Experimental study of flow and sediment distribution at river offtake

Abstract

Off-take is a channel that bifurcates from the main channel. River offtake is one of the complex features in fluvial systems, like braiding, anabranching streams and deltas. In the process of developing offtakes, the distribution of flow and sediments along the branches are still a matter of research. Few works have been conducted in the past, however no notable experimental work has been carried out on bifurcation at off-take channel with special reference to Bangladesh. In this study, an experimental work has been conducted in the Hydraulics and River Engineering Laboratory of DWRE, BUET for understanding the flow and sediment distribution at river offtake by changing the discharge and offtake angles. A total of eighteen test runs have been conducted for three different discharge conditions with three different offtake angles i.e. 200, 400 and 600. Prior to test runs theoretical development has been made using dimensional analysis to obtain a functional relationship among the salient variables. For all the experimental runs, flow velocity, water depth and sediment concentration have been measured both for offtake and main channel. Along with the measurements, flow visualizations have also been carried out at the vicinity of the offtakes. It has been observed that turbulent eddies and zone of siltation were formed in the vicinity of offtakemouth(s). Sedimentation and erosion pattern of the main and offtake channel has also been observed. Water and sediment discharge ratios have been calculated for the offtake system. It has been found that, water and sediment discharge ratios increases with the increase of offtake angle. From the experiment, the measured flow velocity found to be increase with the increase of offtake angle. In order to estimate the sediment discharge three well known sediment transport formulas i.e. Ackers-White, Engelund-Hansen and Van Rijn formula have been used in this study. Comparison with the measured sediment discharge shows that the Ackers-White formula predicts reasonably better compared to other two. Based on theoretical development and experimental data, two relationships have been obtained in this study. These equations predict the water and sediment discharge ratios which are function of channel geometry and offtake angle. In addition, field data from selected river offtake systems of Bangladesh has also been used to verify the obtained relationships. The observation made in the study will help in understanding the flow and sediment behavior and it is hoped that proposed relationship developed in this study will be useful for the estimation the flow and sediment distribution of main and offtake channel providing a guideline for better offtake management of Bangladesh.