Laboratory study of influence of nose angle on sediment distribution at channel bifurcation

Abstract

Bifurcations are typical features in alluvial rivers as. well as in estuaries. The morphological behaviour of bifurcation is not as yet a properly understood phenomenon. This is why river engineers are confronted with this problem. The complexity of the bifurcation lies in the determination of the sediment distribution ratio of the downstream branches. The distribution ratio is determined by the local three dimensional flow pattern. The determination of the distribution ratio is a difficult task. In order to get some idea about how the sediments distribute over the downstream branches, a study has been carried out using a physical model at the Hydraulics and River Engineering Laboratory of Bangladesh University of Engineering and Technology. This study describes the influence of nose angle (the angle between the tip of the nose and the symmetrical line of a bifurcation) on sediment distribution at channel bifurcation. A total of four different noses have been used to investigate the influence. For each nose, three upstream discharges (e.g. 20 lis, 30 lis and 40 lis) have been used. From the experiments, a set of data on q/'h and s/s, (where q1' q, and SI' s, are the discharges and sediment transports per unit width through branch I and 2 respectively) have been collected. These data have been set to the following nodal point relation (the relation between the ratio of downstream discharges and the ratio of the downstream sediment transport rates): !...l ~M(il)k Sz qz where M is a coefficient and k is an exponent. It has been found that the nose angle has a great influence on sediment distribution to the downstream branches. As the nose angle changes, the power and the coefficient of the nodal point relation change to a great extent. The value of coefficient, M in the nodal point relation increases as the discharge increases for nose angles of 6.97°,0° and -3.50°. For nose angle of -10.38°, the coefficient decreases with increase of discharge. For a particular upstream discharge the coefficient M increases as the nose angle changes from negative to positive (-10.38° 5f)~6.97°). The value of the exponent, k in the nodal point relation increases as the discharge increases when the nose angle is held constant. When the discharge is held constant at 30 lis, it is found that the maximum value of the coefficient, k occurs for symmetric nose (8 = 0°). Similar is the case for the upstream discharge of 40 lis. But when the upstream discharge is 20 lis, the maximum value of k is found for 8 = -10.38° It is concluded from this study that the distribution of sediment to the downstream branches is independent of upstream discharge.The nose angle is the major variable for the distribution of sediment.