Small-scale eddies detected by VHF radar in the Western Scheldt, Netherlands

Huyen T. Tran1, Roberto Gomez1, Malcom Heron2, Herman C. Peters3
1Helzel Messtechnik GmbH, Carl-Benz-Str. 9, D-24568 Kaltenkirchen, Germany
2PORTMAP Remote Ocean Sensing Pty Ltd, Australia
3Xi-Advies B.V., Noorderdiep 55, 9521 BB Nieuw-Buinen, The Netherlands

In the modeling approach, tidal intrusion, meandering coastlines, and dynamic bathymetry (e.g., submerged sandbanks) present significant challenges for simulating the hydrodynamics of coastal channel areas at fine scales (on the order of minutes and less than 1 kilometer). These distinctive features can generate strong turbulence and small-scale motions, i.e., eddies, which cannot be resolved at typical modeling scales. Such eddies are potentially hazardous for the operation of ships and vessels navigating through the channels. This study utilizes sea surface current data obtained from WERA, a very-high-frequency (VHF) radar system located in the Western Scheldt channel in the Netherlands, to detect small-scale eddies in the area. The detection approach incorporates multiple criteria, including the Okubo–Weiss parameter, local velocity minima, and the curl of horizontal surface current velocities. Most of the eddies were observed to form behind a submerged sandbank located northeast of the domain and within a deeper area (>12 m depth) compared to the surroundings (~4–8 m). A comparison with water-level time series recorded at the Vlissingen tidal gauge revealed that eddies typically occurred 1–2 hours after high tide and more frequently during spring tide periods, with durations ranging from 15 minutes to over an hour. In this domain, the interaction between tides and dynamic bathymetry generates high curl values, playing a dominant role in eddy formation. The application of VHF radar measurements for this purpose can be operationalized to support port authorities in ship navigation and safety management, where modeling approaches are challenging.