Surface gravity waves are one of the hydrodynamic processes in the nearshore area that play a key role in the transport of plastic, plankton, sediment and other particles. Previous research has studied the combined effect of the Earth's rotation and turbulent mixing on the wave-driven cross-shore velocity profile in both the surf zone and on the inner continental shelf. However, these studies either used eddy viscosity profiles typical for wind-driven currents or determined the turbulence characteristics based on the amount of energy dissipated by wave breaking. This research aims to unravel the connection between the Coriolis force and momentum diffusion in the absence of wave breaking by using a turbulence model that is based exclusively on wave-driven flow dynamics. To this end, the existing one-dimensional wave-averaged model of Lentz et al. (2008) was improved by incorporating a low-Re one-equation turbulence model to determine the eddy viscosity generated by the Eulerian-mean flow. Furthermore, a complementary three-dimensional RANS model was constructed in SWASH.

In an inertial frame of reference, the models predict velocity profiles that are similar to the theoretical inviscid depth-uniform return flow in the inner part of the water column, while in a rotating frame of reference, the results approach the anti-Stokes drift profile based on the theory of Hasselmann (1970). Deviations are observed near the bed, caused by vertical radiation shear stresses, especially in relatively shallow water. The inverse wave Ekman number is shown to be a key indicator of the relative importance of the Coriolis force with respect to turbulent mixing. The results emphasize the importance of including the Coriolis force in nearshore wave-driven flow models. Compared with laboratory measurements, the theoretical model accurately predicts the near-bed velocity profile, indicating that a turbulence model that is based on wave-driven flow dynamics is essential to properly model wave-induced currents. ... Read more

This research focuses on the implementation of the Building with Nature (BwN) approach in the design of artificial coral reefs. The goal is to explore environmental preferences and design tools for coral development, considering both coral habitat potential and coastal protection services. The research also aims to optimize artificial reef design for a case study in Addu City, Maldives.

A conceptual model is developed based on an extensive literature review, incorporating critical engineering and ecological variables relevant to coral habitat potential and coastal protection services. This conceptual model serves as a practical design tool, fulfilling three primary purposes: identifying variables that require further examination to explore coral habitat potential, identifying variables that require analysis to explore coastal protection services, and identifying design variables that influence physical-chemical and biological variables for potential integral solutions.

The research also utilizes OpenFOAM, a numerical modeling tool commonly used in coastal engineering, to design artificial reefs. OpenFOAM accurately models flow and turbulence regimes, crucial for the ecological and biological functioning of coral reefs. An OpenFOAM numerical model is set up, calibrated, and validated for the case study in Addu City. Three design alternatives with different slopes are evaluated using a multi-criteria analysis (MCA), considering criteria such as coastal protection, coral habitat potential, and costs. The numerical model is applied to assess flow regimes and wave transmission for different design variables.

Based on the MCA results and the evaluation of criteria, an artificial reef with a mild slope of 1:3 is selected as the optimal integral solution for the case study. This design performs well in terms of coastal protection, coral habitat potential, and costs. The research demonstrates that the conceptual model and OpenFOAM numerical model are valuable design tools for assessing and optimizing artificial reef design, adhering to the principles of the BwN approach.

In conclusion, this research contributes to expanding knowledge and developing tools for implementing the BwN approach in artificial reef design. By considering both coral habitat potential and coastal protection services, the design of artificial reefs can be optimized to provide multiple benefits, including environmental and socio-economic values. The conceptual model and numerical modeling tool offer practical assistance in the design process, promoting the integration of nature and engineering for sustainable coastal solutions. ... Read more

On the east coast of Romania, at Eforie, coastal erosion manifests. To strengthen the coastal area a large coastal protection project was setup involving beach nourishment combined with the construction of breakwaters. The breakwaters are designed with the well-known modified Van der Meer formulas. To ensure confidence in the breakwaters stability the designs were tested during a 3D physical model test. The designs were stable, however the measured damage to the breakwater was larger than was expected. Investigating the physical model test resulted in three important aspects, that need to be investigated. First, a broad list of configurations, that could responsible for the damage, could be identified. Secondly, excessive wave breaking on the foreshore during the model test resulted in a shift in wave energy from higher to lower frequency waves. These low frequency waves, or so-called infragravity waves became increasingly dominant near the coast. Their effect on the stability of the structure is however unknown. Thirdly, the incoming waves were very oblique and on a shallow foreshore, resulting in a different failure mechanism than is incorporated in the modified Van der Meer formulas. A new stability method has to be formulated to investigate breakwater stability for this physical model test. Due to the broad list of configurations and the possible influence of infragravity waves, real scale or physical model tests are not feasible and a numerical model needs to be used. A reliable and widely applicable method to link breakwater stability to a numerical model has however not yet been composed. Therefore, the goal of this thesis is to propose a method that can link breakwater stability to a numerical model and to assess whether the identified configurations resulted in the damage along the statically stable rubble mound breakwater, measured during the Eforie physical model test.

First, an investigation is performed on the physical model test set-up and observations, resulting in a final list of 5 configurations, that are investigated in this research: The applied offshore transitional slope (1), the assumption of uni-directional waves (2), the slope of the lower foreshore (3), the depth-contour lines inducing wave focusing (4) and the very oblique wave angle on a shallow foreshore (5). Secondly, a method is proposed linking breakwater stability to a velocity signal from the numerical model SWASH. An equation is formulated, based on the theory of Izbash (1935), with a slope factor included, and scaled with the theory of Shields (1936). It requires a velocity signal, that can be obtained from SWASH, to calculate a stone size required for stability.

Thirdly, a numerical model is set up in SWASH, with grid dimensions 3m x 2m, resembling the physical model test. The breakwater is modelled as an impermeable core with a permeable porosity layer placed on top. The thickness of the porosity layer is based on the thickness of the outer armour layer of the original breakwater. The numerical model is validated by comparing the wave characteristics, at several locations along the breakwater, to wave data available from the physical model test. The numerical model shows accurate resemblance of the wave characteristics. Since the wave velocity is linked to the wave height, it is assumed that the wave velocity on the breakwater is also correctly modelled. The model is therefore found valid for the modelling study. In the numerical model along the still waterline measurement points are indicated that provide the velocity and waterlevel signal during a simulation. In the numerical model two layers in the vertical are assumed and tested to be sufficient. The velocity of the top layer resembles the velocity that flows just over the stones. Therefore from the velocity signal of the top layer the governing u_0.2% along the waterline at the breakwater is obtained and from the waterlevel signal the wave spectrum is derived. In the study simulations are performed in which the configurations are tested one by one, and all simulations are assessed on two parameters: the u_0.2% and the wave spectral transformation along the breakwater. The results from the different simulations are compared relatively to identify the relative effect the configurations have on the velocity and wave characteristics.

