Monitoring the morphodynamics of the Zwin inlet. Final report 2019-2022 period

The Zwin tidal inlet and marshland system was nearly doubled in surface in 2019 when a large part of the Willem-Leopold polder was incorporated in the floodable area. The main channel connecting the marshlands to the sea is characterized by a complex 3D morphology with a main channel, sandy bedforms including banks and bars as well as secondary channels. Their presence and movements reflect active sediment transport in the system. The inlet is controlled by hydrodynamics (currents and water leve…

The Zwin tidal inlet and marshland system was nearly doubled in surface in 2019 when a large part of the Willem-Leopold polder was incorporated in the floodable area. The main channel connecting the marshlands to the sea is characterized by a complex 3D morphology with a main channel, sandy bedforms including banks and bars as well as secondary channels. Their presence and movements reflect active sediment transport in the system. The inlet is controlled by hydrodynamics (currents and water level), beach dynamics, tidal prism and sediment supply. This study reports the morpho- and hydrodynamics of the entire tidal channel part from before the opening of the dyke on 04/02/2019 to nearly 3 years after. The results indicate significant morphological changes in the entire channel system where the main channel has become deeper, wider and migrated eastward. Since the opening, the sediment balance is stable in the entrance inlet. In contrast, the inland inlet has lost a significant sediment volume. Short term dynamics in the middle of the entrance and inland inlet unit were studied during field campaigns under calm and energetic conditions. A regime of ebb dominance is found in the center of the channel and to a lower degree at its east side, while the west side of the channel is flood-dominated. In general, the ebb phase lasts about two times the duration of the flood phase. Based on the tidal prism and cross-sectional area analysis, it is concluded that the expansion of the intertidal area in early 2019 meant a very significant increase of tidal prism, which almost quadrupled. The response in the main channel was rapid and strong. A recent sandbank was quickly cleared during the first year after the expansion. As a result, the channel cross-sectional area at the border of the entrance inlet and the inland inlet attained a new equilibrium around 450 m² at the Spring 2020 survey. This section was only reduced by about 10 m² at the Spring 2021 survey. The continued monitoring programme will help to fine tune the insights into the process-response of the Zwin inlet and to estimate the time scales of the expected sanding up in order to guide future management and interventions.