Coastal zones have been a dynamic area and most favoured locations utilized for living, leisure, recreational activities, tourism, commerce and many other human activities. Submerged and floating breakwaters have been used as effective systems to protect these zones from wave attack. However, they are only effectively functional if the incident wave height is relatively low. Under such condition, these systems can reduce the wave transmission with significant wave dissipation and hence achieving a desirable tranquillity in designated areas. Therefore, the focus of this research is mainly to investigate the possibility of using a combination of a submerged porous breakwater (SBW) with a floating breakwater (FBW) as an innovative coastal protection system that can provide adequate calm conditions in the coastal zones with minimum visual impact. This study utilizes the open-source CFD model, REEF3D to simulate such wave-structure interaction. This CFD model is based on the RANS equations coupled with the level set method and the k − ω turbulence model. In the present study, the first section deals with the simulation of irregular wave breaking over an irregular bed profile with the use of wave reconstruction method to generate irregular waves. An excellent agreement between the computed results and the experimental data is obtained showing that REEF3D model is capable of capturing the dominant features of the evolution of the wave breaking process, both in the shoaling region and the surf zone. The second section deals with the simulation of regular wave interaction with the SBW. The simulation is conducted using the VRANS method to resolve the porous flow. The wave interaction with the SBW is validated by comparison with experimental data. An impressive agreement between the numerical results and the experimental data is achieved with very small RMSE values. Finally, the validated model is then used to simulate the combination of the SBW and the FBW. Three different cases are investigated with three different spacing between these structures. For each case, five different configurations related to the geometry of the FBW are simulated. It is found out that an effective reduction of more than 90%, on average, of the incident wave height, can be achieved for this combined breakwaters system. This means that a transmission coefficient (Kt) of less than 10% is calculated across this combination. Besides, it is found out that 1.75 FBW length to wavelength (L/λ) ratio produces a very low transmission coefficient (Kt). Further, an effective distance of 1-2 wavelength between the SBW and the FBW subsystems can also result in lower transmission coefficients (Kt).

In the Netherlands flood protection is immensely important for the safety of the nation. The shocking outcome of the 1953 flooding proves this point. In modern days, the development of socioeconomic and climate change factors casts doubt on the effectiveness of conventional approaches to flood risk management. Consequently, this project explored new approaches to flood risk management. An analysis of climate change effects led to estimation of future loading conditions. Subsequently, a detailed hydrodynamic analysis was conducted. It highlighted the significant levels of uncertainty that climate change introduces into loading conditions. Also, it confirmed the team’s perception, that the Westkapelle case region requires additional safety measures to guarantee an acceptable level of safety in the future. But how to guarantee the acceptable level of safety in the most efficient way? The team adopted the concept of robustness to find an answer. In a keynote publication Mens (2015) describes robustness in the following way: "Robust flood risk systems have some degree of resistance and some degree of resilience: the system can withstand some floods (no response), and for other (larger) floods impacts are limited and the system can recover quickly from the flood impact (response and recovery)." The team set out to include robustness as an integral part of the design process to handle uncertainties. The project shall be seen as an explorative study how this can be done, revolving around Westkapelle as a case study that proves the methodology’s feasibility. Robustness and uncertainty were included on multiple levels throughout the design process. Firstly, the range of uncertainties was quantified. Secondly, the effect, that single parameters have on the magnitude of uncertainties, was assessed. Thirdly, the system’s capacity was analysed to find the required overtopping reduction for guaranteeing sufficient safety. Fourthly, constructive measures were assessed on their robustness potential and satisfaction of stakeholder needs via a Multi Criteria Analysis (MCA). The MCA was then employed to select the type of constructive and non constructive measures to achieve the required levels of overtopping and safety. With the information on uncertainties, the measures were combined to form a robust design, consisting of living breakwater, dike heightening, surface protection and two policy measures. Probabilistic analysis was also done to see the sensitivity of the failure probability to sea level rise in different loading and design scenarios. A thorough comparison between the conventional design, that has been applied to the project location, and the robust design followed. The robust design came out on top. Robustness was found to be an effective tool in countering uncertainties. Where conventional design methodologies are lacking flexibility and precision, the robust design methodology makes use of the system and its resilience to find an optimal solution. Its applicability may not be limited to flood risk management only but stretch out to other civil engineering disciplines.