Winds of change: flooding, drying and flow on the Wadden Sea's intertidal flats
An analysis of wind effects based on measurements and simulation results for the area between Ameland and Holwerd
B.A. Krans (TU Delft - Civil Engineering & Geosciences)
Bram C. van Prooijen – Mentor (TU Delft - Coastal Engineering)
Dirk Sebastiaan van Maren – Graduation committee member (TU Delft - Environmental Fluid Mechanics)
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Abstract
The Wadden Sea is a dynamic coastal ecosystem characterised by broad intertidal flats that are exposed at low tide and submerged at high tide. Exposure time is an essential input parameter for ecotope mapping; the classification of ecological zones based on abiotic factors.
While the interplay of wind, tides, and sediment dynamics has been studied, there remains limited understanding of detailed exposure times, areas and flow patterns under varying wind conditions, particularly in shallow intertidal zones of the Wadden Sea.
This study quantifies how variations in wind direction and magnitude affect exposure characteristics and flow patterns on the Pinke Wad, located between Ameland and Holwerd. A 3Di-calibrated subgrid model, which is well suited for very shallow areas, is used to simulate twelve wind scenarios in addition to a base case without wind. The effects on one tidal cycle are analysed.
The scenarios result in different hydrodynamic responses on flooding and drying. Easterlies cause a non-linear increase in mean exposure time, from 3.67 h in the baseline to 5.51 h under an East-9 storm (≈ 23 m s-1). By contrast, westerly and northerly winds shorten exposure almost linearly for the simulated scenarios: the mean exposure time decreased to 0.95 h (North-9) and 0.77 h (West-9), with exposed area decreasing by 70–82%. A northerly wind of 6 Beaufort decreased exposure on higher flats by up to 49% (∼6 h), whereas an easterly of the same force increased it by 20% (2.5 h). Residual currents responded accordingly, easterly winds reversed the net flow westward from Beaufort 3 onwards. Westerly winds generated the strongest eastward residual currents. Northerly winds also induced westward flow starting from Beaufort 3, though with smaller directional shifts. Wind-driven changes in exposure time and changing flow patterns have clear implications for ecotope mapping, because there is a large variability over time.
The results highlight the substantial and spatially important role of wind in changing hydrodynamics on the Wadden Sea’s intertidal flats. By linking variations in wind speed and direction to changes in residual flow and exposure patterns, the findings provide a better understanding of hydrodynamic-ecological interactions. The use of a (well-calibrated) hydrodynamic subgrid model may offer a promising way forward for calculating exposure times as input for ecotope mapping compared to the current method, which relies on interpolation on water levels between gauge stations.