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Influence of Upstream Discharge on Sediment Distribution at Channel Bifurcation

G. M. Tarekul Islam
Department of Civil Engineering, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

M. R. Kabir
Professor
Department of Water Resources Engineering, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh

Ainun Nishat
Professor
Department of Water Resources Engineering, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh

Abstract/ Executive Summary:

Experiments were conducted to gather information and to assess the influence of upstream discharge on sediments distribution at a channel bifurcation. The test rig consisted of a straight main channel, which bifurcated into two branch channels of different widths. Four different types of noses were used to study the phenomenon. For each nose, three upstream discharges viz. 20 l/s, 30 l/s and 40 l/s were used. It was found that for a particular nose angle and downstream discharge ratio, the sediment transport ratio in the downstream branches increases as the upstream discharge increases.

Keywords: Sediment, Channel bifurcation, Upstream discharge

Location: Bangladesh University of Engineering and Technology, Dhaka, Bangladesh

Start Date:

End Date:

Program Area: Socio-economic and Policy

Non-commodity: Performance

Objectives:

To assess the influence of upstream discharge of sediment distribution at channel bifurcation.

Methodology:

The experimental model of river bifurcation was a mobile bed with fixed banks. The layout of the model comprised of three branches: a main branch (denoted by branch 0) which bifurcated into two branches: branch 1 and 2. The main branch was straight and its length and width were 4.55 m and 1.00 m respectively. The width, length and radius of curvature of branch 1 and 2 were as follows: branch 1: width = 0.40 m, length = 8.6 m and radius = 23.5 m; branch 2: width = 0.60 m, length = 8.40 m and radius = 25.5 m. The detailed descriptions of the model could be found in Islam (1996). The circulation of water within the model was a closed system. From the downstream reservoir the water was transported by means of the pipeline to the upstream reservoir. A sand feeder placed at the beginning of branch 0 provided the supply of sediments. The amount of sediment depended on the equilibrium condition of the branch 0. Using the normal depths and the reference level, the initial bed was prepared for a particular upstream discharge and the downstream discharge ratio. In this experiment, three upstream discharges were used viz. 20 l/s, 30 Vs and 40 l/s. The average sediment loads were 18 kg/hr, 28 kg/h and 44 kg/hr for the three discharges respectively. The longitudinal slope was 0.0015 throughout the whole experiment. Four different types of noses were used in the study. Nose 1 was an asymmetrical nose in which the tip was directed towards branch 1 reducing the inflow area of that branch by 50% with respect to the symmetrical nose. Nose 2 was a symmetrical one, which meant the tip of the nose divided the inflow area according to the widths of the downstream branches. Nose 3 was designed in such a way that its tip divided the inflow area of the main branch equally. Nose 4 was another asymmetrical nose in which the tip was directed towards branch 2 reducing the inflow area of that branch by 50% with respect to the symmetrical nose. The nose angle θ was defined as the angle between the tip of the nose and the symmetrical line of a bifurcation. It was taken as positive when the tip rotated in the counter clockwise direction and negative in the clockwise direction from the symmetrical line. According to this definition, the nose angles of the four noses were 70, 00, -3.50 and -10.4° respectively. The variation of sediment transport ratio in the downstream branches S₁/S₂ with discharge ratio in the downstream branches q₁/q₂ for different nose angles. For a particular nose angle, there is a certain value of the discharge ratio for which the sediment transport ratio is same irrespective of the upstream discharges. This ratio is high when the tip of the nose is directed towards branch 2 and low when the tip is directed towards branch 1. The discharge ratio is 0.15, 0.60, 1.00 and 1.5 for nose angles of 7⁰, 0⁰, -3.5⁰, -10.4⁰ respectively for a particular nose angle and for all values of the upstream discharges. For a particular discharge ratio (q₁/q₂), the sediment transport ratio (S₁/S₂) increases as the upstream discharge increases. The rate at which sediment transport ratio increases or decreases is high in case of higher positive nose angle. A small change in discharge ratio causes a large change in sediment transport ratio. As the nose angle changes from positive to negative, the rate of change of S₁/S₂ with q₁/q₂ becomes low.

Source of Information: Ph. D Thesis, Department of Water Resources Engineering, BUET, Bangladesh.

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

For a particular nose angle and downstream discharge ratio, the sediment transport ratio in the downstream branches increases as the upstream discharge increases. The rate at which sediment transport ratio increases or decreases was high in case of higher positive nose angle. As the nose angle changes from positive to negative, the rate of change of S₁/S₂ with q₁/q₂ became low and vice versa.

Knowledge Management: Thesis