The impact of climate change, particularly the rise in sea levels, obstructs the effectiveness of existing coastal structures. Additionally, climate change can also have an amplifying effect on wind speeds (Takagi & Esteban, 2013). Without proper control, the accumulation of changing environmental boundary conditions could lead to disastrous events.
Currently, the design of rubble mound breakwaters is based on the EurOtop (2018) guideline. However, these guidelines may overlook essential influencing factors. Recent studies by Van Gent et al. (2022) and Irias Mata & van Gent (2023) have contributed to new insights that address these limitations. They aimed to identify numerous influencing factors that enhance the accuracy of wave overtopping expressions for rubble mound breakwaters. However, for several factors a more thorough understanding is still required.
Furthermore, recent insights on the influence of wind have proven to be significant. Previous studies indicate that the mean overtopping discharge has the potential to amplify its quantity several times beyond its initial value (de Waal et al., 1996; Wolters & van Gent, 2007; Van Gent et al., 2023 and Dijkstra, 2023). These investigations included tests on vertical sea walls and dikes.

This research focuses on investigating the maximum effect of wind on wave overtopping for a rubble mound breakwater, considering varying slope angles and crest element designs. The key parameter of interest is the maximum wind effect factor, described as a function of the non-dimensional overtopping discharge. Small scale models of rubble mound breakwaters with mild (1:6) slopes, steep (1:2) slopes, and various crest walls are examined to find their influence on the maximum effect of wind.
An extensive experimental programme was carried out within the Pacific Basin at Deltares. In this basin, a flume was constructed in which the breakwater models were built. Various hydraulic conditions, incorporating the water level and the wave characteristics, were systematically tested. All conditions were repeated, both with and without including the maximum wind effect.

A new overtopping expression is developed, taking the form of an exponential function that includes all relevant parameters. The influences corresponding to the crest wall are thoroughly investigated, while the other factors (roughness, obliqueness, and a berm) are only taken into account. The exponents and constant factors were obtained iteratively and were found to accurately predict the overtopping discharges obtained during the experiments.
The relationship between the maximum wind effect and non-dimensional overtopping, as observed in previous research, was confirmed for these measurements. This observation suggests that a maximum effect of wind increases when the non-dimensional overtopping discharge decreases. Only the configuration with a mild slope and the tallest crest wall stood out by showing a notable influence on the maximum wind effect. Moreover, the wind effect is more pronounced for an increasing water level when similar amounts of overtopping discharges are taken into consideration. Based on these findings, new factors for wind were derived.

In conclusion, this research has contributed to a deeper understanding of the influence of wind on wave overtopping. The presented wind factor expressions significantly improved the accuracy of the expression for each measurement, offering broader applicability across different structure types. Moreover, the amplification factors constructed in previous studies and in this study, have proven to be applicable across various structure types. This enhances overtopping predictions initially obtained for this study.
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In this research, the influence of the permeability and the porosity of artificial reefs on wave transmission and the sheltered habitat of marine life is investigated. A physical model is used to find the answer to this question.
The main finding of this research is that a hollow, perforated structure can act both as an artificial reef that provides a safe habitat for marine life, and as a breakwater that provides sufficient coastal protection. Moreover, a new empirical relation was derived for a smooth impermeable breakwater, rubble mound breakwater, and a hollow perforated breakwater with an impermeable screen inside.
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Climate change affects sea level rise and the safety of the people living behind seadikes. In order to prepare for this, several adaptation measures are available to reduce the overtopping discharge. Wave overtopping can be reduced by including roughness elements or adding a berm. Another possibility is to employ crest elements such as a vertical crest wall, promenade or parapet on the crest wall to reduce wave overtopping discharges.

The overtopping discharge is key when defining the dike dimensions. Moreover, the effect of wind on the overtopping discharge is not included in existing guidelines, whereas wind is complex due to its dynamic behaviour. Also, the knowledge niche regarding the position and height of the vertical crest wall is the reason for performing more research. Physical model tests are conducted to gain more knowledge about the maximum wind effect to obtain a better understanding of how it affects the overtopping discharge and the loading on seadikes.

