Response of sand dunes to variations in tidal flow: Fraser Estuary, Canada

Abstract

The morphology of dunes in rivers and estuaries often lags behind changes in river discharge and neap‐spring tides. This study extends previous research by examining the response of large subtidal dunes in the Fraser Estuary, Canada, to changing flow conditions over a semidiurnal tidal cycle. An acoustic Doppler current profiler (ADCP) is used to measure three‐dimensional velocity profiles and estimate sediment transport and a digital echosounder to measure bed profiles and dune characteristics. Mean flow velocity accelerates from 0.60 to 1.99 m/s and mean flow depth decreases from 15.1 to 12.2 m on the falling tide, followed by a decrease in mean velocity to 0.25 m/s and an increase in depth to 15.25 m on the rising tide. Estimates based on ADCP bottom‐tracking and backscatter, and the sediment transport model of van Rijn (1984a, 1984b, 1984c), indicate that bed‐material sediment transport generally follows the pattern of mean velocity over the tidal cycle. Dune length does not change significantly over the survey period. Changes in dune height, steepness and leeside slope angle precede changes in flow velocity, increasing early in the tidal fall because of scour in dune troughs that is likely caused by increased turbulence resulting from development and expansion of the flow separation/deceleration zone. Erosion of dune crests increases as peak velocity is approached near low tide, resulting in a decrease in dune height. Concentrations of sand in suspension also increase with mean velocity, leading to deposition in troughs and a further reduction in dune height and leeside slope angle. Sand falls out of suspension and drapes the dunes as high tide approaches. Dune length is not in equilibrium with the flow whereas dune height is in equilibrium with the strongest flows that occur around low tide. A comparison of bed‐material transport based on dune migration and the model of van Rijn (1984a, 1984b, 1984c) suggests that the model provides reasonable estimates of bed load in the Estuary, although the model must be used with caution because deposition from suspension also contributes to dune migration.