Low-lying reef-lined coasts are vulnerable to coastal flooding. While fringing reefs usually protect coastal communities from moderate storms by dissipating incoming wave energy, they can exacerbate flooding during strong events with energetic waves. Low-frequency waves (0.001-0.04 Hz) play an important role in coastal hazards, dominating nearshore reef flat wave patterns. Furthermore, low-frequency resonance on reef flats has been suggested to exacerbate coastal hazards. Despite expanding knowledge of wave dynamics on coral reefs, the transient characteristics of low-frequency resonant oscillations on reef flats have not been extensively researched, and no methods currently exist for analyzing this phenomenon in field observations.

This thesis introduces a new methodology for assessing transient characteristics of resonant and energetic low-frequency oscillations using the Hilbert-Huang transform. The study is based on a 5-month dataset from a cross-shore transect of a reef at Roi-Namur in the Marshall Islands, where the reef geomorphology corresponds to a natural frequency in the very low-frequency range (VLF, 0.001-0.005 Hz).

The findings indicate that resonance in the dataset typically had a short persistence, with a median and 90th duration of approximately 5 and 10 minutes respectively. Normalised with the period of the oscillations, which have a typical frequency of 0.0035 Hz, these durations are equal to approximately 1 and 2 wave oscillations. While these results are consistent with previous notions about the potentially short durations of resonance for this particular reef and dataset, they contradict suggestions that resonance is associated with a build-up over several wave periods. Comparison of resonance durations with instances of coastal flooding at the site shows that resonance occurred for about 1 hour within a 24-hour recording period during overwash and large runup events, but also at other times. The duration of energetic VLF oscillations, in general, was more closely linked to the severity of coastal flooding events. While resonance did not directly correspond to coastal flooding, it was observed that energetic VLF oscillations persisted for longer durations under (close to) resonant conditions.

These results suggest the critical role of energetic VLF waves in coastal hazards, while resonance may be of secondary importance. However, it is important to acknowledge the limitations of this study, which focused on a single dataset from a specific reef geometry and included short-duration recordings (34 minutes). Moreover, this study demonstrates the application of the Hilbert-Huang transform for analysing sea surface elevation signals recorded at a coral reef.

The insights gained from this research underscore the importance of considering both energetic VLF oscillations and resonance in coastal hazard assessment. Future investigations should explore factors influencing the persistence of such oscillations, like the characteristics of incoming wave forcing, to enhance coastal hazard prediction. Furthermore, the Hilbert-Huang transform offers a versatile framework for exploring more aspects regarding the time-varying and non-linear behaviour of waves on coral reefs.

The methodology framework proposed in this study facilitates the bulk analysis of extensive field data and longer sea surface elevation recordings, enabling further exploration of reef morphology and hydrodynamic controls on the persistence of energetic and resonant VLF oscillations.

The lock of Hansweert, located in the province of Zeeland in the Netherlands, serves as a crucial inland shipping node between the Western Scheldt estuary and the Rhine Delta. The outer harbour of the lock complex provides the connection to the Western Scheldt and accommodates waiting facilities for inland ships. The Western Scheldt is a vital gateway for maritime traffic, linking the Port of Antwerp to the North Sea. As the navigation channel of the Western Scheldt is located close to the outer harbour of Hansweert, multiple ship incidents at this location are attributed to the water motions generated by passing seagoing vessels on the Western Scheldt. This research investigates how the water motions induced by passing vessels in the Western Scheldt contribute to unsafe situations in the outer harbour and locks of Hansweert, and what preventative measures can be identified to effectively minimize incidents and mitigate risks.

Investigating the incident records reveals that the key contributors are the primary water motions generated by the passing vessels in the Western Scheldt. The phenomenon, experienced as a sudden lowering of the water level and suction forces, can lead to the breakage of mooring lines and uncontrolled movements of inland ships, resulting in a range of safety hazards and operational disruptions. Several documented incidents, field studies and interviews highlight the urgency for effective measures to mitigate the potentially harmful effects of passing vessels on the ships in the Hansweert outer harbour and locks.

A seven-week measurement campaign, involving 1281 passages of so-called oversized vessels, reveals distinct patterns of water level fluctuations during a vessel’s passage. A vessel is considered oversized if its length exceeds 210 metres or if its draught is larger than 10 metres. These patterns are described as a translatory drawdown wave travelling into the harbour, reflecting against the lock complex and oscillating back and forth in the outer harbour until dampened. The key parameter characterizing this wave is the lowering of the water level, referred to as the drawdown height. The average measured drawdown height approximates 6 centimetres, with maximum observations up to 40 centimetres. The main factors influencing the drawdown height are the vessel’s passing distance to the outer harbour, its speed relative to the currents and its dimensions, shown by a correlation analysis between the parameters describing the passing vessel and the generated drawdown height. Extreme drawdown events were exclusively observed during a combination of a relatively high speed through the water of the seagoing vessel and small passing distances relative to the harbour’s entrance.

The impact of the drawdown effects on the inland ships is determined by the forces generated by the pressure difference along the ships, caused by the inclination of the water level. A critical drawdown height of 12 centimetres is set, based on existing force criteria and the linear relation between the drawdown height and water level slope. To improve on the existing drawdown height prediction methods, a site-specific drawdown height prediction equation has been derived. Validation of this equation using the observations made during the measurement campaign yields a Pearson correlation coefficient of 0.81 and an Mean Absolute Error score of 2.2 centimetres.

