Estuarine circulation versus tidal pumping: Sediment transport in a well‐mixed tidal inlet

Abstract

AbstractHigh‐resolution water column observations have been carried out in the Wadden Sea to understand suspended particulate matter (SPM) transport in well‐mixed tidal channels. These observations include more than 4000 consecutive CTD, microstructure shear and turbidity profiles from a free‐falling microstructure probe, as well as velocity data from an ADCP and SPM samples for calibration. A horizontal density gradient was established by a landward temperature gradient built up during an extraordinarily warm and calm spring season. Tidal averaging along σ‐layers (relative depth) provides the first direct observations of along‐channel estuarine circulation in the Wadden Sea, with net inflow near the bottom and outflow near the surface. Increased westerly (up‐estuary) winds during the second part of the campaign weakened and eventually even reversed estuarine circulation and yielded a net barotropic eastward transport. SPM concentrations showed a strong quarter‐diurnal signal with maxima near full flood and full ebb and were generally lower during the calm period and increased during the windy period, mainly due to wave‐related resuspension over nearby intertidal flats. The sediment flux analysis was based on a decomposition of the vertically integrated SPM flux into a barotropic advective component, an estuarine circulation component and a tidal pumping component. As a result, tidal pumping (due to ebb‐dominance weakly seaward) dominated the SPM flux during calm conditions, whereas barotropic advection dominated the strong landward SPM flux during the windy period. Along‐channel estuarine circulation is found to be of minor importance for the net SPM transport in such well‐mixed systems.