Tidal energy has a large potential to contribute to achieving the sustainability goals in the Netherlands, due to the long coastline and many estuaries. The investment costs for the implementation of offshore tidal energy are however, still a drawback. A possibility to lower the investment costs for tidal energy is to implement tidal energy extraction into existing coastal infrastructure. An example of this is the current pilot project in the Eastern Scheldt barrier where five Tocardo turbines are attached to the barrier's geometry. The flow passing the turbines in the barrier is contracted by the barrier's geometry and the weir of the foundation of the barrier. Deltares executed an environmental impact assessment for the pilot project with a measurement campaign and a computationally expensive but accurate blade resolved numerical model. In this thesis, a cheaper computational model is added to the these assessment tools. To do so, a turbine parameterization is introduced in the finite element model FinLab. The relatively cheaper computational model should make it possible to execute more extensive sets of simulations of the pilot project in the future. FinLab solves the non-hydrostatic three-dimensional incompressible Navies-Stokes equations in an unstructured mesh with an optional moving free-surface. The used meshes are refined in certain areas of complex flow, such as the turbine parameterization, near the bed, the recirculation zone of the weir and the wake of the turbine. An extra source term is added to the Navier-Stokes equations on a chosen number of turbine integration points, to implement the turbine into the mesh. The placement of these integration points in a two-dimensional mesh is rather simple, the integration points are evenly distributed over the diameter of the turbine. In a three-dimensional mesh, a Fibonacci series is used to implement an actuator disc (AD) into the model domain. A benefit of the Fibonacci circle is that each turbine integration point represents a similar area. The force in the turbine integration points is determined by three methods: an actuator disc with a uniform thrust distribution (uniform AD), a rotational averaged actuator disc blade element momentum method (AD-BEM) and a non-rational averaged actuator line blade element momentum method (AL-BEM). In the uniform AD method, a measured thrust force is divided equally over all turbine integration points. In the BEM methods axial and tangential forces are determined for each turbine integration point based on the local flow speed and experimental lift and drag coefficients. With these methods, it is possible to estimate the generated power and thrust by the turbine. A tip-correction is added to the AD-BEM method to achieve accurate results of the simulations. Furthermore, in both the AD-BEM and AL-BEM methods a small actuator disc is applied in the nacelle region to reproduce the nacelle's blockage of the flow. The uniform AD, AD-BEM and AL-BEM parameterizations are validated with measurement data of flume experiments of a single turbine, flume experiments with different turbine-weir geometries and flume experiments with multiple turbines. The combination of these last two sets of experiments does very well represent the situation in the Eastern Scheldt barrier. As stated above, the AD-BEM and AL-BEM methods are able to estimate the turbine performance. The results of the AD-BEM simulations show a relative error of the turbine's generated power of within 20% in comparison with the flume experiments. The accuracy of the thrust fluctuates more. The AD-BEM method shows to be able to accurately estimate the location of optimum power harvesting in a combined turbine-weir geometry. Both the power and thrust accuracy depend on the mesh resolution and the turbine's rotational speed. Accurate results can be obtained by fine-tuning these settings. The AL-BEM method should be able to produce at least similar similar accuracy to the AD-BEM method when it comes to power and thrust. The AL-BEM method in this thesis is however, not yet able to do so. The results of the simulations with the three methods are also compared with time-averaged velocity measurements of the flume experiments. In the near wake, the uniform AD method does not imply any wake rotation. The rotation of the wake is represented accurately in a time-averaged sense by the AD-BEM method. The AL-BEM method adds transient features such as the downstream trailing tip-vortices to the flow. The velocity shear and turbulent eddies in the near wake as caused by the different methods influence the TKE in the near wake and the recovery of the far wake. The uniform AD under-estimates the wake recovery. The extra velocity shear in the AD-BEM method improves the accuracy but still under-estimates the recovery rate. Resolving the transient features of the near wake in the AL-BEM method further improves the accuracy of the wake recovery. In the simulations with the weir, the geometry seems to be dominant and the wake recovery rate is more accurate than in the simulations without the weir. The AD-BEM method is applied to the Eastern Scheldt field case. Five turbines are introduced in a mesh which represents Roompot 7 to 9 in a simplified manner. Due to the simplification of the mesh, the resistance of the barrier on the flow appeared to be too low. Therefore, the simulations are executed with an upstream discharge boundary. This discharge boundary makes the model less useful to estimate the environmental impact of the turbines in the barrier and at the moment not yet an alternative for the blade resolved model by Deltares when it comes to estimating these effects. With the discharge boundary, the AD-BEM model predicts the average thrust over the five turbines with a relative error of 3% and the average power with a relative error of 12%. This is only slightly less accurate than the results by the blade resolved model by Deltares, while the computational costs of the Eastern Scheldt field case simulations are only a fraction of the earlier executed blade resolved simulations by Deltares.

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.

Salt intrusion poses a threat to the fresh water supply function of inland waterways. How far inland salt intrusion reaches is dependent on a balance of buoyancy forcing, water depth, discharge of the canal and the amount of mixing. Wind, bottom roughness, and sailing ships contribute to mixing. Currently, little is known about the contribution aof the latter to mixing. It is important to better understand the role of ship traffic in order to make more accurate models and take the right measures to protect the fresh water supply function of inland waterways. The flow field in an unstratified canal has been well studied, however, little is known about the effects that ships have in a stratified canal. Ships move water when sailing through a waterway. This induces three water movements: primary and secondary waves and the propeller jet flow. These processes can contribute to mixing by shear instabilities and internal wave breaking.A sailing ship in a stratified canal has been modelled using a moving grid approach in the 3D non-hydrostatic finite element model FinLab to study these effects in detail. The propeller has been neglected to simplify the model. A parameter study has been performed to observe the influence of several parameters (such as the canal blockage, internal Froude number, and canal bank slope) on the flow pattern and the amount of mixing. The amount of mixing is found to be in the order of magnitude of 1 percent for one vessel over 600m canal length a representative range of parameters of canal geometry, density profile, vessel draught and vessel speed. The actual effect will be larger since the internal wave field is still present at the outflow boundary and since the propeller has been omitted from the model set-up. Processes around the vessel and the internal waves contribute about equally to the amount of mixing in the model domain. The internal waves are estimated to have a contribution of about twice as large if the modelled domain was longer. As the interface comes up behind the vessel, it is likely that the propeller jet can have a large impact on the density field directly behind the vessel. The vessel speed, density profile, relative density difference, layer distribution in a two-layer flow, canal blockage, draught to top layer height ratio, and the slope of canal banks are important parameters in the amount of mixing generated. Due to the cumulative effect of the ship traffic, mixing by ship traffic is estimated to be of large importance on the density distribution in a canal. More research is needed to include this effect in large-scale numerical models and find a good parameterisation. How mixing by ship traffic could finally be implemented will be dependent on the numerical model that is used, and the amount of detail that is needed.