Preventative measures are identified, aiming to minimize incidents and mitigate the risks related to the water motions induced by passing vessels. The predicted drawdown, generated by the passing vessel, is kept below the critical level by recommending a maximum speed related to the passing distance and dimensions of the vessel. Practically, this measure could be applied as a calculation tool or as an overlay on the pilot’s electronic sea chart. Coupling this information with awareness campaigns for pilots will contribute to minimizing the adverse effects on the ships in the outer harbour. The resilience against drawdown-induced risks could be strengthened by restricting the maximum combined width of ships moored alongside. Furthermore, by limiting the excessive slack in the lines of the moored ships, through signage and floating bollards, the movements of the ships will be restricted, reducing the risk of line breakage. Notifications of anticipated critical drawdowns would allow traffic controllers or lock operators to caution the inland ships and delay the lock chamber door openings, whilst alerting the passing vessel. Incorporating the mitigation measures recommended in this research could positively impact the safety of navigation in the Hansweert outer harbour and locks.

The current trend in the offshore renewable energy industry is to install ever larger offshore wind tur- bines (OWT) in ever deeper waters. This results in an increased capacity requirement of the substruc- ture which results in increased dimensions and thus higher hydrodynamic loads.
For the design of OWT the fatigue limit state is next to the ultimate limit state a crucial design loading condition to consider. In the current engineering practice, wave spectra are mostly reconstructed with the JONSWAP spectral parametrization which gives a simplified representation of the actual sea surface based on bulk wave parameters for wave components, such as wind-sea and swell. By using realistic frequency-directional (2D)-wave spectra instead of the JONSWAP spectral model, the wave spectral density distribution can be showed in its most authentic form.
This study investigates the influence of 2D-wave spectra on the wave-induced fatigue loads on OWT’s, as a comparison to the commonly used JONSWAP spectral parametrization. Therefore the relation of different wind and wave parameter as well as of the wave spectral characteristics on the wave-induced loads and the load differences between both spectra are investigated. Wave spectral data as well as bulk wave parameters for a wind-sea and swell wave component were obtained from the DHI MetOcean database for the Dutch North Sea. Based on those the fatigue loads were calculated in the frequency domain using Dirlik’s method.
Results show that the JONSWAP spectral parametrization leads to an overall overestimation in the range of ∼ 12 - 15 % of the total wave-induced fatigue loads compared to those obtained from 2D-wave spectra. The highest fatigue load differences and DEL overestimation between the JONSWAP and 2D- wave spectra were observed for significant wave heights between 2 and 3.8 m (14.70 % overestimation), peak periods between 8 and 15 s (13.63 % overestimation) and wind speeds between 10 and 20 m/s (15.04 % overestimation).
Furthermore, it was observed that narrow frequency and directional width, between 0.1 - 0.3 Hz and 20 and 30 deg, lead to the highest discrepancies for the calculated fatigue loads between the JONSWAP and 2D-wave spectra (∼ 13 % overestimation). For extreme sea states, that correspond to the upper 5 % of the observed data it appears that the JONSWAP spectral parametrization leads mostly to an underestimation of the calculated fatigue loads.
With the distance correlation calculation the dependencies between the wave characteristics, such as the frequency width, directional width and the peakedness parameter, and the fatigue load differences was quantified. But this analysis did not result in additional research insights since the obtained corre- lation coefficients were almost identical and only exhibiting weak dependencies.
Still, it can be concluded that using 2D-wave spectra in the engineering practice and for the fatigue load calculation on OWT’s with monopiles as substructures, physically more representative loads can be obtained compared to those computed with the simplified JONSWAP spectral model. This more realistic load estimation could finally lead to an improved design of OWT’s.

In this thesis, the identification and generation of meteotsunamis on the Southern North Sea are investigated. Although meteotsunamis are well known in other areas, hardly any research has been done on them near the Dutch Coast. Here, nine years of sea level elevation data are evaluated using the classic approach and a subsequent wavelet analysis to identify a list of potential meteotsunamis. Six of those events have been selected, for which the atmospheric conditions were simulated by WRF. To definitively classify them as meteotsunamis, a pressure jump and atmospheric front responsible for the generation are identified. The six events are characterized by computation of their angle of incidence, wave period, and wave height, and are checked for the probability of Proudman resonance. The six selected events were all linked to a pressure jump and an atmospheric front, and thus classified as meteotsunamis. Two events propagated from the North-West and the other four from the South through the English Channel. The wave periods varied between 10 and 25 minutes, and the wave heights varied around 0.20 meters, with a maximum wave height of 0.63 meters. Three events were linked to a front propagating overseas, all of which had a high probability that Proudman resonance contributed to their amplification. These results form a foundation for future research on meteotsunamis on the Southern North Sea.

Dunes often act as the primary line of defence for low-lying hinterland against storm surges with dune erosion in the swash-dune collision regime. In the swash-dune collision regime, an elevated total water level, consisting of tide, surge, and wave setup, temporarily submerges the beach. As a consequence, the incident swash may run up to the dunes and collide with the dune face. The damage to dunes following from these collisions can be severe. In the most extreme of cases, storm surges with dune erosion in the swash-dune collision regime can lead to dune breaching or failure with flooding of the hinterland as a consequence. These floods can have devastating economic effects and potentially lead to loss of life.

The risk assessment of areas protected by dunes is often performed using predictive dune erosion models. Such models first use possible storm conditions as input, then use a set of physics based and empirical equations to model the physical processes based on that input, and finally use a measure of impact to the dunes from the model results to estimate the amount of damage to the dunes. However, not all physical processes are currently fully understood, complicating accurate reproductions in models and the risk assessment of areas protected by dunes.