The results of this research show that breakwater stability can be predicted reasonably well from a velocity signal obtained from SWASH. The velocity signal, obtained from SWASH, results in reliable stone sizes. The configurations could be investigated with the proposed method and the results provide reliable and useful insights. In addition, the proposed method is able to identify the effect of infragravity wave energy on the stability of a breakwater. The method is also tested by calculating the relative obliqueness factors for different incoming wave angles, which shows promising results. It is important to reproduce the breakwater porosity well in the numerical model as it can significantly influence the velocity signal. A decrease/increase of the porosity thickness with 30% or 0.6m can result in an increase/reduction of 20-26% in velocity respectively.

The five discussed configurations provide partial explanations to (in)directly induce the higher breakwater damage in the physical model test. Both the applied offshore transitional slope (1) as the assumption of uni-directional waves (2) result in a slight underestimation of the breakwater stability and therefore a somewhat conservative design along the entire length of the breakwater. The combined effect resulted in a reduction of 0-6% around the head of the breakwater, h/Hs = 2.5-4.8, and a reduction of 16-24% near the shore, h/Hs = 1.1-1.8. Especially near the shore the breakwater is conservatively designed, due to the fact that both the transitional slope as the assumption of unidirectional waves increases the infragravity wave energy in the system. It is found that incoming waves break around h/Hs = 1.7 after which the infragravity waves induce a temporary increase in waterlevel, around h/Hs = 1.1-1.8. This allows the depth-limited short waves to become bigger resulting in higher velocities and more damage on the breakwater. This affects the breakwater stability closer to shore and needs to be taken into account when designing a breakwater in these conditions. The lower foreshore (3) induces the generation of infragravity waves, which affect the velocity closer to shore as described above. The depth-contour lines (4) result in a wave focusing effect increasing the velocity around h/Hs = 1.1-1.8 with 8-11%. Based on the results of this thesis the very oblique wave angle on a shallow foreshore (5) does not induce higher velocities and breakwater instability. It is however assumed that the effect of a breaking plunging wave, inducing acceleration and pressure difference effects on the stones on a slope, is not sufficiently into account, due to the grid dimensions used in the model. As other plausible causes of the increased damage are disproven, it seems likely that the different oblique wave breaking process that is not modelled in detail leads to the increased damage. ... Read more

The design of hydraulic structures like breakwaters and crest walls is often based on empirical formulations, physical models test, numerical models and a fair amount of expert judgement. Each technique has its own pros and cons. The main limitation of the empirical formulas is that often they have to be applied outside their range of validity. Physical modelling also has its own shortcomings. When breaking waves hit the structure, the location of the maximum pressures is still not well known due to the high spatial variability. By using an array with low spatial resolution, the forces estimated in the physical ume will usually underestimate to some extent the actual forces experienced by the wall (Ramachandran et al., 2013). In the last decades, numerical modelling has become an attractive alternative in simulating wave-structure interactions. Nevertheless, estimating loads on crest walls in the numerical ume is still at its early stages. On that account, the present work validates the prediction of wave induced forces on the front face of crest walls on top of rubble mound breakwaters in CoastalFOAM. A scale model of the Holyhead breakwater, located in Wales, is used. The key validation topics are the reproduction of wave-structure interaction when heavily breaking waves reached the wall, the evaluation of the ventilated boundary condition implemented by Jacobsen et al. (2018), the porous ow inside the armour layer formed by Tetrapods units and whether the simplication of not using a turbulence model, as done by Jensen et al. (2014) and Jacobsen et al. (2018), is also valid under heavily breaking waves. Four validation cases were used to test the capabilities of the numerical ume. The results conrm that it is possible to accurately reproduce the wave conditions and the wave induced forces from a physical modelling campaign. Overall, the CoastalFOAM model is able to capture the shape and the order of magnitude of the force events. A calibrated model predicts the highest wave induced forces (forces with an exceedance probability of 0.4% and 0.1%) with errors lower than 20%. Moreover, the results indicate that for practical applications it is not essential to include a turbulence model in the numerical ume to obtain reliable forces on the front face of crest walls for dierent wave conditions. Another outcome of this study is that the implementation of the ventilated boundary condition is required in the interface between water surface and structural elements to mimic accordingly the air-water mixture when the structure is subjected to a heavy wave attack. Nonetheless, there is still room for improvement in this area, where a better understanding of this boundary condition and of the air entrapment during wave-structure interaction needs further research. Despite the large computational time required by the numerical ume when large wave trains must be simulated, a CFD model during design stages of breakwaters and crest walls provides higher spatial and temporal resolution of the wave induced pressures exerted on the wall than a physical test. Therefore, a better picture of the forces and pressure distribution in the front face can be obtained. ... Read more

The Noordstrand of the Marker Wadden has been subject to much more erosion than expected. From our current understanding of this low-energy system we cannot explain what has been the cause of this. The goal of this thesis is to create a better understanding of the different processes responsible for the development of this non-equilibrium beach in a low-energy lake environment. A better understanding of the link between hydrodynamic and morphodynamic processes can be used for the maintenance of the existing Marker Wadden beaches and the design of new sandy protections in this kind of systems. By combining the results of a hydrodynamic and morphodynamic data analysis with the results of the Delft3D Marker Wadden model, it is possible to create a conceptual system description. This can be used to improve our understanding of the Noordstrand development and the responsible processes. In this system description the influence of the general Markermeer system cannot be neglected as the different systems (Markermeer, Marker Wadden and Noordstrand) cannot be regarded separately as the different hydrodynamic processes are linked.