The following research question is covered in this master thesis: What is the maximum wind effect on wave overtopping at dikes with crest elements? The aim of the present research is to examine this wind effect based on physical model tests, which are performed at Deltares in Delft, the Netherlands. Experiments on a small-scale model of a seadike with a smooth outer slope at an angle of tan⁡(α)=1:3 are conducted. In total, four dike configurations were tested, consisting of a crest wall, which is placed at the seaside of the dike crest in one case and on the land side in another. A paddle wheel is used to simulate the maximum effect of wind, based on the idea that all vertical spray exceeding the dike crest is transported over the crest by onshore wind. The maximum wind effect is determined by comparing the tests with and without the use of the paddle wheel.

The results of this investigation show an optimisation of the existing guidelines, such as TAW (2002), for calculating the overtopping discharge. For non-breaking waves, the wave steepness is not included in the TAW (2002) overtopping formula, but the data show a clear dependency on this parameter.

One of the most significant findings from this study is a quantification of the maximum wind effect. It provides an amplification factor on the overtopping discharge. This maximum wind effect is defined as the ratio q_wind/q, which indicates the overtopping discharge with maximum wind effect due to onshore blowing winds (q_wind) over the overtopping discharges without wind effects (q).
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The objective of this master thesis is to investigate experimentally how non-breaking wave loading affects the stability of the MOSES artificial reefs, an artificial reef by the company ReefSystems. Small scale wave flume tests with a length scale factor of 20 were conducted in the Scheldt flume at Deltares in Delft to determine the stability of three physical reef models with varying characteristics under multiple wave loading conditions. Based on this, a prediction of the stability of the reef prototype was investigated.

The experimental tests’ conditions were based on varying parameters (the relative wave height, wave steepness, and water depth) to obtain the broadest possible range of conditions. Variations in regular or irregular waves, with or without foreshore, water depth, wave height, and period were used for the tests. For each experimental wave flume test, it was established whether the reef was stable. The observed stability of the reef was related to the conditions during that test, determined by the resistance-type wave gauges that recorded the height of the free surface.

Based on the data of the experimental flume test, a stability function was determined, giving the influence of three non-dimensional parameters (the relative wave height, the wave steepness, and the relative water depth) on the stability of that reef. The tests were also compared to two stability prediction methods; the Morison method and a prediction method based on the mobility parameter. ... Read more

Over time, degradation processes might cause damage to pattern-placed revetments. Examples of such damages are missing elements, deformation, and the loss of joint filling. However, little is known about the exact consequences of those damages. Therefore, when damage is observed during an inspection, it is estimated based upon experience what the possible consequences are. If the consequences are misjudged, this will lead to inefficient maintenance.
Better insight into the exact consequences of damages will expand the possibilities of how risk-based maintenance can be used to maintain revetments. Consequentially, maintenance interventions can be planned more efficiently, reducing the societal costs incurred due to inefficient maintenance. This study contributes to this topic by investigating whether it is possible to estimate the impact of damages on the stability and reliability of a revetment using a model.
Within the study, first, damages are analyzed based upon a literature review. Next, data of old flume experiments with Basalton and basalt revetments are analyzed to study and quantify damages. This analysis focuses on the uplift of elements, deformation around the wave impact zone (S-profile), and washed-out joint filling. Then, a finite element model (FE-model) is created to simulate the wave impact on pattern-placed revetments. The main focus of the FE-model is to study the uncertainty due to structural changes, which are the damages. The damages quantified during the analysis of the flume experiments are also included in order to be able to assess damaged revetments. Finally, the study uses the FE-model within a sensitivity analysis to study the most important uncertain parameters. Based on the samples used in the sensitivity analysis, response surfaces are fitted to obtain a model that can predict the damage for any set of parameters. Finally, these models are demonstrated in a case study of a coastal dike near Den Helder. An increase of 10 – 100 times of the failure probability has been observed for small deformations, while for medium to large deformations, the failure probability increased by 1000 – 10000 times. For no joint filling or a missing element, the failure probability increased by 10 - 100 times.
This study showed that it is possible to create a finite element model that can estimate the impact damage has on the stability and reliability of a pattern-placed revetment. The obtained results can be used within the daily practise as part of risk-based maintenance as the study provides a way to obtain a first indication of the impact of missing elements, deformation, and washed-out joint filling. Additionally, the developed methodology can be used to obtain the impact of other types of damage. Although Basalton is the primarily investigated type of top layer, analysis of the flume experiments showed that basalt revetments are subject to identical types of damage. Therefore, it is expected that the findings within this study can be applied to a broader range of top layer elements with similar characteristics to Basalton (e.g. basalt, C-Star, and Hydroblocks).
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Rubble mound structures are often constructed to prevent severe wave damage to ships. By constructing the crest at a certain level, the waves are reduced such that safety is ensured. However, a large increase in sea level rise is expected in the next century due to climate change. Because of this, multiple adaptations might be necessary to meet the required lifetime of a rubble mound structure. In current guidelines, the influence of these adaptations is already included. However, for a combination of solutions the empirical equations are not always accurate enough (van Gent, 2019). Accurate guidelines are necessary to correctly predict the overtopping rates for a combination of solutions. In this research, the accuracy of current guidelines is investigated.

