This study investigates the capacity of an open inverted filter to stabilize the interface between the sand fill and core fill in a rubble mound breakwater. The strength of this filter relies on the arching mechanism of the base material, which is directly influenced by factors such as stability ratio and effective stresses. These forces arise from porous flow, and its quantification involves both parallel and perpendicular hydraulic gradients. A full-scale physical model, with a 760 mm open inverted filter, is employed to determine the critical hydraulic gradients required to initiate erosion. The model consists of three compartments, with the central compartment containing the open inverted filter. The two side compartments are filled with water, with one of them featuring a plunger mechanism to induce flow. Pressure sensors and water level gauges are used to measure the hydraulic gradients. In addition to the physical model, a numerical model is developed to gain insights into the flow dynamics within the system. The tests reveal that an increase in effective stress results in higher critical parallel and perpendicular gradients. Furthermore, the addition of fines at 5% or 10% seems to enhance the critical parallel and perpendicular gradients. A negative correlation was found between SR and 𝑖⊥,𝑐𝑟𝑖𝑡, the results were fitted to a hyperbolic function. The findings further indicate that, even after erosion occurs, a stable interface can be reestablished. Therefore, designing solely for the single largest wave is not imperative.

After more than 85 years, the iconic Afsluitdijk is in need of a reinforcement. During the latest safety assessment of Rijkswaterstaat the dike did not meet the standards at two components; the amount of wave overtopping (>10 l/m/s) and armour stability. This research goes into detail about the overtopping on the Afsluitdijk. In the new design a lot a of different factors influence the amount of wave overtopping: the roughness of several elements and a berm. The combination of these different factors makes it difficult to make a reliable estimation of the amount of wave overtopping. In the design new materials are used of which the effect on wave overtopping is unknown. The band width of the design is very narrow so an accurate prediction of the amount of overtopping in the prototype is necessary. Physical model tests of two different dike section are used in this research. These dike sections are tested on different scales (small to large) to validate the final design on prototype scale. Not much is known about the possible scale effects on wave overtopping so this research goes into more detail on this issue.

For the use of open filters in land reclamations new guidelines are needed as no research is currently done for the configuration of a sand layer is placed on top of a filter layer. This research focuses purely on the interaction between cyclic motions and the described sand-filter layer. A numerical OpenFOAM model is used to extract the order of magnitude of the parallel and perpendicular gradients from a numerically designed filter. With these gradients a model setup is designed where five sand-filter combinations are tested. All combinations are tested for both parallel and perpendicular gradients, which resulted in a range of critical gradients for both directions.

It is already acknowledged that Nature-based Solutions can be used to attenuate waves, however it is still uncertain to what extent the vegetation can contribute to decreasing the flood risk. So far mainly small-scale tests have been performed to quantify wave attenuating properties of vegetation. To quantify the effect of more extreme wave conditions (high-water levels and wave heights), full scale tests are required. In the Delta flume of Deltares large-scale physical model tests with a willow forest of 40 meters in front of a dike are conducted. Three different measuring methods: visual measurements, terrestrial laser scanning and video imaging, are used to quantify the wave run-up and wave-overtopping on the dike. This is the first time a ‘Machine Learning algorithm’ is used to obtain the wave run-up heights on a dike from video footages. It is also new that the wave overtopping volumes are determined by a laser scanner without using a wave overtopping tank, which was initially used to collect and measure the real overtopping volume. Results by the laser scanner show an overestimation of the overtopping discharges at high water level for higher crest freeboards, making these results less reliable. However, more research and a thorough validation are required to confirm the accuracy of this method. From results obtained by flume experiments can be concluded that remote sensing: laser scanner and video imaging, are accurate methods to measure the run-up on a dike. Thus, the camera in combination with Machine Learning is an accurate, simple and low-cost technique, to measure the wave run-up on a dike. Results from the camera not only give the run-up height, but also give new insights in the variations of the wave run-up over the dike. It can also be concluded that a willow forest of 40 meters causes a significant reduction in the wave run-up and overtopping, for both willows in summer and winter state. Further research is needed, so these significant reductions can be implemented in the design and assessment of dikes. The most commonly used models for designing a dike are the TAW (2002) and EurOtop. Comparing the obtained results, show that the determined wave run-up from the experiments are underestimated by the TAW (2002). The TAW is an empirical formula, based on a large data set of mainly small-scale experiments. Therefore, the difference between the test results and the TAW could likely be attributed to scale effects. However, more measurements at full scale are needed to confirm this.