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Tidal energy has a large potential to contribute to achieving the sustainability goals in the Netherlands, due to the long coastline and many estuaries. The investment costs for the implementation of offshore tidal energy are however, still a drawback. A possibility to lower the investment costs for tidal energy is to implement tidal energy extraction into existing coastal infrastructure. An example of this is the current pilot project in the Eastern Scheldt barrier where five Tocardo turbines are attached to the barrier's geometry. The flow passing the turbines in the barrier is contracted by the barrier's geometry and the weir of the foundation of the barrier. Deltares executed an environmental impact assessment for the pilot project with a measurement campaign and a computationally expensive but accurate blade resolved numerical model. In this thesis, a cheaper computational model is added to the these assessment tools. To do so, a turbine parameterization is introduced in the finite element model FinLab. The relatively cheaper computational model should make it possible to execute more extensive sets of simulations of the pilot project in the future. FinLab solves the non-hydrostatic three-dimensional incompressible Navies-Stokes equations in an unstructured mesh with an optional moving free-surface. The used meshes are refined in certain areas of complex flow, such as the turbine parameterization, near the bed, the recirculation zone of the weir and the wake of the turbine. An extra source term is added to the Navier-Stokes equations on a chosen number of turbine integration points, to implement the turbine into the mesh. The placement of these integration points in a two-dimensional mesh is rather simple, the integration points are evenly distributed over the diameter of the turbine. In a three-dimensional mesh, a Fibonacci series is used to implement an actuator disc (AD) into the model domain. A benefit of the Fibonacci circle is that each turbine integration point represents a similar area. The force in the turbine integration points is determined by three methods: an actuator disc with a uniform thrust distribution (uniform AD), a rotational averaged actuator disc blade element momentum method (AD-BEM) and a non-rational averaged actuator line blade element momentum method (AL-BEM). In the uniform AD method, a measured thrust force is divided equally over all turbine integration points. In the BEM methods axial and tangential forces are determined for each turbine integration point based on the local flow speed and experimental lift and drag coefficients. With these methods, it is possible to estimate the generated power and thrust by the turbine. A tip-correction is added to the AD-BEM method to achieve accurate results of the simulations. Furthermore, in both the AD-BEM and AL-BEM methods a small actuator disc is applied in the nacelle region to reproduce the nacelle's blockage of the flow. The uniform AD, AD-BEM and AL-BEM parameterizations are validated with measurement data of flume experiments of a single turbine, flume experiments with different turbine-weir geometries and flume experiments with multiple turbines. The combination of these last two sets of experiments does very well represent the situation in the Eastern Scheldt barrier. As stated above, the AD-BEM and AL-BEM methods are able to estimate the turbine performance. The results of the AD-BEM simulations show a relative error of the turbine's generated power of within 20% in comparison with the flume experiments. The accuracy of the thrust fluctuates more. The AD-BEM method shows to be able to accurately estimate the location of optimum power harvesting in a combined turbine-weir geometry. Both the power and thrust accuracy depend on the mesh resolution and the turbine's rotational speed. Accurate results can be obtained by fine-tuning these settings. The AL-BEM method should be able to produce at least similar similar accuracy to the AD-BEM method when it comes to power and thrust. The AL-BEM method in this thesis is however, not yet able to do so. The results of the simulations with the three methods are also compared with time-averaged velocity measurements of the flume experiments. In the near wake, the uniform AD method does not imply any wake rotation. The rotation of the wake is represented accurately in a time-averaged sense by the AD-BEM method. The AL-BEM method adds transient features such as the downstream trailing tip-vortices to the flow. The velocity shear and turbulent eddies in the near wake as caused by the different methods influence the TKE in the near wake and the recovery of the far wake. The uniform AD under-estimates the wake recovery. The extra velocity shear in the AD-BEM method improves the accuracy but still under-estimates the recovery rate. Resolving the transient features of the near wake in the AL-BEM method further improves the accuracy of the wake recovery. In the simulations with the weir, the geometry seems to be dominant and the wake recovery rate is more accurate than in the simulations without the weir. The AD-BEM method is applied to the Eastern Scheldt field case. Five turbines are introduced in a mesh which represents Roompot 7 to 9 in a simplified manner. Due to the simplification of the mesh, the resistance of the barrier on the flow appeared to be too low. Therefore, the simulations are executed with an upstream discharge boundary. This discharge boundary makes the model less useful to estimate the environmental impact of the turbines in the barrier and at the moment not yet an alternative for the blade resolved model by Deltares when it comes to estimating these effects. With the discharge boundary, the AD-BEM model predicts the average thrust over the five turbines with a relative error of 3% and the average power with a relative error of 12%. This is only slightly less accurate than the results by the blade resolved model by Deltares, while the computational costs of the Eastern Scheldt field case simulations are only a fraction of the earlier executed blade resolved simulations by Deltares. ... Read more

Large-scale measures within the river programmes such as Room for the River, Natura2000 and Water Framework Directive have increased the biodiversity in Dutch rivers and have achieved a more natural landscape. However, recent river programmes have shown conflicts between safety against flooding and riverine nature rehabilitation with maintaining navigational water depths. Therefore, an integrated approach is called out upon to improve the river management within these programmes. Conventional groynes are typically used to improve functions within the river programmes, whose primary function is to maintain navigational water depth. Groynes are transverse structures in which large turbulent structures are observed around the groynes. Consequently, significant bed shear stresses are developed, leading to significant local scour. Investigations for improving alternative groyne configurations are often studied for optimizing the groyne structure. The flexible groyne is considered an optimization of the conventional groyne structure. The flexible groyne consists of steep slopes and has a permeable characteristic. Although a fair amount of research has already been carried out on various groyne configurations, detailed flow characteristics around and through permeable, sloped groynes are limited. Hence, this research aims to numerically quantify the effects of permeability and head steepness of groynes. A literature study is executed to investigate the critical processes required for capturing within the numerical model to answer the research question. From the literature study, it appears that the most important processes are the non-hydrostatic effects, adapting large turbulent structures and implementing the porous zone using a non-linear function. Multiple software packages have been analyzed. Fluent is chosen, which is analyzed to capture the required processes adequately.
A new numerical model has been set up for investigating the research question by simulating flow around groynes. The model is validated against three experimental studies for various characteristics. The important mean flow characteristics have been validated within an acceptable range. Still, it appears that the numerical model tends to underestimate the mean streamwise flow velocities, overestimate the Reynolds shear stresses and shift the peak values of the Reynolds shear stresses downstream. Four configurations are identified for the simulations of varying head steepness and porosity. It appears that the increase of the porosity reduces the large turbulent structures and bed shear stresses close to the groyne and shifts the peak values of the Reynolds shear stresses, and bed shear stresses further downstream. The porosity reduces the maximum Reynolds shear stresses and bed shear stresses compared with the Reynolds shear stresses, and bed shear stresses for an impermeable groyne. These reductions are because of the momentum exchange between the free flow region and the flow through the porous structure, which reduces the mean flow velocity in the free flow region. For decreasing steepness, the large turbulent structures and bed shear stresses are observed close to the groyne due to increasing deflection. The flow appears to follow the geometry of the sloped groyne more smoothly. This research is seen as a first approach for modelling a porous, sloped groyne. Further improving and analyzing numerical modelling for porous, head sloped groynes are advised to increase the model's accuracy. Furthermore, more simulations for varying the head steepness and porosity is suggested to improve the relations between the increase of porosity and head steepness for the flow characteristics. In addition, including sediment transport models within the model is expected to increase the understanding of the hydrodynamics and morphology around these specific groynes. ... Read more

