical river flow modelling, accurately simulating confluence hydrodynamics is a significant challenge. In this work, this phenomenon is investigated in a confluence composed of two sub-channels with different strengths through a mathematical model study. The model is simulated in the Brahmaputra River near Bahari, Barpeta from the viewpoint of its complex braiding patterns existent in this stretch. A TVD McCormack predictor corrector technique is used in the mathematical model to solve a modified form of boundary fitted shallow water equations in the MATLAB environment. The stability of the model is governed by the Courant criteria, with the Courant number being less than unity. Through the model study, the variations of the depth-averaged streamwise velocities and the simulated flow depths in the grid points prior to the confluence juncture in the less dominant channel are compared in different timesteps. Alongside, the characteristic of such transitions is examined for different discharge ratios of the two adjoining channels. The study indicated the efficacy of the mathematical model in capturing the “Hamol” phenomenon observed practically in the field.