Secondary Currents around Braid Bar in Brahmaputra River, Bangladesh

Abstract

The pattern of primary isovels, pathways of sediment transport (both bed load and suspended load), and the evolution of channel planform are all strongly affected by secondary currents. The way in which secondary currents affect channel morphology in single-thread meandering rivers is now well understood; however, to date there has been no field-based study of secondary currents in a large, sand-bedded braided river. The River Survey Project of FAP-24, Bangladesh, offered the opportunity to rectify this gap in knowledge through a joint study between the University of Nottingham, Delft Hydraulics, and the Danish Hydraulics Institute. A comprehensive set of field data, including measurements of primary and secondary velocities and channel bed topography, were taken around a braid bar in the Brahmaputra River near Bahadurabad, Bangladesh. An acoustic doppler current profiler was used to measure three-dimensional velocity profiles. The results presented here were taken from 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. The results demonstrate the presence of large-scale secondary current cells at the flow bifurcation and bend apex.