Flow Structure and Transport of Sand-Grade Suspended Sediment around an Evolving Braid Bar, Jamuna River, Bangladesh

Detailed vertical profiles of time-averaged flow velocities and sediment concentration were taken during three periods of mid-channel bar development in the Jamuna River, Bangladesh. Bar growth was initiated downstream from a major flow convergence and generated a bar 4 km long and 1 km wide in a channel up to 15 m deep. Flow velocities and the concentrations of sand-grade suspended sediment were quantified using an acoustic Doppler current profiler (ADCP). Bed morphology was measured using echo-sounding and all positions were located using a differential global positioning system (DGPS). These data reveal no evidence for channel-scale, coherent helical flow cells in either distributary around the braid bar. Instead, the structure of flow is dominated by a simpler flow divergence over the bar head, flow convergence at the bar tail and flow that is usually parallel to the thalwegs in each distributary. During the later stages of bar growth, flow is directed over the bar top from one distributary towards the other as the bar begins to adopt a more asymmetrical morphology. In addition, large sand dunes migrate up the bar stoss side, producing an accretionary dune front at the bar head. These dunes are strongly linked to high suspended bed-sediment concentrations as flow shallows on to the bar top. A shadow of low suspended bed-sediment concentration is located in the bar lee during the early stages of bar growth, this also being a region of small sand dunes. The lack of coherent secondary flows, around large kilometre-scale bars, may be explained through the large width-to-depth ratio of these channels, the low curvature of the anabranches, the complexity of flow over the bar top as it interacts with flow in the anabranches and the significant influence of large-scale dune-bedform roughness. These factors suggest that current models for the processes of mid-channel bar creation, growth and preservation, derived from studies of smaller rivers, require substantial revision before application to kilometre-scale sand-braid bars.