This dissertation aims to study two such physical processes that are relevant to dune erosion during storm surges in the swash-dune collision regime. The first process is the suspension of sediments in the inner surf zone. The total amount of sediment in the water column affects the sediment transport rates. As a consequence, sediment concentrations can have a substantial impact on the magnitude and speed with which sediments eroded from the dune face are transported offshore, and thus on the total amount of dune erosion.

The second process studied is sediment transport due to soil instabilities, i.e. the slumping (or avalanching) of sediments from the dune face. Sediment transport due to soil instabilities leads to dune scarping and a gradual retreat of the dune face. If this type of transport persists for a prolonged period of time, the retreat of the dune face can continue until there is no more dune to erode. As a consequence, the dune breaches and enters the overwash regime, and complete failure of the dune may follow.

To study both processes, a prototype scale field experiment was conducted in the winter of 2021-2022. Two artificial dunes were constructed in close proximity to the high water line on a sandy beach. This increased the probability that the total water level driven by a storm event would result in dune erosion within the swash-dune collision regime. The dunes were monitored for a period of three months and within this period three such events occurred.

The suspension of sediments in the inner surf zone was studied by comparing variability in measured, wave-averaged (i.e. 20 min mean) suspended sediment concentrations. These variations were compared to variability of hydrodynamic drivers that are known from literature to govern sediment suspension during storm conditions. Overall, sediment suspension due to bore turbulence appeared the dominant suspension driver during energetic events representative of storm conditions. During such events, wave energy was saturated in the inner surf zone, and almost all waves were breaking and contributed to the generation of bore turbulence at the free surface. The outcome of the first study suggests that, based on the events analysed, dune erosion models may achieve more accurate results if computations of the magnitude of suspended sediment concentrations were to include a bore-induced turbulence term. If such a term is already included, models should properly address the relative importance of bore-induced turbulence when compared to other drivers.

Sediment transport due to soil instabilities was studied by analysing profile crosssections of the dune face during two storm events. Overall, the morphodynamic behaviour of the upper dune face and dune crest was primarily steered by the morphodynamic behaviour at the dune base. The morphodynamic behaviour (i.e. erosion rate) of the dune base correlated well with the elevation difference between the dune base and the incident total water levels, specifically the square of the total water level that was exceeded for 2% of the time. The slumping events that occurred during both storms likely occurred when sediments from previous slumps at the dune base were nearly depleted by the persisting erosion rate. As a consequence, under similar erosion rates, a new slumping event occurred sooner when the volume of the preceding slump was smaller. A clear relationship could not be established between hydrodynamics seaward of the dune and the volume of individual slumps.

Different model approaches are currently being used to implement sediment transport due to soil instabilities. These different approaches all use the persisting erosion rate of the submerged part of the dune base to steer the erosion of the upper dune face. Therefore, according to the field experiment results, these different approaches may all be able to achieve accurate dune erosion volume magnitudes. Still, depending on the application (e.g. one-dimensional versus two-dimensional modelling), one approach might be more suitable than others.
@en

Coral reefs are considered to be a crucial ecosystem and form of natural coastal protection. They owe their effectiveness to their high structural complexity. This complexity, which positively influences aspects like species richness, coral recovery, live coral cover, and coastal protection, is often quantified using parameters such as porosity and rugosity as has been done in this thesis. However, coral reefs are currently facing decline due to both natural processes and human activities. To address this issue, artificial reefs (ARs) have been proposed as a potential solution. Coastruction, a start-up specializing in 3D printing, has developed a 3D printer capable of creating artificial reefs in any shape. In this thesis, biomimicry-inspired complex ARs were designed and printed to examine the impact of structural complexity on wave attenuation.

Data was collected through physical model tests in a wave flume at Deltares, using Froude scaling with a length scale of nL=4. The experiments involved 1 to 2-meter-long reefs composed of three distinct structures with different geometrical complexity levels: design 1 (medium complexity), design 2 (maximum complexity), and cubes (low complexity). These structures were tested under similar, primarily non-breaking wave conditions. The aim was to determine if the dissipation observed resulted from friction rather than wave breaking.

The results of the conducted experiments showed that structural complexity does not significantly affect wave transmission. Overall, high transmission coefficient were observed across the different structures, ranging from 0.96-0.99. Additionally, no significant differences in dissipation were found among the three structure types, with dissipation ranging from 5.5% to 13.5% of the incoming wave energy. However, increased reef coverage positively impacts transmission reduction and spectral wave dissipation, exhibiting a linear decrease in transmission coefficients and a simultaneous increase in dissipation.

In summary, the various artificial reefs examined in this study demonstrated minimal effectiveness in wave attenuation, with negligible differences among reefs featuring different degrees of structural complexity. Despite the similar wave attenuation effects, the environmental benefits of a highly structurally complex reef suggest that it should be considered a viable solution.

This thesis analyses the importance and application of considering multiple coexisting wave trains in sediment transport predictions.

The wave trains at offshore and nearshore locations are analyzed by wave spectral partitions (Portilla at el., 2009). The temporal variability of consistently occurring wave trains can be analyzed through so-called wave families (Portilla et al., 2015). The wave partitions show that a unimodal wave spectrum occurs 35% of the time in front of the coast of the province of North Holland in the Netherlands. This means an error is introduced 65% of the time as unimodal spectra are assumed in traditional methods that use generalized wave parameters to estimate sediment transport. When the wave field is not unimodal, the angle between wave trains is considerable. Only 39% of the time is the angle between a wave train and the most energetic wave train smaller than 60°. For 43% of the time, the angle is between 60° and 120°. The remaining 17% are wave trains propagating in opposite directions. The difference in wave angle is half of the time larger than 50° when a threshold of one meter of deepwater significant wave height over the whole spectrum, defined by Holthuijsen (2007), is used.

