Tropical cyclones, known as hurricanes in the Atlantic and Northeast Pacific or typhoons in the Northwest Pacific, are intense storm systems characterized by strong rotating winds, heavy rainfall, and low atmospheric pressure. They form over warm tropical waters and are major drivers of coastal flooding in tropical and subtropical regions. Annually, around 50 cyclones reach hurricane strength, causing flooding through storm surges and heavy rainfall, threatening communities and ecosystems. Climate change and human activities exacerbate these risks. Accurately predicting coastal flooding due to these cyclones is challenging due to their complex features, limited historical data, and forecasting uncertainties.

This dissertation aims to enhance the reliability of coastal flood forecasts and risk analysis by improving the descriptions of tropical cyclone wind geometry and pathways. It addresses both operational (short-term) and strategic (long-term) flood risk analyses. Operational risk analysis involves forecasting days before and after a cyclone, while strategic analysis deals with climate variability over decades. Both are crucial for comprehensive climate risk management, offering different time frames for preparedness and prevention.

A key element in both types of analyses is accurately representing tropical cyclone conditions in computational models. By examining historical best-track data, empirical relationships for two tropical cyclone geometry parameters—the radius of maximum winds and the radius of gale-force winds—were derived, improving estimates by up to 25%. This improvement is significant, particularly for cyclones outside the United States. These parameters, either observed or derived, are essential for computing surface wind distributions using the Holland wind model, which is critical for coastal flood evaluations.

Strategic risk analyses often suffer from a lack of sufficient historical tracks for reliable flood hazard assessment. To address this, an empirical track model based on Markov chains was introduced, capable of simulating thousands of synthetic storm pathways. The Tropical Cyclone Wind Statistical Estimation Tool (TCWiSE) generates these tracks, showing good agreement with historical data and extreme wind speeds. This methodology enhances the estimation of extreme cyclone conditions for strategic risk analysis.

The combined data-driven and physics-based methods were used to quantify coastal flooding in the Southeast Atlantic Coastal Zone of the United States. Comparing cyclone-induced flooding to non-cyclonic flooding revealed that extratropical cyclones are responsible for frequent flooding, while tropical cyclones cause the majority of infrequent but severe floods. For example, with current sea levels, extratropical cyclones contributed to half the flooded area, but tropical cyclones accounted for ~96% of the flooded area for 100-year events, affecting significantly more people. At higher sea levels, tropical cyclone-specific flood risk diminished as areas became uniformly susceptible to flooding. This analysis highlights the importance of considering both cyclone and non-cyclone flood factors in future research.

Operational risk assessments, critical for protecting lives and minimizing economic impacts, involve simulating numerous ensemble members to account for uncertainties in cyclone track, speed, and intensity. The Tropical Cyclone Forecasting Framework (TC-FF) integrates major physical drivers such as tide, surge, and rainfall, using Gaussian error distributions and autoregressive techniques. A case study of Cyclone Idai in Mozambique demonstrated the need for a large number of ensemble members for reliable forecasts. TC-FF showed less than 10% difference from operational ensembles, suggesting its utility in data-scarce environments.

This dissertation provides new insights into tropical cyclone wind geometry, pathways, and their role in compound flooding. Future research should enhance data collection, particularly from satellites, to validate models and understand storm characteristics better. Incorporating overlooked processes, leveraging data assimilation, and exploring more efficient methods, including Deep Learning, are essential for advancing flood risk assessment and capturing tropical cyclone variability.

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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.

Small Island Developing States (SIDS) are increasingly under threat of coastal flooding, which challenges the safety of their societies and vulnerable economies. The emergency of this issue, exacerbated by climate change, has alarmed international organisations and national governments that have been demanding for robust risk assessments to guide the development of resilient adaptation strategies. In SIDS, the paucity of local data, required to perform such kind of coastal risk analyses, hinders the application of highly detailed models that therefore need to rely on inaccurate and publicly available data, thus introducing uncertainty in the assessment. This thesis aims to investigate the uncertainty in input data and its impact on coastal flood damage estimates. This study examines prominent uncertainty sources in the coastal flood risk modeling chain, namely: the stochastic variability of (i) significant wave height and (ii) storm surge water level, the quality of (iii) bathymetry data and (iv) digital elevation models and (v) the choice of depth-damage function. To account for risk temporal changes, two other inputs are included, specifically (vi) different sea level rise projections and (vii) socioeconomic developments. A methodology is developed to test the afore-mentioned inputs through global sensitivity analysis, using an ensemble of hydrodynamic models (XBeach and SFINCS) coupled with an impact model (Delft-FIAT). The impacts of these sources on the flood damage estimates are evaluated in a case study on the islands of São Tomé and Príncipe. Model results indicate, for the current time horizon, depth-damage functions and digital elevation models as the inputs with the most significant contribution to the overall damage estimation uncertainty, yielding a variation in the output prediction of a factor 16 and 10, respectively. As future climate and socioeconomic development uncertainties are introduced in the system, sea level rise projection becomes, followed by digital elevation models and depth-damage functions, the most relevant input for the year 2100. Neglecting economic growth in the risk analysis leads to an extremely high underestimation of damages. However, given the constrained intrinsic uncertainty for the projected societal trends, its sensitivity on the risk output is limited. The scarcity of accurate input data proves to have an enormous impact on risk assessments in Small Island Developing States, leading to considerable prediction error and affecting the model outcome uncertainty. New emerging data collection techniques, such as unmanned aerial vehicles, could augment the trustworthiness of risk assessments by providing more accurate datasets for bathymetry and topography. Furthermore, research efforts could be directed towards developing knowledge on the physics of damages and their implementation in a risk modeling scheme. The uncertainty framework presented could be applied in projects with the aim to support risk communication to stakeholders by portraying the implications of the various inputs used and assumptions made, but also to guide the allocation of limited economic resources towards the acquisition of the input data that matters the most in terms of reliability of damage estimates.

