Flow and sediment suspension events over low-angle dunes: Fraser Estuary, Canada

The morphodynamics of large sand-bedded rivers and estuaries are ultimately controlled by the way bed material is moved and the development of large, subaqueous sand dunes that control hydraulic flow resistance. It is widely thought that the primary mechanism for moving sandy bed material in these channels is large-scale coherent flow structures that cause suspension events whose properties vary with flow, especially in tidally influenced environments. Here, we examine mean flow and sediment suspension events over low-angle dunes (lee face angle <30 degrees) in the unsteady flow of the Fraser Estuary, Canada. At high tide, flow nearly ceased and a salt wedge entered the channel, forcing salt water under the downstream-moving fresh water. The salt wedge persisted in the channel until late in the falling tide, causing stratification in the water column and instabilities along the saline-fresh water interface. At low tide, mean velocities peaked and forced the saline water out of the channel. Flow over the low-angle dunes displayed topographically induced patterns previously observed over high-angle dunes, but permanent flow separation was not observed. Large-scale sediment suspension events dominated sediment flux during low tide and became larger scale, yet less frequent, as the tide began to rise. The suspension events appeared to form over the lower stoss of the dunes and grew up over the bed forms and, less commonly, emerged downstream of the crest. Suspension events move similar to 69% of the total sediment in the flow above low-angle dunes when they are present.