Unsupervised machine learning techniques Principal Component Analysis, Maximum Dissimilarity Analysis, and the kmean clustering algorithm are used to group similar wave spectrums over time, spatially group wave families, analyze the wave climate's spatial-temporal variability, relate wave - and weather conditions, and assess their impact on wave-driven potential sediment transport. The Principal Component Analysis is used as a dimensionality reduction technique. This study shows the physical meaning of the Principal Components and their temporal variability in the wave frequency-directional spectrums, where Principal Components describe wave energy in each frequency-directional bin. The kmean clustering algorithm groups similar wave spectrums over time and gives insight into wave conditions, like calm and stormy conditions, called Sea States. The Maximum Dissimilarity Technique changes the focus between common conditions or extreme events. The wave partitions, wave families, and Principal Components give additional insight into the Sea States.

Potential sediment transport estimations are made by traditional methods that use a representative wave frequency and direction and through vectorizing and superpositioning the estimations by wave partitions and wave families. The outcomes show that it is challenging to predict sediment transport for multimodal wave conditions with state-of-the-art sediment transport formulations and models. The results show that the wave period significantly impacts sediment transport. The sediment transport for coexisting wave trains causes about 58% of the time more sediment transport than traditional methods. Here the not considered interaction between wave trains on bedload and sediment suspension will enhance sediment transport. Furthermore, the sediment transport estimations give insight into the wavefield and weather conditions that contribute most to the yearly wind-wave induced potential sediment transport caused by high energetic or consistently occurring conditions.

Mangrove is a coastal vegetation type primarily located in the tropical regions between 5 North and 5 South. This coastal vegetation is capable of reducing the force of incoming waves. This is a result of the obstruction created by the roots, stems and canopies against waves to propagate through. Because of this capability, mangrove vegetation offers coastal protection against extreme annual flood events for 15 million people. Recent studies identified that in 2050, the present-day 100-year extreme sea level will occur annually in a large part of the tropical region. The increase of these extreme events is primarily driven by the projected sea level rise. The hypothesis is that before this chronic flooding is observed, storm-induced flooding might already be observed. The expected increase in the probability of these extreme events is defined as non-stationarity of extreme events. The hypothesis therefore is that the non-stationarity of extreme events could already be observed in historical data.

This study examined non-stationary extreme events between 1987 and 2018 for a total of 5809 hydrodynamic environments distributed around the global mangrove coastline. These mangrove environments defined the hydrodynamic wave characteristics and water levels. The results showed that 87.5% of the mangrove environments have a positive trend in the location parameter of the Generalized Pareto Distribution for the extreme events. The shift of the location parameter indicates an increase of the impact of the extreme events. These extreme events were defined as multivariate extreme events consisting of the significant wave height, the mean wave period and the skew-storm surge, respectively the parameters H_s, T_m and S. This combination is based on the fact that the coastal protection offered by mangroves is depending on the water level and the energy within the long-period waves.

The methodology proposed is capable to perform a non-stationary multivariate extreme value analysis and observe the evolution of the extreme events in these three dimensions between 1987 and 2018. The study showed that the average of the extreme events has been increasing at 70.6%, 72.6% and 64.6% of the mangrove environments in the discussed three dimensions. The average multivariate extreme event in mangrove environments increased by 0.06 m, 0.16 s, and 0.7 cm in three dimensions respectively H_s, T_m, and S. Furthermore, the number of extreme events increased, on average, between the first and second half of the time period, from 4.16 to 4.32 extreme events per year.

The global impact assessment of the non-stationarity of these multivariate extreme events was translated to the 27440 mangrove locations along the global coastlines. Statistical upscaling methods allowed the application of a numerical expensive hydrodynamic model to propagate the offshore conditions to onshore. The impact of the non-stationarity of extreme events was applied to a theoretical framework, introducing the possibility of defining the vegetation width as an optimization parameter. To meet the same safety standard for the 1/40 year design condition in 2018 concerning 1987, the vegetation width of the theoretical framework had to increase on average by 16.8 m. After post-processing the hydrodynamic runs, the results showed that the coastal safety offered by mangrove vegetation is depending on the combined impact of these three parameters, emphasizing the importance of expanding the mangrove vegetation to withstand the non-stationary multivariate extreme event.

The main recommendations from this study are based on the results. First, the study showed that the required vegetation width to ensure coastal safety has increased. It is therefore highly questionable if a stationary extreme value analysis is still valid in these environments, especially when the currently neglecting sea level rise is taken into account. Second, the observed increase in multivariate extreme events and the number of extreme events may reduce the persistence of mangroves and therewith their ability as coastal protection. It is therefore recommended to monitor the condition of the global mangrove forests more closely and to take action when mangrove vegetation is decreasing.

Infragravity (IG) waves are long waves whose frequency ranges between 0.005 Hz to 0.04 Hz. The directional properties of IG waves are essential in various engineering fields, but few spectra reconstruction methods have been tested to be robust for the last forty years. In this study, artificial wave signals were created to check the accuracy and reliability of several wave spectra reconstruction methods. These data were designed as if produced from the REFLEX field measurements. Three commonly used conventional directional wave spectra reconstruction methods, EMEP, IMLM and BDM, are applied and compared over Sea-swell (SS) wave field and IG+SS wave field.

