For both developed and developing nations, coastal zones form an attractive location for urban settlements. With the expected increase in the earth’s population, coastal areas will experience a further increase of inhabitants. Floating city development could therefore be an interesting alternative for land-based urban expansion on land [41]. Expanding an urban settlement towards the ocean however, will make it more susceptible to extreme forces such as a tsunami waves. By generating more knowledge on how a floating structure interacts with a tsunami wave, it can show the potential value that a floating city can bring. If a floating city limits the effect of (extreme) events such as tsunamis, it can protect the coastal zone which is located near a floating city. To provide an answer to this question, this research will focus on how a floating structure can reduce the transmission of a tsunami. With the construction of an analytical model, representing the floating structure and the tsunami wave as simplistic as possible, the system can be understood more quickly. If the problem is solvable by generic programming language, this would mean that it can solve a larger range in the spectrum of the problem. The conceptual model features two options: one where the platform has no freedom of movement, the other where the platformcan move vertically. They both assume that hydrostatic pressure holds during wave propagation and a linearization of the momentum equation describing the water particle interaction. For each option, the transmitted wave height is determined based on varying the floating structure dimensions. This gives an indication on which parameters are of influence in the transmission of the wave. First, the conceptualmodel is analysed by changing the platformdraft and the length for both the motionless and the vertically moving platform. Both options are influenced most by the length of the structure. The situation with the motionless platformshows this effect earlier, by a higher wave attenuation percentage for the same platform length. Whether the draft also has an influence is strongly dependent on the value representing the length of the platform. The difference between the two platform movement options shows that the effect of changing the allowed movement of the platformis significant. Next to the reduction in transmission, the conceptual model shows signs of resonance. The moving platform option in the model is formed by a second order differential equation. Fitting this equation, resonance is evident and therefore visible for certain combinations of the platformdimensions. In addition damping is present, ensuring that there are some parts where, despite the natural frequency pointing there, no resonance occurs. The amount of damping is strongly linked to the platformlength, with a higher level of damping for a longer platformlength. Finally, the results from the conceptual model are compared to the outcome in SWASH. This numerically based model has the possibility to simulate a tsunami wave in its development towards the coast and also features a buoyancy function for structures. This comparison serves as a validation of the conceptual model. In general, the conceptual model always results in a less reduced wave attenuation percentage and can be said to be more conservative. Due to the assumptions of leaving out the non-hydrostatic pressure and a lower level of detail, a maximum of 5% deviation in both model results occured. This however, matches with the fact that it is a less detailed model and adds to the reasoning that the conceptual model provides what it is meant for. Next to the effect of the structure itself, the positioning of the structure is also of large importance. Wave height and intensity of the wave will vary due to the surrounding local coastal features. Next to that, the local water depth is determinant in compressing the wave, therefore decreasing the wave length when the water depth decreases. The maximum wave attenuation that can be achieved according to both models is 10%, considering platform dimensions and location variations. The conceptual model appears to work for which it is intended: modelling the resulting transmission between a floating structure and a linear tsunami. It is expected that modelling programs can be expanded and/or improved, so that more realistic floating structures can be modelled. However, it will remain difficult to accurately model a tsunami as it is complex in behaviour. Yet, this research brought the field one step closer to evaluating the transmission of tsunami waves when interacting with a floating structure of certain dimensions.

Recent analysis on measurements in the North Sea has shown a large amount of wave energy ranging in infragravity bands in the North Sea during storm events. To better understand the generation and propagation of free infragravity waves, the SWAN model is used to inspect the infragravity wave pattern, the infragravity wave origins, and the corresponding contribution from different coastlines in the North Sea. The local equilibrium between sea swell wave and infragravity wavebands is applied to parameterize the source infragravity wave along the coastlines bordering the North Sea following the approach of Arduin et. al. (2014). The empirical parameter α1 representing the radiating energy level from each coastline and bottom friction coefficient χ representing the dissipation strength are combined to calibrate the model. 10 storm events with an observed infragravity wave height over 10cm are chosen to gain detailed insight into the evolution of free infragravity waves under extreme conditions in SWAN. A correlation between α1 and χ is found. A relatively good explanation between measured and predicted infragravity wave height can be obtained as long as a large α1 and a large χ are implemented in the model or vice versa, showing an accuracy of up to 75% in the open sea. Based on a proper combination of two variables, the contribution of infragravity energy from each coastline varies spatially in predicted results. The Danish coastline is the dominant region radiating infragravity waves to the open sea and the reflected free infragravity waves could reach the adjacent coastlines such as Netherlands and UK. Behind the peak of an extreme storm event, the excitation of free infragravity energy is observed along the south of the UK coast originating from Denmark. The north of Holland coasts also radiate large amounts of free IG energy but are unable to have a prominent impact in the center of the North Sea due to energy dissipation. These results imply that the radiated infragravity waves can be influential in distant coastlines in the North Sea during storm events and the changes of local coastal dynamics can be attributed to a different generation region due to the storm characteristics.