'De Slufter' is a nature conservation area located in the Northwest of the Dutch Wadden island Texel which is inundated with seawater during storm events. The landward side of De Slufter is a sand dike which is part of the primary coastal defence ring of the island. HHNK, which is responsible for the coastal safety of Texel, currently relocates the dynamic gully in the slufter mouth every time it reaches one of the dune heads (every 5-6 years approximately) because uncontrolled gully migration might widen the slufter mouth. This could lead to more wave attack on the sand dike, possibly affectingthe coastal safety of Texel. Still, HHNK is planning on ceasing the relocations of the gully. The goal of this thesis is to assess the effects on the present and future coastal safety of De Slufter. First, based on a field data analysis of annual topography and bathymetry measurements it is expected that the gully migration is governed by wave-induced longshore transport, curvature-induced secondary flow due to tidal forcing, overwash from the beach flat into the gully during storms and by the distance to the dune heads (which inhibits migration) To investigate the effect of ceasing relocations of the gully on present and future coastal safety a modelling study was performed with XBeach Surfbeat. 15 scenarios were created to assess the effects of different bathymetry and of sea level and bed level rise
Failure was assessed on two failure mechanisms: 'Grensprofiel', which consists of two aspects. A minimum sand dike volume above storm surge level must be available in every transect alongshore in the sand dike. Also, each post-storm sand dike transect must be large enough to contain a legally defined 'limit profile'. The second mechanism is 'Initiation of flooding', which means that failure occurs when at any point landward of the 'Waterstaatswerk' boundary (the outer border of the primary coastal defence) a depth of at least 20 cm is observed. In all present scenarios no failure occurs for a normative 1/3000 year storm. De Slufter is therefore considered 'safe'. Maximum wave heights did not increase significantly for different bathymetry configurations due to the large amount of dissipation occurring in the slufter valley. No overwash or overtopping occurred in any of the modelled scenarios and morphological impact on the sand dike itself was relatively low. Failure does occur for the 1/3000 year storm with a sea level rise of 1.95 m and 3.17 m. Failure does not occur in the middle of the sand dike where the majority of the wave attack happens but in the southwestern and northeastern parts due to inundation over the ridge there (‘Initiation of flooding’). It is expected that the 'tipping point' of De Slufter, which is the sea level rise magnitude beyond which De Slufter does not adhere to safety standards, is at a sea level rise magnitude of 1.70 m.

Tropical cyclones (TCs) have tremendous impact on coastal communities in terms of damage due to flooding and high wind velocities, as shown by the recent hurricane season of 2017. Coastal flooding due to TCs can be contributed to different types of forcing (e.g. high offshore water levels, rainfall, etc.), or multiple at the same time (i.e. compound flooding). While the impact of TCs increases, there is the need for better predictions and early warning systems (EWSs). Probabilistic forecasting by modelling different ensembles is wanted to account for uncertainties, which requires fast models. Current modelling options are static models (bathtub approach), which are fast but too simple. Furthermore there are advanced process-based models (like Delft3D, XBeach) which are accurate but too computationally expensive. The solution is the use of a semi-advanced model which solves all relevant processes, but does that in a computationally efficient way. The semi-advanced SFINCS model is developed to solve all necessary processes with computationally efficiency in mind, but is still in its development phase.

This research assesses how compound flooding due to TCs can be modelled in an accurate and computationally efficient way. Besides assessing the relevant physical processes, the implementation in a semi-advanced model is tested. In multiple tests it is shown that for conceptual situations a semi-advanced model can give accurate results within certain limits. Also it is shown in what conditions the advection term of the momentum balance needs to be solved and that a swash zone model approach with an indirect random forcing can give realistic results in 1D runup tests.

A case study of the compound flooding at Jacksonville, Florida, during Hurricane Irma (2017) shows that using a semi-advanced model, a similar accuracy compared to Delft3D can be achieved while being two orders of magnitude faster. Furthermore, a first test with wave-driven flooding in a real case study at Hernani, the Philippines, during Typhoon Haiyan (2013) shows that wave-driven processes have to be explicitly included to model all types of compound flooding. The approach to include these processes with the semi-advanced SFINCS model gives reasonable results compared to XBeach, although they can be further improved with more research. The resulting computational efficiency seems to get in the right range as needed for EWSs.

One sixth of the world's coastline consist of coral reefs and provide natural flood defence for the people who live in the coastal region behind the reef. However, a rising sea level, changing wave conditions and degradation of corals threaten the coastal safety of these reefs.Numerical models can be applied to study the reef-hydrodynamics and the effects of coral degradation on the reef-hydrodynamics. When non-linear processes are important or the individual waves need to be determined, a phase resolving model is preferred. Within this thesis two issues regarding the application of non-hydrostatic models to coral reefs were studied.

Due to the large bottom gradient in front of a reef, the offshore boundary has to be located in deep water, which means that frequency dispersion becomes important. The accuracy of frequency dispersion within non-hydrostatic models depends on the number of vertical layers. However, the addition of a vertical layer increases the computational time extremely. Therefore, a reduced two layer non-hydrostatic model (XBeach-nh+) was developed with the assumption of a constant non-hydrostatic pressure in the lower layer. In theory, XBeach-nh+ is capable of modelling the wave transformation from deep to shallow water, but the applied boundary conditions cannot force deep water waves. On top of that XBeach-nh+ has never been properly validated for reef environments.

Furthermore, the corals (growing on the reef flat) have a large effect on the reef-hydrodynamics by dissipating a large part of the wave energy. There exist different formulations to include vegetation into a non-hydrostatic wave model, but these formulations are mainly applicable for cylinder shaped geometries, whereas corals are more complex in shape. Apart from the shape, the in-canopy velocity can be significantly different from the free stream velocity. Therefore, a porous in-canopy model was implemented to model the in-canopy velocity, which was used to determine the canopy-induced force on the depth-averaged flow computation.

