Sediment distribution at channel bifurcation using two-dimensional curvilinear model

Abstract

The morphological behaviour of bifurcation in rivers is a poorly understood problem with which many river engineers are confronted. Bifurcation can be found in deltas, in estuaries and in braided rivers. In one-dimensional mathematical simulation of a river reach that contains bifurcation, some nodal point relation is needed to obtain its sediment distribution in those branches. Determination of this nodal point relation is a problematic task. While the sediment distribution at channel bifurcation is a threedimensional phenomenon, a two-dimensional mathematical model for morphological simulation with the inclusion of helical effect or secondary flow in the vicinity of bifurcation omits the need of nodal point relation to obtain its sediment distribution in the bifurcated channels. The present study deal the two-dimensional mathematical modelling under the morphological module of the sediment distribution at channel bifurcation with the use of curvilinear grid along its main and bifurcated channel. For this study purpose, software called MIKE21-C developed at Danish Hydraulic Institute, Denmark has been used. In the model set up, an additional programme for the generation of grids has been modified and adapted to fit the intrinsic curvatures occurring in the bifurcation. A grid number of 75 in the x-direction and 25 in the y-direction give a resolution of 0.16 m by 0.04 m in the specified directions, respectively. The model has been calibrated against the available physical model data conducted by Hannan (1995), Roosjen and Zwanenburg (1995) and Islam (1996). They conducted test runs in a bifurcation model constructed in the laboratory of Water Resources Engineering Department, Bangladesh University of Engineering and Technology. They used both symmetrical and asymmetrical noses for three discharge conditions, i.e., 20 lis, 30 lis and 40 lis. The present mathematical model reproduced the physical model results for symmetrical nose satisfactorily. After the calibration, the model has been applied for different sensitivity runs. The scenarios of the sensitivity analysis include different Chezy's number (25 m1 / 2/s, 30 mI/2/s, 35 mI/2/s), different sediment transport formulas as Engelund-Hansen and van Rijn, raised boundary water level in branch 2 by 5 mm and different grain size diameters (dso) such as 0.19 mm, 0.27 mm and 0.35 mm. From the two-dimensional mathematical model simulations, a set of data of discharge ratio and sediment ratio have been calculated. This data have been set to the nodal point relation 2= k(!iLr 'where k is the coefficient and m is the exponent. It has been found Sz qz that the value of exponent m increase with the increase of upstream discharges and is greater than 5/3 for all three upstream discharges which explain well with the findings of Hannan (1995) and Roosjen and Zwanenburg (1995). Sensitivity of the Chezy's number gives higher sediment transport with increase in Chezy's number for Engelund-Hansen formula. Sensitivity of different grain size has been tested using 0.19 mm, 0.27 mm and 0.35 mm diameter and it has been found that the present setup is valid for grain size greater than 0.19 mm. When testing sediment transport formula, with van Rijn sediment formula, a lower sediment transport volume has been found compared to Engelund-Hansen. Influence of boundary water on the sediment distribution at the bifurcation has been tested by raising the boundary water level at branch 2 by 5 mm. Due to raised boundary condition, flow through branch 2 is reduced and also the sediment transport and in the branch 1 flow and sediment transport has been increase and thus increase the discharge and sediment transport ratios compared to the same boundary level in the two branches. It is concluded from the study that sediment distribution at channel bifurcation is independent of upstream discharge for one type of nose and two-dimensional curvilinear modelling is a good tool for this type of study where several options and scenarios could be tested easily.