Climate change puts under pressure existing and future coastal interventions. Growing threats like sea-level rise and intensity of storms require solutions to be adaptable and resilient. Nature-based solutions have shown to tackle these challenges while providing social, environmental, and economic benefits. The role of vegetation in coastal protection is increasingly recognized. Aquatic vegetation reduces erosion, storm surge, and incoming wave height.

Large-scale modeling of waves with spectral wave models such as SWAN is indispensable for the design of coastal structures and the assessment of flood risk. Wave dissipation due to vegetation can be modeled in SWAN as increased bottom friction (implicit modeling) or as an additional dissipation function (explicit modeling). The second assumes that vegetation can be represented as rigid cylinders or plates (canopies) with different properties. While some studies concluded that implicit modeling reproduces the spectral evolution of field measurements more closely, others concluded the opposite.

Within the BE-SAFE project, field campaigns measured the spectral energy distribution over salt marshes in the Dutch Wadden Sea during several winter storms. The vegetated foreshore in front of the coastal dike got submerged over 2 m of water during high tide and storm surge. The measurements deployed wave gauges over the study transect, which was defined between the pioneer zone marsh edge and the near-dike location (300 m behind the salt marsh). Calibrating the implicit and explicit models in SWAN brought the modeled total wave energy decay closer to the measurement. Nevertheless, the spectral shape, which describes the energy distribution over frequencies, still showed significant and not yet understood differences near the dike.

A methodology was executed to investigate the mechanisms that could reduce the spectral mismatch between the SWAN wave model and measurements over vegetation. First, the literature highlighted possible mechanisms that could be incorporated for this purpose. Next, a new frequency-distributed explicit dissipation model of Jacobsen et al. (2019) was implemented in SWAN and compared to implicit and explicit models using lab and field measurements.

The results showed that the newly implemented model accurately captures the physics and the change of spectral shapes for all experimentally tested wave conditions and submergences. In contrast, the existing implicit and explicit dissipation models in SWAN reproduce the spectral evolution only under certain circumstances. In the validation and comparison to the field measurements with a much larger water depth than the vegetation height, the model of Jacobsen et al. (2019) correctly captured the vegetation's physical representation and the dissipation on the wind-sea frequencies. Nevertheless, the amount of energy on low frequencies was largely underpredicted by all frequency-distributed models. Therefore, the model of Jacobsen et al. (2019) was modified to include flexibility in a frequency-dependent reduction factor that reproduced the energy decay of the measurements in all frequency regions. Other mechanisms that could be responsible for the mismatch before and over the marsh are the redistribution of energy by non-linear triad interactions, generation of infra-gravity waves, and near-shore currents caused by horizontal variations on the vegetation properties.

The present research provides the range of conditions in which the tested explicit and implicit energy dissipation functions in SWAN are able to simulate the spectral evolution over rigid canopies and flexible salt-marsh vegetation. A new version of SWAN includes a new frequency-distributed explicit model that performed more accurately than existing models for rigid canopies. The physical insights from the research contributed to developing additional versions of SWAN, which performed closely to the energy distribution of the measurements over deeply submerged and flexible salt marsh vegetation species.

References:
Jacobsen, McFall, Van der A (2019). A frequency distributed dissipation model for canopies. Coastal Engineering, 150, 135-146. ... Read more

Over the recent years, flood risks and losses have been increasing for coastal cities due to climate change, subsidence, population and economic growth. Hong Kong and Macau are two cities located in the Pearl River Delta that experience a significant flood risk due to storm surges. The increased losses and risks has sparked interest around the world for efficient and accurate flood forecasting. At the moment coastal flooding events are often simulated with difficult hydrodynamic models that reproduce the physical phenoms. Over the last decades there has been more interest in other methods to forecast storm surges, namely neural networks. Other than hydrodynamic models a neural network is capable is making predictions in seconds, while the model can take hours to finish simulation. fast and accurate storm surge forecasting is of importance for disaster and evacuations management strategies and will only become more important in the future. During this research the main goal is to develop a neural network capable of prediction maximum water levels due to storm surges in case of an approaching tropical cyclone. The neural network is trained with data that is obtained from hydrodynamic simulations. A synthetic storm database is used to provide the necessary data to conduct 1000 simulations of which the results are used in the neural network. The first focusses on developing a hydrodynamic model capable of accurately simulation tropical cyclone induced storm surges in Hong Kong and Macau. The model is calibrated in a way to reproduce the real world as close as possible. It accounts for the real life bathymetry, topography, astronomical tides and wind forcing. The storm surge model is validated extensively based on three historical tropical cyclones. The errors between the observed and simulated water level are below 20 cm, after calibrations of different physical parameters within the model. The second step of this research is to use the validated storm surge model to run synthetic storm simulations. Instead of using historical storms who only have been recorded for 40 years, a synthetic storm database is used containing 10000 years worth of data. From that storm data, 1000 synthetic tropical cyclones are selected that come close to Hong Kong and Macau. With the data obtained from the previous two steps, it is finally possible to training the neural network. In this network a total of seven input parameters (tropical cyclone track parameters) are used to estimate the maximum water level that will occur during the tropical cyclone. The input parameters considered are: latitude, longitude of TC eye, maximum wind speed, minimum eye pressure, radius of maximum winds, forward speed, forward propagation direction. During the development of the network, three different types of configurations are tested. Extensive validation and calibration shows that neural networks are capable of making maximum water level predictions for a large number of cases. However, variety in the quality of the prediction is observed. Improvements still can be made for more accurate predictions. ... Read more

