Ferry-ADCP observations of tidal currents in the Marsdiep inlet and how they are affected by the motion of the ferry

Abstract

The Wadden Sea is a dynamic tidal system with multiple inlets. In the western Dutch Wadden Sea large amounts of fresh water coming from Lake IJssel are being exchanged with saline water coming from the North Sea during a tidal cycle. This study analyzes the tidal currents in the Marsdiep inlet which is located between the mainland of the Netherlands and the island of Texel. The velocity measurements are obtained via two Acoustic Doppler Current Profilers (ADCPs) mounted underneath the hull near the bow and the stern of the Texels Eigen Stoomboot Onderneming (TESO) ferry. The use of two ADCPs allows to investigate not only the spatial and the temporal variability of the velocity field of the flow but also the turbulence during tidal cycles. Features of tidally induced turbulence are identified by using the difference of the measurements of the horizontal velocities of the two ADCPs which were found to differ slightly. In this study, the vertical profile of the turbulence as well as its relation with the strength and the phase of the tide are investigated. Regarding the vertical velocity of the flow, it is observed that the ADCP at the bow of the ferry always measures a downwelling motion while the ADCP at the stern of the ferry measures an upwelling motion of the flow. This observation leads to the speculation that the ferry’s motion influences the velocity measurements and the measured velocities are not the real ones. A correction of the vertical velocity component for the ferry’s motion is introduced in this study and through this correction a physical interpretation of the vertical velocity is given by identifying physical features in the inlet. Finally, we expect that the component of the velocity along the sailing direction is also affected by the ferry’s motion. As the ferry sails, there is a displacement of a large amount of water backwards underneath the hull due to a pressure gradient created by water level differences. However, instead of a backflow, a forward flow is observed in the direction of the ferry’s motion and possible origins for its presence are given in this study.