The research performed in this thesis is divided into two parts. First, multiple solutions are derived to adapt a breakwater and ensure safety for a rising sea level based on existing empirical equations. Safety is ensured if the mean overtopping rate remains less than 50 l/s/m. Beyond this point, the ships behind the structure might become prone to large wave attacks. Secondly, the accuracy of current guidelines (i.e. the empirical overtopping equations) is tested in an OpenFOAM model. OpenFOAM is a so-called open-source Computational Fluid Dynamics (CFD) software that can solve complex fluid flows.

Several solutions are possible to ensure safety against severe wave overtopping. The four solutions applied in this thesis are the addition of a berm, the addition of a crest wall, an increased foreshore and the implementation of a low-crested structure. By combining these solutions, the overtopping rate remains below the maximum of 50 l/s/m. The combination of solutions forms a path, all paths together form a pathway. The adaptation pathways are a guideline for the moment in time at which a certain solution should be implemented. Therefore, a structure is not unnecessarily expensive and can be managed easily. The paths are rated based on the implementation costs of the combination of solutions.

In total two empirical equations are applied to derive an adaptation pathway. The first pathway is based on the overtopping equation proposed by the TAW (2002). The TAW is a Dutch advisory committee on flood defences. Based on the applied theory, the economically most attractive solution consists of a low-crested structure, a foreshore and a berm. As this equation does not account for the influence of a berm in non-breaking waves, an adapted TAW equation is applied as well. The adapted equation was proposed by Krom (2012) and includes the influence of a berm. Based on the adapted equation, the economically most attractive solution consists of a foreshore, a crest wall and a berm. Once the economically most attractive solution is derived, the accuracy of current guidelines is reviewed in a phase-resolving model. It is found that, there is a large discrepancy between the results calculated with the empirical equations and the results from the model. As no physical data is applied in this research it is hard to interpret and analyze the exact numbers. Therefore, the relative effect of an adaptation is compared. It is found that in contrast to the TAW overtopping equation for non-breaking waves, a berm decreases the overtopping rate by at least 30% for the case study applied. Furthermore, the current method to account for a crest wall proposed by the TAW overestimates the reduction (73% compared to 40% in OpenFOAM). Finally, it is concluded that the addition of a low-crested structure decreases the overtopping rate by a larger value than based on the applied guidelines (74% in OpenFOAM compared to 35% in theory).

Based on the performed research a realistic combination of the adaptation measures consists of a combination of a berm, a crest wall and a shallow foreshore. Therefore, it is advised to focus further research on the combination of these measures. It is necessary to improve the guidelines for combinations of these adaptation measures since the existing ones seem to be either incorrect (TAW, 2002) or require a better validation (Krom, 2012). ... Read more

