Impact of Tidal Energy Extraction in the Eastern Scheldt Storm Surge Barrier on Basin Hydrodynamics and Morphology

Abstract

The Eastern Scheldt basin is protected by a semi-open storm surge barrier that was completed in 1986. In 2015 a set of five turbines was installed in one of the barrier openings as a pilot for tidal energy extraction. Due to the construction of the barrier, the average tidal range and velocities have significantly decreased throughout the basin and therefore tidal and meteorological processes driving sediment transport over tidal flats are no longer in equilibrium with the bathymetry. Working towards a new equilibrium, tidal flats erode and channels fill up. This is expected to continue over a long period of time. Turbines installed in barrier openings will lead to an increase in effective resistance at a barrier opening and therefore a further drop in tidal range and velocities throughout the basin. An available two-dimensional Delft3D model (Pezij, 2015) covers relevant hydrodynamics and morphodynamics of the basin with barrier, openings, channels and tidal flats. This model was modified to account for tidal energy extraction. The tidal turbines were parameterized through a momentum sink that increases the flow resistance locally. This sink term was calibrated using a state-of-the-art three-dimensional model of one opening with five turbines. Upscaling scenarios were specified, in which different sets of the barrier openings are equipped with tidal turbines. Numerical simulations over one spring-neap cycle were performed for these scenarios. Results show small deviations in tidal range, volume and discharges throughout the basin due to tidal energy extraction, compared to changes that have occurred due to the construction of the barrier. Reductions in tidal range and volume appear to be near-linearly increasing with the number of turbines installed. Deviations in tidal range increase in landward direction. Between the barrier and mid-basin, peak discharges decrease in channels directly behind the barrier section with turbines and increase in channels behind sections with no turbines. From mid- to end-basin, discharges are not affected by positioning of turbines, only the amount of energy extraction. Simulation results indicate that the emergence time of tidal flats decreases due to tidal energy extraction as a result of an increase in mean low water level throughout the basin. The increase in low water level due to tidal energy extraction would only be a fraction of the increase in water level due to sea-level rise in coming years. Additionally, the reduction in acreage was estimated based on hypsometric curves of the three largest individual tidal flats. The reduction in acreage of these flats due to the rise in mean low water level resulting from tidal energy extraction is relatively small compared to the ongoing loss in acreage due to both sea-level rise and erosion resulting from the barrier construction. Further work is required to improve basin and turbine modeling and evaluate long-term morphodynamics (sediment transport). Separately a comprehensive socio-environmental evaluation is required to compare benefits from (renewable) tidal energy extraction to the (incremental) ecological impact resulting from further reduction in acreage of tidal flats that support animal and bird life.