When a breakwater is under heavy wave attack, the concrete armour units will occasionally move, causing a collision between two concrete armour units. This process is called rocking, and induces stresses in the concrete, that may lead to breakage of the concrete armour units. This MSc Thesis provides a probabilistic method to predict breakage of concrete armour units, focussed on Xbloc®. The impact velocity is based on the forces on a unit under wave attack. This impact velocity is used as input to determine the impact force, based on an energy balance. The stresses at a critical location in armour unit will then be determined from a strut-and-tie model. With an estimation of the distribution of several stochastic variables, the eventual result is a probabilistic prediction of breakage of the concrete armour units.

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.

Witteveen+Bos as part of construction consortium Nautilus, has prepared the design of a sea defence system in Middelkerke. The very shallow water conditions (h/Hm0 = 0.3) and complex geometry of the structure, i.e. a high berm and stepped revetment, caused uncertainties in the usage of the empirical equations from the EurOtop Manual (2018) for wave overtopping discharges. Therefore, small-scale experiments have been performed to asses the overtopping discharges. Physical model experiments can be used to determine wave overtopping discharges, but can be expensive and time consuming. Recent studies show promising results for the use of SWASH as tool to predict overtopping discharges for relative simple geometries, but SWASH is suspected to be less accurate for more complex cross-sections. Two methods (SWASH, EurOtop) are used to predict wave overtopping discharges for the new boulevard Middelkerke and the results are compared with measurements from small-scale experiments conducted in Ghent. The goal is to asses the feasibility of using both methods for this complex structure with stepped revetment in shallow water conditions. The EurOtop Manual (2018) uses influence factors to account for roughness at the slope and the presence of a berm in the structure. To use the empirical formula to determine the average overtopping discharge for this specific complex cross-section, a berm influence factor was added to the equation for shallow water conditions developed by Altomare et al. (2016). Recent studies of Schoonees et al. (2021) found that the influence of the roughness of a stepped revetment on overtopping discharge mainly depends on: a characteristic step height, relative overtopping discharge and the wave period at the toe of the structure. Using their research as guideline, a roughness influence factor of γf = 0.75-0.9 was estimated for the stepped revetment for this case study. Although not validated for this specific configuration, using the (adjusted) equation resulted in very comparable average overtopping discharges compared to the physical experiments in Ghent. In this case study, the usability of SWASH to predict the average wave overtopping discharges has been evaluated. First, the SWASH model was calibrated based on the incident wave conditions at the toe of the structure (Hm0, Tm-1,0) with the data the physical experiment in Ghent. This were the target wave conditions of this study since they are generally used to determine the average wave overtopping discharges (e.g. EurOtop) and to exclude the wave-structure interactions. Thereafter, the structure was added to the bathymetry with a smooth slope and local friction was added to represent the stepped revetment, as resolving the small steps would require an overly fine grid. SWASH was able to reproduce the target incident wave conditions of the physical experiment very well (Hm0<3%,Tm-1,0<5%). However, when the structure was added to the bathymetry, larger differences were seen for the wave conditions at the toe compared to the physical experiment, for which no explicit explanation was found. Contrarily, Hm0 and Tm-1,0 include the full wave field, primary waves and infragravity waves, and the limited number of overtopping waves made it difficult to asses the influence of the difference in wave spectrum on the overtopping discharges. The overtopping reduction due the added roughness/friction compared to a smooth slope reference test, was eventually related to a Nikuradse roughness height for the stepped revetment. A Nikuradse roughness of 1-1.5 times the characteristic step height of the stepped revetment, resulted in very similar average overtopping discharges compared to measurements from the Ghent experiment. Both the EurOtop Manual (2018) and SWASH were able to reproduce the average wave overtopping discharges from physical experiments, but the results need a wide confidence band since the number of overtopping waves were limited and discharges small (q < 1 l/s/m). Each method has their own benefits and limitations. The EurOtop Manual (2018) is easy to use, but the validity of the empirical equations are uncertain for more complex geometries. SWASH gives more detailed information than only an average q: spatially and temporarily varying surface elevations and velocities along the domain, by which direct attack on the slope and structures (extreme values of u and F) can be calculated. SWASH could also be used as a ’numerical laboratory’ to further parameterize the influence of roughness on wave overtopping for a wide range of boundary conditions and structural configurations.

