Coherent flow structures and suspension events over low-angle dunes: Fraser River, Canada

It is increasingly obvious that dunes with low-angle lee-sides (< 30°) are the most common bedform morphology in large sand-bedded alluvial channels. Flume studies have revealed much about flow and sediment dynamics over high-angle (~30°) dunes, however little is known about low-angle dunes. This study examines flow and suspension events over low-angle dunes in unsteady flow of the Fraser Estuary, Canada. Dune field bathymetry was mapped using a multibeam echo sounder while an acoustic Doppler current profiler simultaneously provided flow and suspended sediment measurements over a range of flows through tidal cycles. Mean flow and sediment transport varied through the tidal cycles. River flow dominated at low tide and large-scale suspended sediment events emerged as important sediment transport agents. I estimate that 69% of the total sediment is carried by 50% of the flow during low tide suspension events dominate the flow field.