Many coastlines fronted by a coral reef are threatened by wave driven flooding. For this reason, waves in coral reef environments has been topic of many studies. However, most are focused on a 1D simplification of the coral reef environment. These studies neglect the impact of 2D processes, for example wave directional spread. In this study an attempt is made to understand and investigate the influence of wave directional spread on wave runup. XBeach Non-Hydrostatic with an additional vertical layer is used to perform a great amount of digital experiments. A synthetic database is created consisting of 540 different model runs. The analysis showed that wave directional spread influences the wave runup by affecting the wave setup and low frequency runup. So, for the most accurate runup predictions it is advised to include directional spreading in the analysis. Furthermore, the complete exclusion of directional spreading in the runup analysis will lead to an over prediction of the runup, so current 1D models give conservative estimations of the runup. Lastly, it was found that the exact wave directional spread is not that important but it is advised to obtain information about the wave field that is dealt with and take that into account in the runup analysis.

The present thesis describes the development of a hybrid behavioural / process-based and wave-averaged model (XBeach Surfbeat) that successfully predicts the recovery of the subaerial beach at Narrabeen Beach, Australia, following a severe storm erosion in April 2015. Two model innovations were developed. Firstly, a behavioural model was developed to predict berm growth during calm conditions, as well as erosion during episodic storm conditions. A second innovation calculates a re-distribution of the sediment transport in the upper swash zone, to account for incident band swash-induced sediment transport, which is not resolved in XBeach Surfbeat, but is of major importance, especially on reflective beaches. The model successfully predicts the behaviour of the beach throughout the recovery period, and shows good potential for long-term simulations.

'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.

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.

Spurs-and-grooves (SAG) are a common and impressive characteristic of shallow fore reef areas worldwide. Although the existence and geometrical properties of SAG are well-documented ever since the 50’s, the literature concerning specifically the hydrodynamics around them is sparse. This study provides a characterization of the 3D flow patterns found on SAG formations, and a sensitivity of that flow for a set of short wave and SAG geometry parameters, as well as for alongshore and long wave forcing. Its main interest is to provide scientists predictive capability of the flow conditions for a set of conditions commonly found on coral reef systems with SAG formations. Delft3D-FLOW coupled with SWAN/XBeach (3D phase-averaged) was applied to model schematic SAG formations.

Shore-normal shoaling waves on top of SAG formations are shown to drive two circulations cells, the first in deeper waters with offshore spur and onshore groove depth-averaged velocities (offshore cell), and the second in shallower depths with offshore groove and onshore spur depth-averaged currents (onshore cell). In the offshore cell, the cross-shore velocity profile shows vertically monotonic currents - onshore to grooves and offshore to spurs -, except for the bottom, at which velocities are always onshore. In the onshore cell, the velocity profile shows offshore surface velocities and onshore bottom currents for both spur and groove, with resulting depth-averaged offshore groove and onshore spur velocities.

The mechanism driving this flow results from the wave forcing being mostly balanced by pressure gradients both in the cross-shore and alongshore, and the mismatch between those is balanced by horizontal turbulent forces, that are higher in deeper waters, and friction, larger in shallower waters. Variations of this pattern are associated with changes in the velocity profile, that fundamentally depend on the wave, SAG geometry and alongshore forcing parameters.

The waves are the main driving of the SAG flow, and as such wave parameters play a fundamental role in the SAG hydrodynamics.

Wave heights are the most important parameter associated with the flow strength - higher waves induce significantly stronger circulation cells. When wave heights start breaking due to depth limitation, the SAG circulation cell is lost, and the velocity profile shape starts having onshore surface and undertow with maximum values at mid depth.

Wave periods have moderate influence on the velocity values found on SAG circulation cells - higher wave periods induce slightly higher velocities. When the wave steepness reaches the breaking limit, the whitecapping results in changes of the velocity profile similarly to the case of depth-induced breaking waves.

