SECONDARY CURRENTS AROUND BRAID BAR IN BRAHMAPUTRA RIVER, BANGLADESH

The pattern of primary isovels, pathways of sediment transport (both b ed load and suspended load), and the evolution of channel planform are all strongly affected by secondary currents. The way in which seconda ry currents affect channel morphology in single-thread meandering rive rs is now well understood; however, to date there has been no held-bas ed study of secondary currents in a large, sand-bedded braided river. The River Survey Project of FAP-24, Bangladesh, offered the opportunit y to rectify this gap in knowledge through a joint study between the U niversity of Nottingham, Delft Hydraulics, and the Danish Hydraulics I nstitute. A comprehensive set of field data, including measurements of primary and secondary velocities and channel bed topography, were tak en around a braid bar in the Brahmaputra River near Bahadurabad, Bangl adesh. An acoustic doppler current profiler was used to measure three- dimensional velocity profiles. The results presented here were taken f rom two cross sections: at the flow bifurcation upstream of the study bar and at the bend apex of the left anabranch around the study bar. T he results demonstrate the presence of large-scale secondary current c ells at the flow bifurcation and bend apex.