Though the existence of bound IG waves weakens the accuracy and reliability of these methods, they may be used to reconstruct the IG wave spectra with reasonable results. It is found that the EMEP outperforms the BDM and the IMLM since it yields accurate results with low sensitivity to noise in most cases. The BDM is reliable in moderate wave conditions, but would fail in case of high wave height or narrow directional spreading. The IMLM tends to produce less accurate results than the EMEP. The EMEP is recommended because of its overall reliability and low sensitivity.

Infragravity (IG) waves (0.005-0.04 Hz) are surface waves that can dominate the nearshore hydrodynamics and can impact various coastal processes (e.g., run-up, overwash). A proper offshore description of incident IG waves is required for storm impact models, which generally assume a local equilibrium between sea-swell (SS) waves (0.04-0.33 Hz) and the resultant nonlinearly excited bound IG waves. The contribution of free incident IG waves and the directional properties
of the IG wave field are neglected, though they play a critical role in IG wave variations. Various research focuses on IG wave dynamics in shallow waters, but a detailed understanding of IG wave variability in intermediate water depth is lacking. Furthermore, the directional spectra of bound and free IG waves in the field are usually unavailable since bound IG waves do not follow linear dispersion relations, which is assumed by commonly applied directional spectra reconstruction methods.

In-situ observation data (surface elevation, pressure, and velocity) of IG waves from November 2021 to April 2022 at three cross-shore locations in intermediate water depth (∼ 6 − 14 m) off the Dutch coast are analyzed to study generation, propagation, and directional properties of IG waves. The bispectral technique is applied to quantify the contribution of bound IG waves to the total IG wave field. A newly developed method is validated and used on the offshore (∼ 14 m) data to reconstruct directional spectra of bound and free IG waves separately during storms.

The results show that IG wave heights are best predicted (correlation coefficient R² up to 0.94) with an offshore forcing parameter that includes peak period H_ssT_p² , which indicates SS wave energy flux. The growth rate of IG waves is in between the shallow water equilibrium solution (for bound waves) and Green’s Law (for free waves), indicating the total IG wave field consists of bound and free components. The relative contribution of bound IG energy (up to 76%) is correlated to SS wave energy, whereas it decreases dramatically during intense storms when SS waves break. The bound IG waves that are assumed to follow the weakly nonlinear wave theory, have a similar peak direction and broader directional spreading to SS waves. In contrast, the directional spectra of free IG waves are nearly isotropic during calm conditions, but may have diverse peaks during storms, which incident IG waves from remote sources and edge waves can
influence. A better understanding of the complex pattern of free IG waves requires more detailed observations and/ or modeling.

Many coral reef islands are low-lying, which in combination with population growth, sea level rise and possibly more frequent extreme weather events is likely to result in increased coastal risk (e.g. Storlazzi et al., 2015). On smaller scales of O(10 km) wave-driven coastal inundation can be accurately predicted with advanced models such as XBeach (Roelvink et al., 2009), at already high computational costs. For larger scales, larger number of islands, for scenario modelling, and for implementation in early warning systems, computationally faster methods are needed. Reduced physics models, which neglect some of the processes (e.g. non-hydrostatic pressure gradient term and viscosity), are a potential solution. However, their accuracy and the best method to force them has not been established. In this research we propose a new methodology to model wave-driven flooding on coral reef-lined coasts. A look-up-table (LUT), composed of XBeach model runs, is combined with a reduced-physics model, SFINCS (Leijnse et al., 2021), to achieve high accuracy predictions at limited computational expense. The LUT consists of pre-run 1D XBeach simulations for several reef profiles from Scott et al. (2020), forced with different offshore wave and water level conditions. Wave conditions close to the shore as predicted by the LUT are used to force SFINCS which then simulates the wave runup, overtopping and flooding. These are forced in SFINCS using random wave timeseries from an interpolated parameterized wave spectrum following Athif (2020). The accuracy of the method is investigated for 6 distinctive cross-shore profiles from Scott et al. (2020), for two wave scenarios (gentle swell and stormy conditions). Results of complete XBeach simulations are compared to LUT-SFINCS simulations with different boundary forcing locations. The sensitivity analysis shows that the most optimal boundary location to initialize the SFINCS model is at a water depth of 0.5 m, shore-ward of the reef edge. Interpolation of the forcing conditions at the boundary is investigated with 9 different interpolation methods. Results reveal that the most accurate method to interpolate spectral parameters (the amount of high frequency wave energy, the amount of low frequency wave energy and the frequency peak of high frequency part of the spectrum) and wave setup at the boundary is the Inverse Distance Weighting method with a power of -2. Errors introduced by the interpolated parameterized boundary conditions lead to runup estimation errors of 10-20% on average with the maximal errors up to 60%. Simulating wave runup with XBeach LUT – SFINCS couple leads to about 50-times higher computational speed compared to the XBeach model simulating hydrodynamic processes along the entire reef profile.

Coral reefs are vital to the prosperity of the world and the local communities by providing food and coastal protection. Coral reefs are home to 25% of marine life and have therefore gained the nickname ’rainforest of the sea’. However, the reefs are being damaged by climate change and human intervention, resulting in an alarming rate of degradation. The combination of higher water levels on reef flats and the reduced friction due to reef degradation lead to greater risks of flooding and overwash on low-lying islands.