Around the world, there are 58 Small Island Developing States (SIDS). What these SIDS generally have in common is that they are very susceptible to different natural hazards, among which coastal flooding. In order to address this issue, adaptation measures are needed. In order to prioritize adaptation efforts when considering these islands at a large scale, it is important to obtain an overview of which islands are most vulnerable to coastal flooding. This requires a large-scale assessment of coastal flooding for these islands. Such a large-scale assessment introduces different challenges, that were looked into in the hydraulic engineering part of this thesis. A first challenge is the availability op topography data for these islands. For most islands, only satellite-based DEMs are available. The accuracy of different satellite-based DEMs was assessed by comparing them to more accurate elevation data for 11 different islands. It could be concluded that in general, the TanDEM-X DEM is the most accurate for terrains with milder slopes, and the ALOS DEM for terrains with steeper slopes. Furthermore, the DEM error was found to be strongly correlated with forested and builded areas. Especially the DEM error in builded areas poses a problem for coastal flood assessments, as these are the main areas of interest in such assessments. Therefore, a building-correction method based on data from Open Street Map (OSM) was proposed and implemented. It could be concluded that the building-correction can both in- and decrease the DEM error, depending on the other error sources that are present in the DEM. A second challenge that is introduced when upscaling coastal flood assessments, is the used method for flood modelling. In the context of this thesis, a simple flood model (called the JBIW model) was developed. This model is a combination of the Janssen-Battjes model for short wave dissipation and the IW-method, which was originally developed for large-scale river flooding calculations. The JBIW model was tested in 1D and compared to other flood models. The results indicated that the simple flood model can be used to obtain a first estimate of coastal flooding, but is not accurate enough to predict exact values of the maximum water levels. Furthermore, the JBIW model was implemented in 2D for the island of Ebeye. The model was combined both with accurate elevation data and with building-corrected satellite-based DEMs. The results indicated that the error introduced by the use of the simple JBIW model were much smaller than the errors introduced by the use of the satellite-based DEMs. This indicates that further research focusing on the upscaling of coastal flood assessments for reef-fronted SIDS should focus on obtaining more accurate elevation data for these islands.
Apart from allocating resources to the SIDS in the most efficient way, it is important that the resources that are allocated to a certain island are used effectively. There are different ways to help these islands. An interesting approach is to focus on increasing the adaptive capacity of the SIDS. In the context of the science communication part of this thesis it was investigated whether the simple JBIW model could be used to increase the adaptive capacity to coastal flooding in the context of São Tomé. In order to do this, a theoretical framework was developed that aims to provide practical guidance in the assessment of the (barriers to the) adaptive capacity of a certain system. This framework was applied to the context of São Tomé to map the adaptive capacity to coastal flooding of the system and obtain an overview of the most important barriers to this adaptive capacity. Subsequently, it was assessed which barriers could be addressed with a tool based on the JBIW model. These barriers were used as the starting point for an initial design of the tool interface. ... Read more

To calculate tsunami forces on coastal structures, the wave type in front of the coast is of great importance. Hence this paper aims to find ways to predict the type of tsunami wave breaking. Based on literature review, video footage, analytical reasoning and numerical modelling (SWASH) it can be concluded that both the continental shelf slope (alpha_2) and the bay geometry (beta) have a significant influence on the transformation of a tsunami wave near the coastline. After conducting 1D and 2DH wave simulations, a distinction is made in three types of tsunami waves; a non-breaking front (surging), a breaking front (plunging) and an undular bore breaking front (spilling). Tsunami waves transform into these three wave types for a steep continental shelf, an intermediate sloped continental shelf, and a gentle sloped continental shelf respectively. A new tsunami breaker parameter (xi_tsunami) is proposed to predict the type of wave at the coastline in a quantitative way.
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A main challenge port engineers had to tackle in recent years is wave penetration inside a harbour, as it determines vessels’ safe sailing and mooring, possibly causes unwanted vessel movements, and unequivocally regulates the execution of port operations. A physical scale model can describe wave penetration in a complete way. However, the construction of a physical scale model is expensive and time consuming. For this reason, nowadays several numerical models are used to describe wave penetration in ports, affected by multiple processes such as diffraction, partial reflection, etc. In this study, the simulation of wave penetration with the non-hydrostatic model SWASH is examined. To validate the numerical model, output of an open benchmark dataset of Deltares (Deltares, 2016) is used, consisting of physical scale model tests of schematic port layouts. As wave penetration is a summation of physical processes, each process should be described accurately by SWASH. This thesis focuses on assessing how SWASH models wave penetration per wave process, first separately by means of simplified models and then combined in a model describing the full harbour layout resulting to the final wave field inside a port. The main topics of interest of the formulated research questions are the ability of SWASH to simulate wave propagation, wave celerity and the effect of two dominant wave processes: reflection and diffraction. As the amount of processes influencing wave penetration increases for higher layout complexity, the research was targeted at the simplest port layout considered in the benchmark dataset (Deltares, 2016). Moreover, only regular waves were taken into account, as in this case the differences between the measurements and the computational results are most easily identified. To better understand the influence of reflection the waves, two simplified one-dimensional SWASH models were designed. The first model simulated reflection in front of a gravel slope, located outside the harbour basin, and the second model reflection in front of the harbour basin end, consisting of a gravel slope and a concrete quay wall behind it. The results suggest that outside the basin the reflection off the gravel slope has a minor effect in comparison to the reflection off a vertical quay wall. Inside the harbour basin, wave reflection played a dominant role on the resulting wave field there. The SWASH models were proven to be robust as the wave height in the computational domain did not change considerably for an increase or decrease of the porosity of a gravel slope by 10%. It should be emphasised that the standing wave heights were altering fast within a short horizontal distance. Therefore, the precise wave height values were strongly influenced by the exact location of the output points examined. The importance of diffraction inside the harbour was demonstrated by a simplified two-dimensional model, in which reflection off the harbour end was not included. The information that could be obtained from the measurements about the wave height changes due to diffraction was limited. However, the initial trends due to diffraction were also identified in SWASH. From the comparison of the wave height in the SWASH model, influenced only by diffraction, to the respective measured value, it was confirmed that the total measured wave penetration inside the harbour was significantly influenced by diffraction. The comparison of the measurements to the results of the final SWASH model, which included the full version of the simplest physical model, showed that the overall wave field pattern is in agreement. The numerical model was able to reproduce the diffraction and reflection patterns observed in the measurements. At many output locations in SWASH the measured wave height values were simulated with high accuracy. On the downside, at other locations the measured and the modelled wave height deviated significantly. The large deviations can be explained by the fact that the standing wave patterns change within a short distance and thus the wave height can vary significantly at the area close to a specific output point. It may be possible that the measured wave height at a specific point can be identified in SWASH in the region close to the exact point coordinates. All in all, it was concluded that for non-breaking, relatively low waves, with wave height to water-depth ratio lower than 0.2, the accuracy of SWASH in modelling the wave processes of reflection and diffraction is sufficiently well for engineering purposes. For relatively high waves and/or breaking waves, numerical instabilities were detected. It is assumed that the numerical instabilities can be attributed to the relatively low number of grid cells per wave length. This study advances our understanding of the wave penetration simulation in SWASH. The approach followed allows investigating the ability of the model to simulate, separately and combined, two wave processes which predominantly contribute to wave penetration in harbour: reflection and diffraction. With further validation to guarantee the model stability, the strategy of this thesis can be a useful tool to understand the performance of SWASH in modeling wave penetration per wave process and in combination. The knowledge obtained enlightens the possible reasons leading to deviations between the measurements and the model outputs. This can be valuable assistance in the course of further improving the model accuracy. ... Read more

