Investigating Intermittent Flow Stagnation at Channel Confluence of Braided River

Abstract

<p>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 <em>“Hamol”</em> 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 <em>“Hamol”</em> phenomenon observed practically in the field.</p>