These aforementioned effects already present a threat to small islands that are naturally fronted by coral reefs. However, there is one more threat of damage to these islands: resonance. The characteristic bathymetry of the coral reefs can induce this type of threat, which is likely to be enhanced through climate change. Fringing reefs can display harbour-like resonance under the right conditions, because of their generally steep fore reefs and shallow horizontal reef flats. This has been shown to occur during various occasions, but the conditions that lead to are still unclear. What drives the occurrence and magnitude of resonance?

Resonance has been observed during high-energy events on various pacific islands, and though it occurs only 3.6% of the time, its damaging potential is larger than any other form of infragravity (IG) wave propagation. Various numerical models have been set up to study the behaviour and drivers of resonance, yet the defining characteristics and drivers of the phenomenon remain enigmatic.

To get a better understanding of the parameters that influence the occurrence and magnitude of resonance in fringing reef environments, a SWASH model is used. The amplification of the incoming IG wave is determined
by comparing the incoming IG wave energy at the beach toe for a beach boundary to an absorbing sponge layer boundary. If the ratio of incoming IG energy is greater than unity, resonance is expected to occur.

The magnitude of the AR is largely dependent on the characteristics of the incoming SS wave field and coral reef bathymetry: a great modulation and a gentle fore reef slope lead to a large amplification. Additionally, a lot of incoming SS wave energy corresponds to a limited amplification of the incoming IG wave, likely due to the interaction of the released bound long wave and the IG wave generated through break point forcing, which are 180° out of phase. Therefore, the largest amplification occurs for a strongly modulated wave field without being too energetic. There is no clear relationship between the incoming SS wave periods and the amplification, the height of the incoming IG wave is similar for various situations. The amplification appears to depend on the combination of the fore reef slope and incoming SS wave periods. Different SS wave periods
lead to different amplifications depending on the slope of the fore reef they pass over. The height of the incoming IG wave is largest for the steepest slopes under consideration. Thus, for these conditions, the risk of flooding is largest. However, the greatest amplification occurs for the gentler slopes
under the considered conditions. It should be noted that this relationship is heavily dependent on the combination of incoming SS periods and the steepness of the fore reef slopes.

The period for which resonance occurs is always higher than the theoretical fundamental eigenperiod, which is found through water depth integration from the location of minimum IG wave height to the beach toe. Integrating the water depth from the most offshore point of break point forcing leads to a smaller deviation of the theoretical fundamental eigenperiod from the observed period, yet this method needs further exploration.

The insights acquired by this research can be used as a base for site-specific modelling, which can provide circumstances under which certain areas are at risk of flooding due to IG wave resonance.

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.

Sandy beaches can be found all over the world and are on the interface between the sea and the land. Important functions of beaches are the protection of the inland to the forces of the sea and providing local opportunities in recreation. The impact of storm events on the beach is therefore an important topic of research especially with future climate change predicting more extreme events with the influence of Sea Level Rise expected to result in a worldwide decrease in beach area.The famous Copacabana beach, located at the South-Atlantic ocean is one of the most popular tourist attractions in Rio de Janeiro with thousands of visitors per year. The beach is characterized by its parabolic shape with rocky headlands on either sides. In July 2019 a storm event occurred at the beach with a 7-day period of energetic waves. This resulted in significant erosion along the whole beach up to 40 meters leaving not more than 10 meters of beach width in the South part of the beach. The period of erosion was followed up by a period with year-round average wave conditions resulting in rapid natural recovery with the beach returning to its original beach width within a period of 4 weeks. The focus of this research is on the cycle of erosion with subsequent recover which is important in having a long-term sustainable beach cycle.The history of Copacabana beach is marked by one major nourishment in 1970 which resulted in the 55 meter widening of the beach parallel avenue and an average widening of the beach of 35 meters. From 1970 onwards historically available satellite images show a stable beach behavior with the equilibrium profile of the beach showing smaller beach widths in the South compared to the North. A dataset of high resolution Sentinel 2 is analyzed in terms of beach width for a period of 4 years. This clearly shows the short-term variations in beach width of which most are the result of the impact of storm events. This highlights the impact of the July 2019 storm event showing rapid recovery in terms of beach width.Storm events are characterized by a 2 to 7-day period of energetic swell-dominated waves often from in between the South and SSE. The maximum wave height during the July 2019 storm event was of a yearly return period in combination with an erosional impact which was of lower frequency according to locals. What caused the big erosional impact was the long 7-day duration of the storm in combination with an extraordinary wave direction from the SE. Under this wave direction the South part of the beach is most vulnerable due to a convergence of wave energy as a result of bottom refraction. In combination with a lower equilibrium width in this part of the beach due to the lack of sand placement during the 1970 nourishment this part of the beach is most vulnerable to storm impact. The subsequent beach recovery process shows rapid beach width recovery with recovery rates up to 1.4m/day. This is the result of mild wave steepness due to the swell-dominated wave climate in combination with the equilibrium beach state characterized by an attached sandbar. Both these system characteristics are positively related to the recovery rates (Phillips et al., 2017). However, structural erosion is visible in terms of backbeach elevation in the South part of the beach which as of 16 months after the July 2019 erosion event shows no signs of recovery.To further test the beach vulnerability, the July 2019 storm and subsequent recovery period are modelled subsequentially with the XBeach Surfbeat and- stationary mode. With the use of scenario modelling an attempt is made to test the vulnerability related to wave characteristics, erosion/recovery duration and the frequency of storms. Judgement of the model performance resulted in good model applicability and realistic model results for the erosion simulation. The results confirm the highest impact is in the South part of the beach under a SE wave direction. Besides this, the impact of an increased wave height (resulting in a 21% increase in erosional volume with a 10% increase in wave height) is more significant along the whole beach compared to an increased extreme event duration (resulting in a 9% increase in erosional volume with a 20% longer duration). During periods of recovery the swash zone processes become more important. These processes are not well represented in XBeach. To compensate for these effects the Bermslope model can be used forcing the slope in the swash zone to a pre-defined value (Roelvink & Costas, 2017). The model results however still shows a limited interaction between the beachface and the sub-aquatic part of the beach resulting in accretion further offshore than is observed in reality. From this it is concluded that it is not possible to assess the beach vulnerability in relation to recovery rates with XBeach.The third and last part of this research looks into the future changes in beach vulnerability taking into account the effect of climate change. Local long term climate trends are analyzed with the use of multiple data sources. This results in a clear positive trend showing a future increase of mean wave height. For the other climate parameters like the extreme wave height, storm frequency and wave direction a wide range of trends is found. This often shows both a positive and negative trend among the available data. Within the range of future climate trends there is a clear indication of a future increase in beach vulnerability. Both an increase in the mean wave height as a potential increase in extreme wave heights has significant implications on the erosional quantities judging from the model results. Where a 10% increase in extreme event wave heights results in a 22% increase in erosional quantities according to the model results. With a possible eastward change of mean wave direction chances of SE directed storm events increases resulting in increased beach vulnerability in the South under convergence of wave energy. For Sea Level Rise the impact is relatively highest in the South of the beach with beach decay predictions being approximated at a maximum of 8.4 meters as of 2070 with the use of the Bruun rule (Bruun, 1962). From this it is concluded that the vulnerability of the South part of the beach is bound to increase the most in the future with also taking into account the structural backbeach erosion as a consequence of the July 2019 erosion event. Future interventions with the goal decreasing beach vulnerability should focus on either widening or further protecting the South part of the beach.