Mineral oil spills at sea can have many negative consequences. For both preventive and responsive purposes, it is essential to accurately forecast oil spill evolution. Shear diffusion (in this context, i.e. the combined effect of vertical mixing and differentiated horizontal advection of mass) determines for a significant part the evolution of an oil spill. These processes are partially forced by waves. A viscous fluid layer attenuates the waves throughout the area the layer covers. In this thesis, surface oil slicks are modeled as a continuous viscous fluid layer. It is investigated to what extent the wave-forced shear diffusion of the oil is affected by the oil-induced attenuation of the waves. For this purpose, the spectral wave model SWAN is extended with a module for energy dissipation due to a viscous fluid layer. The stationary, 1D wave energy balance is solved for uniformly forced waves in deep water. A high cutoff frequency (5 Hz) is employed to include the wave frequencies at which the dissipation is active. Also, special attention is paid to the choice of the wind and whitecapping formulation. Simulations are performed in full factorial setup, varying wind speed, oil layer thickness and oil viscosity. The results are compared to a no-oil case. Based on the difference, functions are fitted for the reduction of two key wave properties: the whitecapping dissipation rate and the surface Stokes drift velocity. The reduction functions are included in the oil spill module of the particle tracking model OpenDrift, which is subsequently used to calculate oil spill evolution due to shear diffusion for 2DV cases. The results of oil spill simulations with and without the implemented reduction functions are compared. Idealized cases (only wave-forced) show that for sufficiently thick layers (h+ ≥ O{10^-3} m) of sufficiently viscous (ν+ ≥ O{10^-3} m^2/s) oil, the Stokes drift reduction can significantly affect the wave-driven evolution of an oil spill in two ways: the average forward transport is reduced and the skewness of the oil mass distribution is increased to ‘less negative’ or even positive values. If simple sheared wind drift and ambient vertical turbulence are added, however, the relative importance of these effects becomes smaller. In none of the cases, a difference is found for the distribution of the oil mass between surface and subsurface, which implies that the whitecapping reduction hardly affects the results. It is recommended that further effort is put into obtaining a detailed understanding of the (differentiated) forward transport of the surface and near-surface oil, so that wave and wind effects can be distinguished, and modeled independently, more accurately. ... Read more

All over the world, coastal protection measures are taken, which can be soft (e.g. sand nourishments on sandy beaches) or hard (e.g. seadikes or seawalls). Special care has to be taken to design the transition between these hard and soft flood defences, as they are often vulnerable components of a coastal defence system. This study therefore aims to give more insight into the morphological processes around hard-soft transitions, in order to make recommendations on design and nourishment plans for hard-soft transitions in the future. To this end, the hard-soft transition of Maasvlakte 2 (MV2) was studied. For the case of MV2 the limited knowledge of hard-soft transitions resulted in a conflict of the assessment method of the flood defence and the nourishment strategy according to the client requirements. The most important factors governing the morphological behaviour were studied using different numerical models (UNIBEST-LT/CL , SWAN and XBeach. From the results of the case study of Maasvlakte 2, it was found that the most severe erosion at the hard-soft transition is found during periods with strong southward longshore transports, which causes positive longshore transport gradients at the transition. Cross-shore processes such as storm erosion also indirectly contribute to this by transporting sediment from higher in the profile to lower in the profile. Regarding the influence of wave reflection, preliminary results were found. Using SWAN and UNIBEST, the effect of short waves was investigated, which showed that reflection of short waves is larger at the deeper located part of the hard flood defence. The preliminary results from XBeach showed that bound long wave reflection is larger closer to the hard soft-transition, and that these reflected waves also reach quite far offshore. As regards to the role of the tide, it was found that the dominant northward directed flood tidal current along the soft flood defence strengthens the supply of sediment from south during northward transport. On the other hand, the dominant southward directed ebb current along the hard flood defence hardly picks up any sediment and therefore it does not contribute to the morphological changes at the hard-soft transition.
Some generic observations in the morphological behaviour of MV2 are likely to be found at other hard-soft transitions: - A strong dynamic variability is usually present at the hard-soft transition, in the form of coastline retreat which gives a rotated coastline shape. - The reflection of short waves will likely be larger at the deeper part of the hard flood defence. Closer to the transition zone, where sediment from the soft flood defence is deposited, short wave reflection will be smaller, but long wave reflection will be larger. - In general the highest erosion will be observed in periods with oblique wave incidence where sediment is lost due to longshore sediment transport gradients. Based on these findings, several recommendations are proposed to design a hard-soft transition. Moreover this study presents some other design examples to minimise the nourishment frequency. Finally, recommendations are presented for further research. ... Read more

