Freek Brils1*, Bob Smits2 , Roy van Weerdenburg2, 3
1 Rijkswaterstaat-WVL; 2 Deltares; 3 TU Delft
* Corresponding author: freek.brils@rws.nl
Introduction
The tidal channel Boontjes in the Dutch Wadden Sea is an important connection for navigation between the Afsluitdijk locks at Kornwerderzand and Harlingen. The dimensions of the channel have been maintained over the past years by extensive dredging, up to a yearly dredging volume of 360.000 m3 in 2019. In 2025, a pilot is started to reduce the amount of maintenance dredging and its impact on the Wadden Sea. During the pilot, the maintenance depth of the channel is reduced from -3.80 m NAP to -3.30 m NAP. The effects of the pilot are being monitored by several monitoring campaigns, focusing on hydrodynamics, morphology, and benthic communities.
Objective and Methods
Goal of the pilot is to reduce impact of dredging activities on the Wadden Sea. The measurement campaign is designed to help determine whether this reduced maintenance depth is an effective measure for this goal.
Prior to the pilot execution, a baseline assessment was performed, consisting of four measurement frames were placed in the research area between 8 October 2024 and 16 December 2024. These frames were equipped with ADCP, ADV, OBS and CTD sensors. Additionally, ship measurements are performed along four cross-sections in the channel. These measurements are performed during 13-hour periods in October. In May 2025, October 2025 and May 2026, the measurement campaign is repeated with the pilot measures (i.e. reduced maintenance depth) in place.
Results
The result is several months of hydrodynamic data, split among multiple seasons. The data can provide new insights into the dynamics of tidal channels, especially when combined with morphological data. We are exploring possibilities to incorporate the data into future projects with academic and applied research partners. Goal is to increase knowledge of tidal channels, improve dredging operations and better streamline maintenance on the natural processes.
Location of the frames (in black) and of the ship measurements (in red). Source: Smits (2025).
References
Smits (2025). Analyse van T0-meting bij pilot verondieping vaarweg Boontjes https://open.rijkswaterstaat.nl/publish/pages/218321/analyse_t0_pilot_verondieping_vaarweg_boontjes.pdf