The main objective of this thesis is to increase the understanding of the effect of shellfish presence on the stability of loose rock revetments and to investigate the possibilities in the design. Pacific oyster (Crassostrea gigas) and blue mussel (Mytilus edulis) presence on loose rock revetments was quantified. C. gigas and M. edulis are common shellfish species in the port area and act as ecosystem engineers. They require a certain salinity level, water temperature, phytoplankton ratio, and sufficient submersion time for daily feeding and respiration. The current presence of shellfish in the port area was predicted based on salinity levels generated by an OSR model. Outcomes were compared to qualitative measurements of C. gigas and M. edulis presence performed in the port area. C. gigas is present on revetments where salinity levels exceed 16‰, which is the case at Maasvlakte 2, Beerkanaal, and Calandkanaal. M. edulis were hardly observed in the port area on revetments. M. edulis can move relatively easily and detach from the surface after mortality occurs in contrast to C. gigas. They are, therefore, not considered a reliable structural addition, so only the effect from C. gigas was studied. C. gigas presence leads to binding of stones in revetments. This binding of stones will increase the stability of stones in the revetment. The relationship between the presence of oysters and stability upgrading is described using a connectivity model. The relationship between the presence of oysters and binding of stones is measured in the port area, these measurements are used to validate the model. Measurements showed that the cumulative coverage ratio of dead and living C. gigas decreases as one moves up to the intertidal zone from the mean low water level (MLW). The binding of stones increases when the coverage ratio of C. gigas is larger. This shows that there is a relationship between the coverage ratio of oysters and the binding of stones, and this assumes that there is a relationship between the coverage ratio of oysters and stability upgrading. The presence of oysters comes with structural, environmental, and ecological impacts. The conditions in the Port of Rotterdam as a system itself can pose various opportunities and threats for oyster coverage, such as global warming, pollution, and future port plans. Complementary measures can be implemented to mitigate the limitations and threats. The maintenance demand, and associated costs, decreases when oysters are present. Oysters are usually present naturally; their presence can be stimulated by enhancing C. gigas populations at locations with conditions that would allow potential oyster growth. Enhancement strategies depend on local conditions. Application is most promising when bow thruster or ship wave impact is normative because oysters are mainly found up to NAP +0.5 m. Additional enhancement costs are approximately equal to the expected reduction in maintenance costs. Possible ways of application in the Port of Rotterdam are summarized in a flowchart.

Mangroves can provide coastal protection by attenuating waves, currents, and trapping sediment (Menéndez et al., 2020; Bao, 2011). The effect of mangroves on the hydrodynamics depends on their size, location, density, distribution and morphology of the vegetation (Mendez and Losada, 2004). Mangrove loss during storm events will therefore impact the coastal protection. The soil-root interaction and influences of soil properties in the resistance of a mangrove tree against failure is neglected in most studies of flood protection by mangroves, due to a lack in understading. This thesis improves the understanding of the soil-root interaction by deriving the schematization for multiple failure mechanisms in comparison with field observations. Firstly, the wind and wave forces are modelled to determine the different loads in the forest. Secondly, a novel schematization was developed of different failure mechanisms considering the soil properties and root system. All results are developed for a case study of a fringe forest in Demak, existing of Avicennia marina rooted in a silty, saturated soil. To be able to determine the difference between the seaward and landward edge of the forest, the width of the forest is increased to 500 meter. Also, due to a lack of information on the mechanical strength of Avicennia m., the mechanical properties of Rhizophora m. are used, which will overestimate the resistance due to the higher wood density of Rhizophora m. (Manguriu et al., 2013). The results show that the drag forces were largely influenced by the tree architecture (such as vegetation width and height), forest density and inundation of the tree. Larger wind and wave forces, and therefore a larger moment occured if the width of the vegetation increased. If the height of the tree increases, a smaller part of the canopy is submerged. This leads to a decreased area that is exposed to waves and therefore a declined wave force. A higher tree does enlarge the area subjected to wind forces, resulting in an increased total moment. Overall, the 2.8-meter tree analysed in this thesis experienced the largest horizontal forces and moment at the seaward edge of the forest due to the relatively large water depth at this location. Furthermore, the maximum horizontal force and moment shift to the landward edge for larger trees due to the increase of wind contribution. To investigate the stability under these loads, the different failure mechanisms need to be inspected. The failure mechanisms of root breakage and slippage are not important because of the high safety factors and no observations in the field. The failure mechanism trunk breakage also showed a high safety factor, contradictiory to field observations pulling willow trees. The model results and field observations showed that a combination of upwind soil uplift and bending of roots was found most likely to occur. For this failure mechanism, the roots are schematized as beams with a spring-support on the leeward side of the trunk. The roots are exposed to the weight of the overlying soil and the overturning moment. The model showed that an increase in the participation angle α or root diameter, or increase in root length results in larger stresses in the windward roots. The amount of uplift enlarges when the root diameter or root length increases or the participation angle decreases. The modulus of subgrade reaction and modulus of elasticity influence the distribution of the overturning moment over the leeward and windward roots. The largest difference between the moment inside the leeward and windward roots is found with a high modulus of elasticity or high modulus of subgrade reaction. This difference is in agreement with the contrasting reaction between soil types as stated in literature (Dupuy et al., 2007). The results show the dependence of the different failure mechanisms on soil and root properties, but also on the forest architecture. This research shows that the incorporation of mangrove stability in wave attenuation models cannot be neglected. Using an effective stress approach, instead of a total stress approach as in this thesis, would provide extra information, such as soil behaviour in time during loading. It is therefore recommended to measure pore pressures under static or cyclic loading, resulting in the progression of the effective stress over time. The inclusion of more accurate soil descriptions would enable reducing uncertainty in nature-based solutions and enables to describe the soil-root reaction to loading over time.

