Graduation of Kamilla Guijt

16 January 2018 11:00 - Location: Room E, Faculty of Civil Engineering and Geosciences - By: Webmaster Hydraulic Engineering

"Impact of Tidal Energy Extraction in the Eastern Scheldt Storm Surge Barrier on Basin Hydrodynamics and Morphology" | Professor of graduation: Prof. dr. ir. Z.B. Wang, supervisors: Dr. ir. B.C. van Prooijen (TU Delft), Dr. ir. R.J. Labeur (TU Delft), Ir. A.C. Bijlsma (Deltares), Ir. V.M. Gatto (TU Delft)

The Eastern Scheldt basin is protected by a semi-open storm surge barrier that was completed in 1986. This barrier includes 62 openings with gates between piers, which are only closed during extreme storm surges. In 2015 a set of five turbines was installed in one of the openings as a pilot for tidal energy extraction. The construction of the barrier significantly changed the basin hydrodynamics (tidal volume, range and velocities) and associated basin morphodynamics (sedimentation and erosion patterns of tidal flats and channels). This thesis investigates the incremental impact of tidal energy extraction in the barrier on the hydrodynamics and morphology of the basin, covering scenarios with turbines installed currently and potential upscaling of tidal energy extraction. 

Due to the construction of the barrier, the average tidal range and velocities have significantly decreased throughout the basin (Louters et al., 1998), 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 block and shear local flow and increase turbulence levels (Verbeek et al., 2017). As the flow passes through the blades, they exert a thrust force on the flow, thereby losing momentum. This 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 semi-stationary three-dimensional model of one opening with five turbines. To evaluate variations in resistance, a sensitivity range was defined as input to the numerical simulations. Upscaling scenarios were specified, in which different sets of the barrier openings are equipped with tidal turbines (varying between 2 to 17 openings with turbines out of the 62 gate openings of the barrier).

Numerical simulations over one spring-neap cycle were performed for these scenarios. The results show small deviations in tidal range, volume and discharges throughout the basin due to tidal energy extraction, especially compared to changes that have occurred due to the construction of the barrier. The 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, in both magnitude and absolutely. 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.

The simulation results show that the emergence time of the tidal flats decreases due to an increase in mean low water level throughout the basin for different turbine installation scenarios. Based on hypsometric curves of the three largest individual tidal flats, the reduction in acreage is estimated. This reduction in acreage due to tidal energy extraction of these tidal flats is relatively small compared to the ongoing loss in acreage due to the construction of the barrier.  

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.