The relation between shear velocity and suspended sediment concentration over dunes: Fraser Estuary, Canada

Abstract

Two types of dunes develop in the Fraser River Estuary, Canada. Symmetric dunes have stoss and lee sides of similar length, gentle lee-side slope angles and rounded crests, while asymmetric dunes have superimposed small dunes on their stoss sides, sharp crests and steeper lee sides. Symmetric dunes appear to be near-equilibrium features at higher flows. Asymmetric dunes are transitional between symmetric dunes and the small superimposed dunes that are in equilibrium with the lower velocity and sediment transport conditions. Spatially averaged velocity profiles over dunes consist of upper and lower log-linear segments. Upper segments of profiles reflect the total stress of the flow. The lower segment on symmetric dunes reflects skin friction from sand particles, but for asymmetric dunes it is skin friction plus form roughness from the superimposed dunes. Predictions from the Rouse equation indicate that sediment suspension is controlled by total stress for symmetric dunes, whereas for asymmetric dunes sediment suspension is related to stress associated with skin friction plus form roughness. It is the equilibrium bedform — the large symmetric dunes, and the small superimposed dunes in the asymmetric case — that controls sediment suspension over dunes.