As a result of growth in the offshore energy infrastructure, the connection between the offshore environment and the mainland is increasing in importance. This leads to an increased use of the seabed for cables and pipelines as part of our energy infrastructure. Cables or pipelines are placed in trenches and covered by sediment, to be protected from any damage from activity near the sea bed. During execution, the trenches will refill with sediment prior to the placement of the cable or pipeline, which is known as siltation. The siltation rates are increasing when entering the breaker zone near the shoreline. The predictability of the siltation rates are a crucial step in efficient and safe realisation of new connections between the offshore energy infrastructure and the mainland.

Inclusion of nearshore processes is missing in existing quick-assessment siltation tools. Complex process-based models, like Delft3D or XBeach, are capable of predicting siltation volumes in the nearshore environment accurately. However, these models demand large computation capacities. This makes them unsuitable for probabilistic modelling, requiring large numbers of calculations. Probabilistic modelling however is a crucial step in identifying and quantifying uncertainties and related risks in the execution. This research presents a quick-assessment tool that includes nearshore processes. Ensuring low complexity makes quick-assessment tools suitable for probabilistic modelling of siltation predictions, reducing and quantifying uncertainties within nearshore trench siltation.

This research presents an approach to include wave transformation and wave-driven currents into an existing siltation prediction tool (SedPit). The resulting SedPit Nearshore tool allows fast predictions of siltation volumes in the nearshore zone. The performance of the SedPit Nearshore tool is assessed by comparing it to data from a field case, and comparing the accuracy to the accuracy of the existing SedPit tool. The SedPit Nearshore tool gives accurate predictions on the total siltation volume, and gives good insights in the spatial distribution of siltation volumes. The potential of the SedPit Nearshore becomes most evident when comparing it to the existing SedPit tool. A great improvement compared to the existing SedPit tool is seen. For the test case, the SedPit Nearshore tool reduces the absolute error in redicting the total siltation volumes by 82% compared to the existing SedPit tool. The SedPit
Nearshore tool predicts the total siltation volume with an error margin of 7%, while the existing SedPit has an error margin of 41%. The largest improvements compared to the existing Sedpit are seen in the most onshore regions, as this is the zone where most wave-driven currents are generated. The computational speed of the tool has proven its applicability for analyses on model sensitivity and uncertainty quantification. Computation times are reduced by factor 9,000 when comparing it to XBeach, a complex process-based model. Bottom roughness ks and wave roller steepness β were identified as most influential free variables in driving nearshore siltation volumes. Calibrating the model to obtain likely values for a range of free variables has helped to reduce the 95% confidence interval of peak siltation rates by 36%.

The inclusion of wave-driven currents into existing siltation prediction tools has shown a great improvement in the accuracy of siltation predictions in the nearshore zone. Although the SedPit Nearshore tool is calibrated on one specific field case, the method and the workflow of the tool show potential to help as general prediction tool of nearshore trench siltation. The power of the SedPit Nearshore tool lays in its simplicity, making it a fast, efficient, and accurate tool, suited for probabilistic modelling. ... Read more

A possible way of generating a stable and predictable base load of renewable energy, supplementing the unpredictable wind and solar energy, is dynamic tidal power (DTP). DTP is generated with the use of large dam, which is built in the sea, under an angle with the propagating tidal wave. A dam of sufficient length can create a phase difference between the propagating tidal waves on either side of the dam, creating head differences over the dam, which can be used to generate renewable energy using large turbines in the dam.
Previous studies considered a DTP dam built perpendicular to the coast and the direction of the propagating tidal wave. However, a structure of this size extending into the North Sea interferes with other functions of the area such as shipping. Instead of a perpendicular dam, an oblique dam off the Dutch delta coast has been proposed to reduce this interference. This design has the additional benefits that it could increase the coastal safety of the surrounding area and potentially increase the sediment budget of the Voordelta area in front of the coast.

In this study, a first-order assessment of an oblique DTP dam has been carried out. This assessment focuses on three aspects: the expected energy yield and how this compares to the expected energy yield of a perpendicular dam under the same conditions; the impact this dam has on the coastal safety of the surrounding area. To this end, an analysis of the change in hydrodynamic and morphological processes in the Voordelta as a result of the construction of such a dam has been carried out.

The propagation of the tidal wave around an oblique DTP dam in the North Sea has been modelled using the FINEL modeling software, applied as a two-dimensional flow model. The reference layout of the DTP dam has been determined beforehand based on several requirements concerning the location and length. Using a turbine module integrated into FINEL, the discharge through each turbine has been determined, base on which the energy output of the DTP dam has been calculated. A comparison was made of the energy output of the oblique dam and that of a perpendicular dam, as used in previous studies. To do this, the same model has been run with a dam positioned perpendicular to the Dutch coast.

A DTP dam with a length of 62.5 km and a southwest orientation starting at the Maasvlakte 2 was found to have a maximum power output in the order of 8 * 10^2 MW and a yearly generated energy yield in the order of 2 TWh. Both of these values were approximately a factor five lower than the power output of the perpendicular dam with the same length also starting at Maasvlakte 2. This is because, in the case of the perpendicular dam, a phase difference in the tidal wave over the dam was created, creating a large head difference over the dam. In the case of the oblique dam, the phase difference was lower, significantly reducing the water head over the dam. As opposed to the perpendicular dam, the head difference was created as a result of amplification of the tidal wave within the estuary.

Subsequently, a SWAN wave model has been coupled to the FINEL flow model. In this coupled model, a design storm with a return period of 10000 years has been simulated. Of this storm, the wave characteristics and water level in the area were compared between the scenarios with and without oblique dam. A large decrease in both significant wave height and peak period behind the dam was found across nearly the whole Voordelta area, with the largest decrease offshore immediately behind the dam. However, the significant wave height increased on the outside of the dam and near Westkapelle. The same result was found for scenarios where 25 cm and 80 cm mean sea level rise has been applied. This is because the incoming waves from a northwestern direction are blocked by the dam, resulting in locally wind-generated waves behind the dam becoming dominant in the area. On the offshore side of the dam, significant wave heights are increased as a result of reflection of incoming waves. The maximum water level during the storm decreases in the nearshore area.

As the area behind the dam is transformed into an area resembling an estuary, the dominant hydrodynamic and, consequently, the morphodynamic processes are changed. The dam blocks all incoming waves from the west and northwest, which are the dominant wave directions in the area. Because of this, the wave energy within the area decreases significantly under storm conditions. At the same time, the tidal amplitude is expected to be amplified within the area. These changes cause the area to become more tide-dominated. As a result, a decrease in the onshore directed sediment transport into the area is expected. The entrance of the estuary is expected to become more flood-dominant, as a result of which more inflow of sediment into the created estuary is expected. Dominant sediment transport mechanisms with the estuary are also changed, but for a more accurate view of the extent to which this happens, a complete morphological model is necessary, which has not been applied in this study.

In summary, a DTP dam oblique to the Dutch coast can provide a substantial amount of energy, although it is significantly less when compared to a perpendicular dam and to previous studies. A further advantage of this dam is that the area behind is sheltered against storm wave conditions and extreme water levels, thus increasing the coastal safety. It is foreseen that the area attracts more sediment by the creation of the dam, but this needs to be confirmed by a future morphological model assessment. ... Read more