Atolls and coral islands in and around the Pacific and Atlantic Ocean largely rely on coral reefs for their coastal protection. On these reefs a large roughness is created by different coral species. The dissipative character of these species reduces the incident wave height, thereby reducing the amount of run-up on the coast. In our current society we are confronted with large issues due to climate change such as global sea level rise and temperature rise. Because of these issues, coral reefs are more prone to bleaching and degradation. Research has also shown that the wave climate will intensify due to sea level rise. As the sea level will rise globally, the water level above the rough coral elements will increase leading to less wave dissipation and more extreme wave conditions at the coast. It is therefore of vast importance to gather more knowledge about the dissipative character of corals to quantify the risks coral reef islands and atolls will face in the coming decades due to climate change.

For this master thesis, an experimental study was performed in the wave flume of the Fluid Mechanics Laboratory of the Technical University of Delft, investigating dissipation rates due to complex corals. 384 corals were reproduced and exposed to 21 different wave conditions. It was found that the presence of corals significantly enhanced dissipation rates. Shallower water depths, higher wave frequencies, and higher wave heights positively influenced the dissipation rates due to the corals. By increasing the amount of corals in flumewise direction and over the cross section, rates also increased.

Lowe et al. (2005b) and Lowe et al. (2007) developed a canopy model that is able to estimate in-canopy flow velocities and dissipation rates due to coral canopies. In the final part of this thesis, this canopy model was assessed and its output was compared with the experimental results. In general, the model predictions were in good agreement with the experimental results.

The research presented in this thesis is a contribution to the understanding of hydrodynamic processes around coral reefs. The good performance of the analytical model of Lowe et al. (2005b) and Lowe et al. (2007) can be of value for quantifying risks atolls and coral reef islands will face in the coming decades. The model can also be used to estimate the impact of coral restoration projects, when coastal protection is of the essence.

The resiliency of coral reef islands to changing environments associated with climate change is controlled by the delicate balance between the import and export of sediment. The majority of the sediment is derived from coral reefs for which the stability of these islands is directly related to reef health. Understanding the sediment signature and its drivers is essential to assess island resiliency. We performed a study on sediments from the islands Eva and Fly in the Exmouth Gulf, Australia. We analysed the grainsize distribution and the abundance of sediment producers in order to desribe and discriminate the spatial distribution of these sediment characteristics and performed statistical analysis to identify corresponding key environmental drivers. We found that the sediments were typically course sand-sized (500 − 1000 [μm]) and the dominant constituent is reef-derived sediment. The median grainsize of Eva island (549 [μm]) is nearly equal to the median grainsize of Fly island (540 [μm]). The standard deviation of the grain size distribution of the sediments from Eva island was much larger than at Fly island. However, analysis of variance showed there were no significant differences between islands (Eva/Fly), hydrodynamic regimes (high/low), distance to shore (inshore/offshore) and local habitat (reef/no reef). Furthermore, a distance-based redundancy analysis showed no key environmental driver responsible for the distribution in grainsize and composition of the sediment. The environmental factors which were analyses were depth, temperature, pH, dissolved oxygen content and the oxidation-reduction potential. The spatiotemporal scales that were studied are potentially smaller than the scales on which climate change effects act, which explains the absence of significant spatial differences or key environmental drivers. Based on these findings it is not possible to assess the resiliency of Eva and Fly islands, however a study from Perry et al. (2011) found that islands with their particular characteristics (sand-sized and coral-dominated) are expected to undergo major morphological change under a range of predicted climate change scenarios. This research provides a baseline for future studies to assess the stability of Eva and Fly islands or sedimentological research other reef-derived islands.

