A field study of coherent flow structures over low angle dunes: Fraser Estuary, British Columbia

Aqueous dunes are present in nearly all sand bedded alluvial channels and can significantly influence flow resistance and sediment transport and deposition. The geometry of these bedforms can take on a high angle asymmetrical or low angle symmetrical shape. While advances have been made in understanding the mean and turbulent flow over high angle dunes, far less progress has been made in detailing flow over low angle dunes, commonly observed in large rivers, due to difficulties measuring near the bed and quantifying the turbulence over these bedforms. This field study documents the flow over low angle dunes in the Fraser Estuary, British Columbia, using an acoustic Doppler profiler (aDcp) to measure 3-D flow characteristics and a multi-beam echo sounder (MBES) to provide high-resolution bed topography. Measurements were made over a dune field (~1 km long and ~0.5 wide) through two semi-diurnal tidal cycles during the 2010 freshet. We examine the coupling between the bedform morphology and the generation of coherent flow structures. Bedforms in the dune field range from low-angle symmetric to higher angle asymmetric and vary over tidal cycles; however, none display the classic angle of repose geometry. Mean flow velocity increases on falling tide while it decreases the rising tide. At lower tides, large scale motions caused by topographic forcing emerge on stoss slopes and rise up over the crest producing variations in suspended sediment over the bedforms. Our analysis is intended to contribute insight into what controls the occurrence of low angle bedforms in rivers.