Studying mangroves in the field can be very difficult; therefore it is in some situations better to model in a flume. The question is, to what extend are the situation in the field and the situation in the flume comparable with each other? This study focusses on reflection in the wave flume. Wave energy in a flume can be damped by placing a wave absorber at the back of the flume, which absorbs part of the wave energy; part of the waves is reflected. In this study rock is used to build the wave absorber. Two aspects are studied: the influence of the lay-out of the wave absorber on the reflection in the flume, with as goal to find an optimal lay-out, fitting on a maximum horizontal length of 3.00 m, to minimise reflection; and the method of measuring the amount of reflection in a wave flume.

Contrary to a breakwater outer slope, rear-slope revetment does not have to withstand breaking waves. However, generally, an equal dimensioned revetment is used on both the inner- and outer slope. Although with low-crested breakwaters severe overtopping occurs, the application of similarly sized armour on both sides still results in an over-dimensioned inner-slope revetment. An (economical) optimisation is possible in this regard. The tests executed by Hellinga (2016) in this topic, showed large scatter in the found stability results of single-layer cube revetment at the rear slope. This scatter was party introduced by the large number of influencing factors in the test scope (Hellinga, 2016). By developing an overtopping simulator, the scope of the model-tests is delimited to solely the breakwater crest and rear slope. Moreover, larger scale tests are feasible in the same flume, reducing scaling errors. The main objective of this thesis is a) determine if the approach of a (semi-) small scale overtopping experiment using an overtopping simulator is increasing the reliability of the test results, and (b) to increase the understanding of failure mechanisms of single-layer cube revetment on the crest and rear side of low-crested breakwaters. To design and construct a simulator that is able to generate realistic waves, the relevant mechanisms of overtopping waves were determined and boundary conditions were composed. The behaviour of the simulator was tested in combination with a ‘mock’ breakwater. Seven types of realistic breakwater models were installed to investigate both the hydraulic behaviour of the overtopping volume and the stability behaviour of the revetment. The determination of the layout of a configuration is every time well substantiated by the results from the former configuration. The water level inside the Wave Overtopping Simulators (WOS) reservoir showed a skewed drop during the release of an overtopping volume. Consequently, the water level measured at one single point was not accurate to determine the volume (change) inside the reservoir, and hence the discharge and velocity. Different test configurations failed, where others remained stable during exposure to simulated waves in the test range. Compared to stability numbers of the theoretical front side, the cubes are relatively stable. The transition of the crest and rear slope showed to be a weak point, and outwash of the rock material filling the gap is a significant possibility. Improvements on the WOS are possible to enhance the ability to simulate the characteristics of a realistic overtopping wave volume and to give a more detailed evaluation of the magnitudes of these overtopping characteristics. The normal forces between the cubes at the slope are more important for the stability than the own weight of the cubes. The lack of interaction between the cubes at the crest results in a significantly lower resistance to hydraulic loads compared to the cubes at the rear slope. The layout of a sharp inner crest-line transition, if well designed, showed a stable alternative for a rounded transition.

