Monitoring the morphodynamics of the Zwin inlet. Interim report. 2 years after the extension works

The Zwin is a relatively small tidal inlet connected to the North Sea through a tidal channel. The inlet is flooded twice a day and it is characterized by the presence of intertidal area with sand banks and salt marshes. From August 2016 until the opening of the dyke in February 2019, large intervention works were carried out to reduce the sediment silting and accretion processes by increasing the amount of water flowing in and out of the inlet (i.e. tidal prism). On 4 February 2019, the interti…

The Zwin is a relatively small tidal inlet connected to the North Sea through a tidal channel. The inlet is flooded twice a day and it is characterized by the presence of intertidal area with sand banks and salt marshes. From August 2016 until the opening of the dyke in February 2019, large intervention works were carried out to reduce the sediment silting and accretion processes by increasing the amount of water flowing in and out of the inlet (i.e. tidal prism). On 4 February 2019, the intertidal area was nearly doubled when the International Dijk was cut and a part of the former Willem-Leopold polder was made intertidal. This project aims to evaluate the effects of the intervention work and to improve our understanding on the morphodynamics of the Zwin. Detailed topographic and hydrodynamic measurements were carried out in the entrance and inland inlet units over the period covering the time before the opening of the dyke which was on 04/02/2019 and up to 1.9 years afterwards. The results indicate significant morphological changes in the entire inlet system where the main channel has become deeper and wider. Also, it has experienced an eastward migration along the entire inlet. Although a high spatial variability is noted in the morphological evolution, with a clear onshore mobility of the sandy bedforms in the channel especially in the entrance inlet, the sediment balance is stable there, suggesting that sand bypass dominates there. In contrast, the inland inlet has lost a significant sediment volume over the monitoring period. It has also experienced two clear morphological trends with accretion at the west side with a vertical gain of the sand bank and bars, while erosion dominates in the middle and at the east side. There, the channel has become deeper and wider. Both trend means a migration of the channel toward the east. Since the interventions, the water discharge during the flood and ebb phase has significantly increased. It is linearly related to the geometrical characteristics of the tidal basin and water level. Future topographic and hydrodynamic monitoring will allow to fine tune our insights into the morphological response and to estimate the contribution of the different processes controlling the tidal prism in the Zwin inlet.