Water transport at subtidal frequencies in the Marsdiep inlet

Long-term time series of subtidal water transport in the 4-km wide Marsdiep tidal inlet in the western Dutch Wadden Sea have been analysed. Velocity data were obtained between 1998 and the end of 2002 with an acoustic Doppler current profiler that was mounted under the hull of the ferry ‘Schulpengat’. Velocities were integrated over the cross-section and low-pass filtered to yield subtidal water transport. A simple analytical model of the connected Marsdiep and Vlie tidal basins was extended to include wind stress and water-level and density gradients and applied to the time series of subtidal water transport. In accordance with the observations, the model calculates a mean throughflow from the Vlie to the Marsdiep basin. The mean water transport through the Marsdiep inlet consists of an export due to tidal stresses and freshwater discharge and an import due to southwesterly winds. In contrast, the variability in the subtidal water transport is mainly governed by wind stress. In particular, southwesterly winds that blow along the main axis of the Marsdiep basin force a throughflow from the Marsdiep to the Vlie basin, whereas northwesterly winds that blow along the main axis of the Vlie basin force a smaller mean water transport in the opposite direction. The contribution of remote sea-level change to the water transport, or coastal sea-level pumping, has been found to be much smaller than the contribution of local wind stress.