Many tropical coastlines are fronted by coral reefs and are increasingly exposed to wave attack and wave-driven marine flooding. This problem demands immediate attention as safe habitability of and social and economic activity in reef-lined coastal regions are under serious threat, while these regions are known to have some of the world’s highest population densities.
High runup events and flooding on coral reef-lined coasts have been subject of recent studies, and some valuable insights are gained. It was found that the recent increases in wave attack, high runup events, and coastal flooding are primarily due to high offshore water levels coinciding with high energy swell events, circumstances which will become more frequent with sea-level rise. Increasing water depth over the reef changes the hydrodynamics across the reef in such a way that larger incident-band and infra gravity-band waves reach the shoreline, causing high runup levels and flooding of the land behind the shoreline.
However, little is known about the influence of longshore variations on runup and flooding of reef-lined coasts, while most show significant longshore variations, of which shore-normal paleo-stream channels are a prevalent one. This study aims to fill in that knowledge gap by examining the influence of paleo-stream channels on runup along reef-fronted shorelines, specifically during extreme wave conditions.
With a system analysis we determine the range of naturally occurring topographies of reef-channel systems and determine a representative reef. Results of this analysis are a useful starting point for future studies on this subject.
With a parametric study using the numeric model XBeach, we show that the presence of a channel results in a strong circulation on the reef flat and significant longshore variation of runup. Depending on the geometry and forcing, runup levels are increased next to the channel or inside the channel. This impact of the channel increases for higher incident waves, lower incident wave steepness, wider channels, a narrower reef and shorter channel spacing. Longshore variation of infragravity wave height is responsible for large scale variations in runup, while setup, short waves and very low frequency wave heights cause a local increase of runup inside the channel.
Results of the parametric study are valuable as they provide insight in which locations on a coast are most vulnerable to high runup events, using only widely available data such as reef geometry and offshore wave conditions. This is relevant for prediction of coastal hazards and to guide coastal management policies. Furthermore, this study provides insights for future studies on flood risk of reef-lined coasts, as it illustrates the importance of accounting for longshore variations while schematizing a coastline to predict high runup and coastal flooding, for instance to assess when a 1D schematization is sufficient and when a 2D model is required.

Coral reefs are degrading at an alarming rate, affecting not only the precarious coral ecosystem but also human habitat. The combination of coral degradation, sea level rise and its exacerbated effect in the tropics, and the possible storm intensification increases the flood vulnerability of low-lying tropical islands. To protect reef fronted coasts against flooding, coral restoration is put forward as measure to increase coastal safety. This study addresses the effects and efficiency of coral restoration in four steps. First, a simple reef hydrodynamic model is formulated to identify important drivers of runup at a reef fronted beach, to be used as a quick scan tool before using more intensive process-based models. Next, 30.000 US reef profiles are classified to gain insight into the prevalence of different reef shapes. Four dominant reef profiles are then used in the subsequent modeling step, in which the hydrodynamics and flood vulnerability of different reef shapes are investigated using the XBeach process-based model. With the knowledge gained, the hydrodynamic effects and runup reduction potential of different restoration configurations at various reef morphologies can be assessed.

Combined wave-current flow is defined as flow in which waves and currents are present together. One of its occurrences in nature is at a tidal inlet system, where incoming waves meet either a current in the same (following) or opposite (opposing) direction. This type of flow is different when compared to wave- or current-only flows. As a result, different effects are expected on, amongst others, the (suspended) sediment transport. In this study, we are modelling combined wave-current flow with the non-hydrostatic wave-flow model SWASH. The goal was to extend the present SWASH model to facilitate the modelling of combined wave-current flow, validate model results with experimental data and use the new model to analyse waveforms in combined wave-current flow. Model results for spatially uniform and non-uniform currents showed that the evolution of the wave height over the domain was approximately constant. Validation results of the vertical structure of the flow were in general poor except at the boundary. However, the model is capable of producing qualitatively the same trends as observed in measurements of the horizontal structure of the flow. In particular, the spatial evolution of the skewness and asymmetry was captured fairly accurate by the model. It is recommended to expand the newly developed methods to 3D domains for further validation and analysis.

Low-lying islands are highly vulnerable to wave-induced flooding, with low-frequency waves (frequency <0.04Hz) being one of the main drivers. The impact of these inundations can increase due to wave resonance over coral reefs, which has been observed in the range of low-frequency waves. This study aims to understand the reef resonance phenomenon along with processes that could limit its resonant amplification over wave height and wave run-up. A numerical experiment was carried out based on a 1D SWASH numerical model. A cross-shore profile of a schematized fringing coral reef was built, and resonance was forced over this bathymetry for the first two resonant modes and two water depths. The offshore forcing was designed as a simplified wave climate, with small amplitude regular low-frequency waves. Resonance was found to occur in a bandwidth of periods for each resonant mode, generating two resonant amplification peaks (one for each resonant mode). The periods leading to the maximum resonant amplification inside each resonant bandwidth are the modeled resonant periods, which were found to be longer than the theoretical resonant periods (based on reef flat width and water depth). The resonant amplification over wave height and run-up were found to be more significant for the fundamental mode than for the first mode, decreasing for both resonant modes when increasing the water depth. Moreover, for the wave conditions modeled in this experiment, the relative resonant amplification was found to be stronger for smaller wave heights than larger wave heights. Reef wave resonance presented a build-up behavior, needing a minimum number of waves to reach a maximum resonant amplification, which varied depending on the wave period, wave height, and water depth. Resonant amplification was found to increase for a larger amount of trapped wave energy over the reef and lower friction dissipation. Frictional dissipation was found to be the most effective process to counteract wave resonant amplification. Thus, increasing coral reef bottom friction is essential for enhancing low-lying island coastal safety.