The role of varying wave directions and directional spreadings could not be accurately evaluated due to uncertainties related to the importance of refraction and diffraction using a phase-averaged model. An initial assessment of their importance with a model neglecting refraction, thus with unchangeable wave direction, was performed. Results showed that oblique waves result in alongshore transport systems, i.e., cross-shore currents become significantly lower than in the alongshore. In those cases, the SAG offshore cell is lost, and the onshore cell gets wider and stronger.

The SAG geometry has a very important role associated with the resulting SAG hydrodynamics. Overall, the spur height, SAG wavelength and the SAG shape provide the biggest influence on the hydrodynamics.

The spur heights have significantly influence in the strength of SAG circulation cells - higher spur heights are associated with stronger flows.

The SAG wavelengths moderately influence the strength of the flow, with longer SAG wavelengths resulting in not much stronger SAG circulation cells. Shorter SAG wavelengths do not present the offshore SAG circulation cell, due to higher alongshore mixing of momentum that gives offshore spur and groove currents in that zone.

The shape of the SAG formations is, together with the wave heights, the most important parameter influencing the strength of the flow. SAG formations with peak spur height located further onshore (Buttress type) have SAG circulation with higher velocities involved and the zonation of the SAG circulation cells changes accordingly, i.e., lower peak spur height depths have circulation cells shifted onshore, with widening of the offshore cell.

The reef slope, without significant interference in the strength or velocity profile shape, also affects the zonation of SAG circulation cells, with steeper slopes providing wider SAG offshore circulation cells.

The groove width, the differential roughness between spur and groove, and the reef flat widths were shown to have a minor role in the SAG hydrodynamics.

The alongshore forcing leads to an alongshore transport system. The degree of the alongshore dominance is directionally proportional to the alongshore forcing. In the cross-shore direction, the onshore SAG circulation cell was persistent, while the offshore cell can be undermined with large alongshore forcing.

Long waves were shown to result in negligible influence in the mean SAG hydrodynamics, associated with the low long wave forcing observed in the SAG zone. They are primarily more important as approaching and within the reef flat, and the water exchange between this and the SAG zones was concluded to have limited influence in the SAG flow.

In terms of coral growth and health, bottom shear stresses were found to be systematically higher over spurs than grooves, resulting in a higher potential for coral development over them due to increasing water motion. Accordingly, sediment transport potential is higher over spurs, for which alongshore currents are higher than grooves, thus sediments would tend to drift towards the grooves, where they would more likely deposit due to lower shear stresses. The fact that SAG with distinct shapes - with significant different peak spur height depths - experience similar bottom shear stresses suggests the existence of a range of ideal hydrodynamics conditions for coral development.

Waves in coastal waters play a role in forecasting storm conditions, as boundary conditions for the assessment of coastal risk and in coastal structure design. Assessment of their impact can be made through numerical modeling. In a forecasting context, the CPU time of the wave model is decisive in its suitability. Recent years wave model approaches have been to describe both linear and non-linear generation, dissipation and transfer processes in the wave spectrum explicitly. This resulted in computationally demanding tools. Such models are very suitable for research purposes or for design and assessment if time is available, but are rather heavy tools for forecasting applications on larger scales. Parametric models describe the wave spectrum through characteristics in a smaller solution space compared to spectral models, which makes them potentially faster solvers. In this thesis the suitability for forecasting of a spectrally integrated approach to the wave action balance is investigated. The stationary wave drivers from the storm impact assessment model XBeach were taken as a starting point. A second balance equation was identified to model the evolution of the mean period. In estuaries and behind shoals, locally generated waves can be dominant over offshore wave conditions. As part of the project, a wave generation mechanism is developed for the modified wave drivers based on prediction relations for growth of wind-waves under fetch-limited conditions. The two-equation approach was discretized and tested in a 1D Matlab model and the solution was proven to converge towards the literature prescribed growth curves for total wave energy and mean wave period. This approach was then implemented in the unstructured Frequency Integrated wave drivers in D-Flow FM. The implementation has been verified through semi-1D test cases and showed convergence behavior equal to the Matlab model. A hindcast of a 1982 storm in the Haringvliet was performed to compare the model skill of the unstructured frequency integrated model (FIM-FM) to the spectral wave model SWAN. In this hindcast the model skill of FIM-FM is comparable to SWAN for significant wave height and mean wave period for all locations apart from directly behind a shoal. Depth and wave height dependence of bottom friction coefficients should be studied in more validation cases to improve the prediction of wave steepening over shoals. Further research should also look into the iterative method in the solver for stationary solutions of the model equations to realize faster CPU time of a stationary problem. Although not established in this thesis, faster CPU time compared to a spectral model like SWAN should be possible based on the reduction of the solution space.