Living breakwaters are designed to protect the coast against flooding and erosion, whilst at the same time they enhance the local ecological system by incorporating natural reef components. This study investigates the design of a modular artificial reef developed by the company called Reefy. Reefy breakwaters will consist of interlocking blocks with holes inside and rounded corners. For the configuration of the breakwater no proper design guidelines exist yet which incorporate both the hydrodynamic and ecological functionalities, as both this field of engineering and this reef system are relatively new. Therefore, this study aims to provide insight and develop preliminary design guidance on how to design a hybrid living breakwater from Reefy blocks under wave loading. 
To this end, an experimental study was performed in the Eastern Scheldt wave flume of Deltares investigating the impact of different design variables on both the 2D hydrodynamicand ecological performance under wave loading, in shallow water conditions. Both irregularand regular wave conditions are tested. This thesis focuses on submerged structures and therefore the freeboard is defined to be positive for submerged structures. In total, 15 different designs are tested amongst which 7 are 2DV configurations and the other 8 are complex 3D configurations. Single as well as double 3D structures are tested and the space between a double structure is referred to as ”channel”.
For the hydrodynamic performance, the impact of several design variable on the transmission coefficient (Kt) and reflection (Kr) is quantified. In this study, Kt is defined as the transmitted waveheight behind the structure divided by the incoming waveheight at the same location without a structure. These coefficients are based on the incident wave signals. As most of these tests had shallow water conditions, with large Ursell numbers, the usual methods to determine the incoming wave did not work. Therefore, to obtain the incoming wave signal, a new method was used, based on a combination of the existing techniques. Lastly, from the irregular waves the transmitted- and reflected energy density spectra are investigated and compared to the results from the regular waves. The results of this study reveal that for the same number of blocks, a more complex structure can be built without making a compromise in the hydrodynamic performance parameters.  Furthermore, the existing formulae for the Kt and Kr are compared to results of the hydrodynamic performance as measured. The ones with the best correlation are optimized using a non linear regression analysis. 
For the ecological performance, the stream-wise peak velocities are investigated in the wake behind the structure and in the channel. The performance is investigated based on a tranquility index Tr. Tr increases if the flow is more tranquil. The outcomes of Tr showed that in general, Tr increases for an increase in the design variables that were inversely proportional related to Kt .
In conclusion, the performed tests and analysis provide insight into relevant physical processes and design parameters for artificial reefs, and therefore assists the designer of artificial reefs.


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Coastal areas around the world have attracted settlements and human activities since the early stages of the history until nowadays. This has introduced continuous modifications to the natural characteristics of these coastal regions by means of coastal structures and engineering interventions. The design of such coastal structures has evolved significantly since the first quarter of the XXth Century, when more scientific design methods and formulae were developed. Nevertheless, further research is required given the stochastic nature of the environmental loads involved, the remaining uncertainties regarding the response of these structures to the applied loads and the growing impacts of climate change and sea level rise. Four knowledge gaps are identified regarding the “Damage assessment of coastal structures in climate change adaptation”. Based on these, the objectives of this thesis are summarized as follows. First, climate change adaptation: demand for validated upgrading alternatives. Second, damage characterization concepts: demand for unified damage characterization concepts. Third, damage characterization parameters: demand for universal and more accurate damage characterization parameters. Fourth, damage characterization measuring techniques: demand for validating the suitability of innovative survey methods. These knowledge gaps are addressed using physical modelling results from two test campaigns (UPorto deep water and Deltares shallow water tests. In consequence, this study includes the validation of the damage criteria required for a precise assessment of a coastal structure (second knowledge gap), the validation of an universal damage parameter for rubble mound structures (third knowledge gap), the validation of the benefits of innovative measuring techniques when carrying out physical modelling tests (fourth knowledge gap) and the validation of upgrading alternatives for climate change scenarios (first knowledge gap). Thus, it can be stated that with these definitions, parameters and measuring techniques, a complete method for damage characterization of coastal structures is presented. It was also defined how this damage characterization method can be used to precisely and accurately describe the damage to conventional and non-conventional coastal structures. Furthermore, this method was also used to describe the effects of not only current environmental forces acting on the structures but also future and more energetic scenarios. For such future scenarios, adaptation alternatives for coastal structures were evaluated and berm configurations are recommended for their upgrading. Future research is needed in order to evaluate, adjust and generalize the conclusions made in this thesis, considering additional structure configurations and environmental loading conditions. ... Read more