Firstly, the inclusion of the second reduced layer improves the dispersion relation up to a relative depth ($kh$) of 5 for linear waves. A simulation of biochromatic waves over a plane beach showed that XBeach-nh+ is capable of modelling the energy transfer between the major wave components. Both steeping and reflection of the sub-harmonic were modelled according to the measurements. Furthermore, the validation of random waves over a fringing reef showed the capability of XBeach-nh+ to model the reef-hydrodynamics for different wave conditions (rel. bias of -0.003 for total wave height, -0.081 for LF-waves and -0.103 for the setup). Moreover, the addition of the second reduced layer gives a more robust prediction for all modelled wave conditions, whereas the one-layer model contains more scatter.

Secondly, the in-canopy model captures the canopy-induced force when the canopy parameters were known. Both the in-canopy flow of unidirectional and oscillating flow fields was accurately modelled when the results were compared to the measured velocity though cylinders and corals. Although, the canopy parameters were not always known, it was shown that an un-calibrated in-canopy model, based on porosity and canopy height, gives a competitive result compared to a fully calibrated shear stress formulation. The applicability of XBeach-nh+ in 2-dimensional domain with a coral covered reef flat was shown by modelling a 5 day Swell event at Ningaloo Reef. Reasonably accurate results were achieved when using the in-canopy model, based on the canopy properties.

Many small island developing states (SIDS) are among the most vulnerable to climate change (e.g. sea level rise) and seasonal to inter-annual climate variability, and subsequently experience flooding due to swell waves and wind waves, coastal erosion and salinisation of freshwater lenses. To counteract this, reef flat mining for sand and aggregate offers a possible solution to the material demand for coastal protection infrastructure and other engineering projects. However, the knowledge on the effects of these pits is limited to only two studies at a single reef: near-shore wave transformation based on measurements (Ford et al., 2013) and a numerical modelling study (Yao et al., 2016). This MSc thesis provides insights on the effects of pits, related to hydrodynamics and wave runup, on a large variety of fringing reefs, by using a 1D and 2DH process-based wave-resolving hydrodynamic model (XBeach non-hydrostatic+, “XBnh+”). Model results indicate that excavation pits cause a decrease in amplitude for waves in the infra-gravity (IG) frequency band. For the majority of the modelled reefs, this was mainly due to a modification of the longest natural frequency of the reef caused by the pit, resulting in a decrease in resonant amplification. Both pit width and cross-shore location have a strong influence on this mechanism. The observed changes in variance in the high frequency (HF) band can be partly explained by a decrease in wave dissipation, as well as a decrease in wave-wave (triad) interaction, both associated with (locally) increased water depth, which causes an increase of the HF peak and a decrease of the HF tail. Of all modelled reefs, there is a 15% chance of an increase in wave runup due to the presence of a pit. This probability is lowest for pits with narrow width and/or located close to the reef crest. The change in runup ranges from +10% to -20%. An increase in runup is caused by a combination of increased wave energy at peak frequencies, as well as smaller reductions in resonant amplification. Moreover, the effects of pits on mean water level near-shore of the excavation can cause circulation patterns that potentially result in coastal erosion and entrapment of sediments.

With the implementation of new legal aspects for dune safety assessment in 2017, the current method, Duros+, is no longer feasible to represent the desired processes. This 1D deterministic, volume-balance model cannot simulate processes like inundation and overwash, which are crucial to fulfil the new requirements for dune safety assessment. Therefore, the implementation of a 2DH XBeach model for dune safety assessment is evaluated. A transformation from 1D to 2DH becomes necessary and several hydrodynamic effects come into play, which are not relevant for a simulation in 1D. Additionally, the definition of a new limit state is a challenging task in order to define failure properly: The legal definition needs to be transformed into a technically feasible one. In this thesis five different limit states have been defined in order to evaluate the influence of a limit state on its consequences. These five limit states are then applied in a case study on the Delfland coast and the consequences of the different definitions are evaluated. Additionally, the safety development of the Delfland coast over the last two decades was analysed. It was found that all nourishment strategies applied at this coastal stretch contributed to an increasing safety. This includes the placement of the Sand Engine and its on-going alongshore spreading.

Coastal defense mechanisms are an integral part in the safety of infrastructure and communities residing on coastlines around the globe. In long temporal and spatial scales, traditional “hard structures” for coastal defense can become infeasible. Incorporation of soft engineering methods for coastal defense can then be a viable solution. Vegetation belts along the coasts can be a prominent soft engineering application.
Many previous studies have been done to identify the protection offered by coastal vegetation. This thesis aims to identify mangrove vegetation interactions under hazardous wave conditions. Moreover, the study includes a comprehensive parameter space by accommodating the variation in wave climate, vegetation and bathymetry observed in the field.
The hydrodynamic-mangrove interactions are analyzed from the perspective of coastal hazard mitigation by vegetation. The study focuses on wave attenuation, setup variation and runup reduction by mangroves.
The investigations are carried out using a numerical modeling scheme, XBeach-Surfbeat.
Mangrove vegetation can substantially mitigate the effects of coastal hazards by waves (wave energy, wave induced flooding) faced in the hinterland. However, the level of mitigation depends on several factors. The most important factors are, vegetation density, mangrove forest width, wave height and water level.
Denser mangrove vegetation and wider forests increases the wave attenuation while deeper water depths in mangrove forests reduces the attenuation capacity.
The improved understanding of the hydrodynamic-vegetation interactions gained in this study can be used as a foundation for a Bayesian network. A better understanding of the effect of the different parameters on flood mitigation can then be attained.