Sediment transport processes at the mouth of the Western Scheldt estuary

Abstract

Understanding the sediment transport between the sea and the estuary is crucial for sustainable estuarine management. Sediment fluxes in estuarine environments are controlled by multiple interactive forces like tides, waves, wind, density differences, bathymetry, and trends in sea level rise. Tidal sediment transport is relatively constant while high wave events occur episodically and can transport large amounts of sediment. The sediment transport processes at the mouth of the Scheldt estuary will be studied and the following research questions will be answered: does the sediment transport depend on a) the location in the mouth, b) the position in the vertical and c) the tide, waves and wind? Measurement frames are used to measure flow velocities and suspended sediment concentration (SSC) at two locations at the mouth of the Western Scheldt estuary, southeast of the Vlakte van de Raan. One measurement site is in a flood channel (F1) and the other in an ebb channel (F3). The measuring frames contained: two Acoustic Doppler Current Profilers (ADCP) & (AQD), two Acoustic Doppler Velocity meters (ADV) and four Optical Backscatter Sensors (OBS). With these measurements, flow velocity profiles and suspended sediment concentration (SSC) profiles were created over the vertical, to calculate the sediment transported at the two locations. The influence of the waves and tidal currents were studied by calculating the wave and current induced bed shear stresses. To further analyse the influence of the tides, a Godin filter was used to remove the tidal signal from the flow velocities and the SSC, leaving the influences of the tidal asymmetry and the residual currents. The net sediment transport is landward directed in the flood channel and seaward directed in the ebb channel. The near bed sediment transport is larger in the flood channel than the ebb channel (29032 kg/m vs 18507 kg/m). Most sediment transport occurs during spring tides. The direction and magnitude of the net sediment transport is mainly determined by the tidal component of the sediment transport, being four (F1) and tree (F3) times larger than the residual components. The cause of the direction of tidal sediment transport was the difference in SSC between ebb and flood, caused by peak flow velocity and sediment type differences. The asymmetry of the tides is flood dominant in both locations, and the residual currents are seaward, with larger residual currents at F3. The main driving mechanism of the residual currents is the tidally induced residual flow due to differences in bathymetry. During the measurement period there were no large wave events, therefore, it is difficult to draw conclusion about the role of large wave events on the net sediment transport. Higher waves did cause higher SSCs, but the resulting increase in sediment transport was only slightly visible in the cumulative sediment transport. This study presents the main sediment transport processes in ebb and flood channels located in a estuary mouth. Because of the highly dynamic nature of the estuary mouth, projecting these findings to other regions is difficult. Thereby, only part of the sediment transport between the sea and the estuary is solved. But with a better understanding of the sediment transport processes, sediment transport models covering larger areas can be improved.