Using data made available by George Mason University (GMU), a U.S. university situated in Virginia. Field data is used to validate both a Delft 3D and SWAN standalone model. The paper not only shows the difference between the implicit manning roughness approach and the explicit cylindrical vegetation approach, but also provides a comparison between different drag coefficients as well as dissipation rates for Spartina Alterniflora under storm conditions. ... Read more

For the offshore wind industry it is essential to further reduce costs to be competitive to traditional energy resources. One of the options to achieve so is optimizing support structure design. Using XL-monopiles allows for larger turbines, deeper water access and consequently can provide for a more cost effective support structure. Within this market, Energieonderzoek Centrum Nederland (ECN) operates with its in-house developed software package to evaluate turbine responses. To further improve the model, a better insight is needed in breaking wave impacts on structures.
This thesis is focused on developing an enhanced method to evaluate breaking waves on monopile structures to identify the effect of slamming waves on XL-monopiles. Based on wave tank measurements, a method to identify slamming wave impacts is developed and tested. Later, those wave tank measurements are reproduced and the identification method verified.
Wave tank experiments were carried out at the Atlantic Basin at Deltares within the joint industry research project WiFi. For the experiments, two monopile scale models were placed in the wave tank, both equipped with multiple load and pressure sensors. During the tests the monopiles were exposed to series of wave trains, the irregular wave trains with a total of approximately 5000 waves, created a large sample size needed for the experiment. The wave measurements from the tank are analysed numerically to identify breaking waves. In this thesis a slamming wave event definition is proposed as follows:
• Front crest steepness 푆 should reach breaking limit
• The slamming impact of the wave should be more than 4 times the standard deviation of the force time series
The numerical computations are carried out using the potential flow solver OceanWave3D to generate a sea state with comparable characteristics to the wave tank measurements. At first, the generated sea state appeared to lack the necessary wave height. By adjusting the input in the OceanWave3D program, a sea state was found that matches the measurements. Wave energy dissipation is checked throughout the measurements wave tank and compared to dissipation in the numerical model. It was shown that both the wave tank and numerical model show resembling dissipation.
Hydrodynamic loading on the monopile foundation is assessed using the kinematics obtained from the OceanWave3D model. Several methods were combined to come to the total wave loading. First the Morison approach was used to account for the non-slamming part for the wave load. Using OceanWave3D, the force coefficients are calculated applying DNV guidelines. The second part, the slamming part of the wave load, is obtained in multiple phases. First, based on the above mentioned slamming wave criteria, wave steepness and impact forces, the individual waves are evaluated and scored as potential slamming. Secondly, using conservation of momentum and kinematics derived from OceanWave3D, the slam load is calculated. This slam load is determined evaluating the impact velocity of all waves individually and added to the Morison load if identified as slamming. Finally, when the calculated impact loads from the numerical model are compared to the recorded loads in the wave tank measurements, large similarity can be noticed.
After comparison of the developed slam load representation to the DNV method of slam load estimation, the result is a less conservative slam load approximation. This is due to the evaluation of slamming impact velocities per wave, opposed to the assumption of a single impact velocity for the whole sea state. The results of this thesis can further implemented in turbine response evaluation tools by ECN resulting in a more optimized calculation model. It will enable the design of more (cost) efficient XL-monopile structures. ... Read more

The Tohoku Tsunami of 2011 in Japan flooded a large part of the coastal area of Japan. The tsunami was caused by an earthquake with a magnitude of 9.0 just of the coast of Tohoku. The inundation height of the tsunami exceeded the design height of the tsunami barriers. This event led to thousands of fatalities.
The aim of this study is to find the characteristics of the incoming tsunami waves for the design of coastal defence structures. This incoming tsunami wave close to the shore or on the shore is influenced by a lot of offshore factors that will change the wave and its behaviour. The tsunami wave will either develop into a bore or just run up the coast. This has large influence on the forces on the barrier. Potential influencing factors are examined on if and how they influence the tsunami wave when it travels to the coast.
A numerical one-dimensional SWASH model is used throughout this study to simulate the tsunami wave. The tsunami factors and the factors that influence the wave were studied in several steps.
The factors that have the most influence on the wave are used to simulate bores. From all these bores the important characteristics for the design of a barrier are investigated. These are the bore height, the bore velocity and the corresponding Froude number. With the simulations a new definition of the bore height is introduced. This is the height at the maximum velocity of the bore. The bore characteristics are also tested with an existing formula for the impact forces on a structure.
The behaviour of the breaking wave is studied and a breaker parameter [ξtsunami] for tsunami waves is made. This breaker parameter defines if the tsunami wave develops into a bore before it reaches the coastline or that the wave runs up the coast without breaking. This is important for the location of the coastal structure.
This breaker parameter and the Froude number of the bore give a relationship between the important parameters that influence the development of a bore and the characteristics of the incoming tsunami bore.
Finally, physical tests were performed at the Waseda University in Tokyo, Japan, to simulate the bore attack on a coastal defence structure with a dam-break. The bore of the tests is compared to the bore from the SWASH simulations. This resulted that the velocities of the tests seem too high. However, with a new method to find the bore front characteristics is a Froude number constructed. This Froude number matches very well for the tests and SWASH simulations. The Froude numbers of the test represent a bore at the coastline.
... Read more

