Distribution of suspended sediment transport between asymmetric bifurcated channels in mobile bed condition

Abstract

Bifurcations are a fundamental feature in alluvial rivers and estuarine systems, contributing to the complex dynamics of fluvial environments, alongside braiding, anabranching streams, and deltas. Despite extensive research, the distribution of flow and sediments at asymmetric bifurcations remains inadequately understood. To achieve in-depth understanding of flow and suspended sediment dynamics at asymmetric bifurcations, this study has been conducted in the Hydraulics and River Engineering Laboratory at the Department of Water Resources Engineering, BUET. The experimental investigation focuses on varying discharge rates and nose angles in asymmetric bifurcated channels to explore their influence on flow behavior and sediment distribution.

In order to achieve the objectives of this study, a total of thirty-six experimental runs have been conducted comprising three distinct nose angles: nose type-1 [θ = (+) 3.5°], nose type-2 [θ = (-) 4.2°], and nose type-3 [θ = (-) 7.3°]. Each nose angle has been tested under three different upstream discharges (40 l/s, 50 l/s, and 60 l/s) with respective rate of sediment feeding. Throughout the experiments, key parameters such as discharge, flow velocity, water depth, bed level changes, and suspended sediment concentration have been measured at specific locations across the channel. Flow visualization has been also observed, particularly near the bifurcation.

The results reveal that nose angle plays a vital role in suspended sediment distribution, with turbulence near the nose tip significantly increasing sediment in suspension. As the nose angle changes from positive to negative indicating flow area of nose tip or branch mouth opening narrower to wider of a bifurcated channel, the suspended sediment transport ratio increases for a given upstream flow discharge in that branch. Velocity variation patterns have been observed in relation to nose angle and discharge variation. Bed level changes indicates that erosion intensifies with increasing discharge, and the erosion-deposition pattern varies depending on the nose angles. The relationships among flow patterns, velocity variations, suspended sediment concentration, and bed level changes are examined with consideration of downstream boundary conditions.

The bifurcation geometry, especially the branch mouth shape, plays a critical role in sediment dynamics. The nose angle is the significant factor that governs the distribution of sediment and discharge in river bifurcations. Nine nodal point relations of two bifurcated branches have been found for three different nose angles and three upstream discharges. The parameters of the nodal point relations, namely coefficient M and exponent k, reveal a defined pattern corresponding to variations in nose angle. However, these parameters do not demonstrate a significant linear correlation with variations in discharge. The suspended sediment transport ratio of the bifurcated channels is directly proportional to the discharge ratio and inversely proportional to the quasi-offtake bifurcated angle. These results have been compared with the relevant studies and found satisfactory. Nose angle is a vital factor in determining the distribution of suspended sediments between asymmetric bifurcated channels. Therefore, this study provides valuable insights into the interrelationship between suspended sediment transport and nose angle variations under asymmetric conditions, offering a deeper understanding of bifurcation dynamics in the fluvial systems.