Across the world, the presence of humans in coastal regions is always increasing. Hydraulic forcing from extreme events is a large risk around the global coastlines, the risk being complicated by the increased human presence along the coasts.
The knowledge that vegetation can be used as a coastal protection measure is not something new. The benefits of vegetation have been seen throughout history and are being researched on till date. If we know to a certain confidence what the wave heights are at the shoreline, coastal defence structures, used as a hybrid protection measure, can be designed accordingly. Not much research has been done on the wave behaviour on shallow foreshores.
Localised studies have been previously done on tropical vegetated coasts, but there is a lack of efficient and accurate analysis on a large scale. As we bring more clarity on how waves transform and attenuate in a typically vegetated coast, some questions get answered, and some more questions arise, which also happened during the research done for this thesis.
Observed data, be it laboratory or field, is very crucial in validating numerical models. A laboratory experiment was done in the TU Delft Laboratory of Fluid Mechanics flume, for a complete vegetation-free profile, where the surface elevations were observed for different wavemaker input conditions.
A lot of numerical models have been developed that predict wave transformation and dissipation through vegetated foreshores. However, these models lack validation from observed data. This thesis first focuses on understanding the wave transformation for two unique (and mainly theoretical) wave conditions: a regular sinusoidal wave and a bichromatic wave. It was checked if the transformation is reflected in the models – SWAN and SWASH, which they did.
The research proceeded on to validating the models by comparing the wave heights observed in the laboratory experiment versus when the models were inputted with the same conditions, including inputting the observed data into the models. When the laboratory conditions were replicated, the SWASH results obtained correlated quite well with what was observed in the laboratory. The same was not true in the case of SWAN.
When a spectral analysis was done for the observed data, a presence of very low frequencies (VLF) as well as some minor higher frequencies was noticed. To check its effect, if any, on the model results, they were filtered out. Both the original and filtered data was inputted into the models. The difference in the foreshore region was more distinct in the filtered case, i.e., making a bichromatic elevation input purer resulted in more pronounced undulations in the wave heights than what was predicted in the unfiltered data. This result does not fit well with the existing knowledge on wave dissipation processes. It is widely known that the presence of VLFs and higher frequencies are the driving mechanisms that result in the undulations in the foreshore region, but the predicted results were exactly opposite to this knowledge.
What can be thought of from the anomaly is that the presence of various frequencies (that is, waves with different periods) counteract each other’s effects and make the undulating wave heights milder, but when the signal is made purely bichromatic, it leads to more distinct undulations. This proposition is also backed by the similar SWASH results for the laboratory condition-replicating theoretical inputs. This anomaly needs further investigation.
Another interesting observation was that the changes happening in the offshore region did not affect the results in the foreshore region, for varying parameters in SWAN.
SWASH can be concluded as a better model for predicting wave heights, especially in the foreshore region. SWAN could not predict the fluctuations in the wave heights. Obtaining the wave heights at the shoreline with SWAN, and designing a dike with those results, for example, will lead to disastrous consequences, as SWAN underestimates the wave heights.
The study is limited by the consideration of hydrodynamics only, and by the many simplifications made to simulate the conditions. One of the recommendations formulated is to obtain field data and to make a similar comparison with the models to corroborate (or correct) the observations made.
This study tried to see the correlation between the models and observed data in the laboratory, for simple (and somewhat purely theoretical) cases. It is, nonetheless, a starting point for more complicated cases, the basis for which can be laid on this study. ... Read more

Shore-normal breakwaters are constructed in coastal zones both for beach protection (erosion reduction) and port development (wave sheltering). These breakwaters have an effect on the waves, the (wave-driven) currents and hence, the sediment transport along the coast. The waves and tide force an equilibrium sediment transport along the coast in a natural unaltered coast. When breakwaters are constructed this sediment transport in the alongshore direction is (partially) blocked. This results in accretion at the updrift side of the breakwater and coastline retreat at the lee-side. Coastal erosion behind a breakwater can result in floods, destroy property or simply narrow the beach. Not only the short term effects of these structures (what happens during and short after construction) is important to know, also the long term effects need to be known; because the breakwaters are build for decades coastal influence of these breakwaters needs to be known for these time-scales as well. Therefore it is relevant to investigate the scale (in time and space) of these adverse effects beforehand.

In coastal engineering, numerical models are used to predict the impact of coastal constructions like breakwaters. High detailed models which take for all physical processes into account will result in accurate predictions but result in large computation times. Simplified models have smaller computation times and are more suitable for coastal impact predictions on larger spatial (10 - 100 km) and temporal (decades) scale. The objective of the thesis is to improve the coastline change predictions of models at decadal scale by reviewing the common practice and a new, very fast, module for lee-side wave computations and investigating different wave processes that can improve the wave modelling. This research will focus on (1) understanding of what wave processes are important for the coastline change and (2) advise on what models to use for which conditions and how to the improve model predictions.

The approach of this thesis is triple. First five different modelling approaches for wave modelling are compared, varying in computation time and usability, to a very accurate model that will be used as ground truth. The wave conditions will vary in direction and both wind waves and swell waves will be modelled. From this step de accuracy of these models for different wave conditions can be analyzed. The second step is to get an better understanding of the processes that are involved with the wave modelling. This information can be used to improve the accuracy of model approaches with high computation times. The last step is to see how this relates to sediment transport and thus the coastline change at the lee-side of a breakwater.

This can be concluded after the investigation of 3 wave model (SWASH, SWAN and the Kamphuis module)


SWAN model approach is for cases with wind waves a good model approach: the wave height, wave direction and the sediment transport are well representative. Also the setup differences are very similar to the ground truth model.
SWAN is not very accurate for cases with a small directional spreading. For these cases the diffraction is more important and there is not enough wave energy in the sheltered area nor is the wave direction well represented.
Kamphuis with Snellius (refraction) is for the case with a wide directional spreading, given the computation speed, pretty good. For the cases with a small directional spreading the wave height is not accurate.


The conclusions related to the influence of the wave processes are:


The refraction is investigated by comparing the Kamphuis module (which does not incorporate refraction) with the ground truth model. When Snell's law was applied to the kamphuis module the model results did show good results. Therefore the refraction is (especially outside the sheltered zone) very important to the wave direction.
There is a large difference for cases with small and large directional spreading. The SWAN model gives results can can be expected when no diffraction is present. Directional spreading both influences the wave height directly behind the breakwater as well as the influence lengt of the breakwater.
Diffraction does not play an important role for wind waves. The large directional spreading results in much smaller energy differences between the sheltered and the non-sheltered zone. Even SWAN without diffraction gives a good representation for wind waves. For swell waves however this is very different.Then diffraction is very important. There is a much larger difference in wave energy between the shelterd and the non-shelterd zone and for a good representation a model that can coop with diffraction is a must.
Current induced refraction of the waves has influences for waves of an incoming agle 0 to 30 degrees because this will result in rip currents along the breakwater that turn the waves up to an extra 10 to 15 degrees.



The relative sediment transport of the five modelling aproaches was also analyzed. The sediment transport proxy for the five modelling approaches showed that SWAN computes the wave height and direction well for wind waves. For swell waves however the model did not show good results. The reason is that there is no diffraction in SWAN, the diffraction computations with SWAN did not show much improvements either.

The sediment transportation proxy for the Kamphuis module with Snellius where expected to be good for wind waves, because the wave height and wave direction where well represented. However the length of the erosion pit was much smaller and also the shape of the erosion pit is very different from the other two models. ... Read more