The added drag on a buoyant body traveling in a solid-body rotation flow has been rigorously studied over the last century with G.I. Taylor being the first one to describe it in 1922, a hundred years ago. A recent publication by Duinmeijer (2021) investigated the capacity of free-surface vortices to transport solids as a practical application to avoid solids accumulation in wastewater pump sumps. In that research, the assumption was made that a Taylor column exists upstream and downstream of the solids body, aiding the downward motion in the vortex core. This has however never been experimentally confirmed for free-surface vortex flows. The goal of this research is to experimentally check whether or not the Taylor-column induced drag force is the main mechanism for the downward motion of buoyant particles in a free surface vortex core. An experimental set-up consisting of a 600 x 1000mm (diameter x height) Perspex tank is used, in which controlled vortices can be generated. A novel combination of Laser Doppler Velocimetry (LDV) and Particle Tracking Velocimetry (PTV) is deployed simultaneously to obtain synchronised data of both the flow velocities and particle motion. The LDV device measures the tangential and axial flow components in a small measurement volume. Many point-measurements along a horizontal line through the vortex enable us to create the tangential and axial velocity profiles. A deconvolution process is applied on the LDV data to account for the spatial averaging effect of the LDV measurement volume and the wandering of the vortex core. The PTV system is used to determine the particle location over time, from which the velocity is obtained. For simplicity, only one type of particle - a buoyant sphere of 25 mm in diameter - is considered in this study. To obtain repeatability of the experiments, a particle dropping device is introduced to insert the sphere in the vortex core without entrapping air. A PID control system is added to the set-up to keep the discharge through the system constant. The first part of this research focusses on the performance of the measurement system, where the measurements of the flow characteristics of the free-surface vortex in the absence of a buoyant particle are compared to the results obtained by Duinmeijer (2020), repeating one of the experiments from that thesis without a buoyant particle. The tangential velocities measured with LDV, coincide well with the tangential velocity profiles found during PIV measurements by Duinmeijer. The axial velocity profiles however do not agree. Duinmeijer measured maximum axial velocities around the vortex core radius where in this thesis an axial velocity profile with two maxima is found; one in the vortex centre and one at 60-70% of the vortex core radius. Several additional experiments were performed to rule out possible causes for this difference, from which it is concluded that the axial velocity is non-zero in the vortex centre. The second part of the thesis focusses on the characteristics of the flow in the vortex core in the presence of a buoyant sphere. With the synchronized deployment of the LDV and PTV-system, the flow mechanics and the motion of the buoyant sphere can be simultaneously characterized. Such that the influence of the sphere's presence on the tangential and axial velocity components is quantified. New findings were obtained from these measurements, with the most striking one being a significant difference that is observed in the tangential velocities of the flow just above and below the buoyant sphere. Although this effect is described in theory e.g. by Moore and Saffman and Maxworthy (1970; 1968), it has not been measured before in any lab experiment before and therefore it was not expected to be observed so clearly during the experiments. The clear tangential velocity discrepancy experimentally confirms the presence of the Taylor column above and below the sphere. The last part of the thesis is dedicated to quantifying the Taylor column induced drag force, derived from the tangential velocity difference of the flow above and below the particle. This difference in tangential velocities induces a pressure difference over the particle leading to a downward pointing force. The magnitude of this force is shown to be 75±17% of the total drag on the particle, making the Taylor column induced drag the main downward transport mechanism for the buoyant sphere in the free-surface vortex core given the conditions used in this experimental set-up. It is recommended to investigate further the effect of the vortex core radius/particle characteristic length ratio on the Taylor column induced drag. The axial free-surface vortex flow is not radially uniform, with a region of high axial flow around the core radius. This axial flow being pushed into a Ekman layer on the sphere surface is generating the pressure difference over the particle and thus the downward force. It is therefore suspected that vortex core/particle size ratio strongly influences the Taylor column induced drag magnitude and thus be researched further.

In 2050, the entire energy consumption in the Netherlands is foreseen to come from renewable sources. Offshore wind energy has the potential to enable this transition to a CO2-emission-free energy supply. Consequently, the development of offshore wind farms in the Dutch North Sea has been expanding enormously in the last decade. The Dutch Government is also ambitious to achieve biodiversity goals in the Dutch North Sea, by restoring ecosystem functions. The European flat oyster is identified as a key species when restoring the biodiversity in the North Sea, because they embody a distinctive benthic community that provides a range of valuable ecosystem services. The environmental conditions in offshore wind farms have shown to be suitable to act as a habitat for the European flat oyster reefs, because of hard substrate and undisturbed areas. Multiple oyster recovery initiatives have been introduced in offshore wind farms, to kick-start oyster reef restoration. No larvae source is nearby, therefore oysters need to be introduced for oyster reefs to develop in offshore wind farms. These oysters are introduced by applying oyster brood stock structures, which are structures to which adult oysters are attached. The brood stock structures are installed by onboard cranes present at vessels, on the scour protection at offshore wind farms. This crane installation method has shown to be complicated and costly, due to the vessel type and equipment required. These complications result in a challenging and therefore limited application. Hence, there is a need to look for an alternative installation method, that ensures a simple and cost-effective application. Deployment of brood stock structures via dropping from a vessel manually is selected, which ensures easy deployment, because it does not require a crane (and a vessel that can hold this crane). This would decrease the engaged expenses significantly. Such a new installation method requires a different design for the brood stock structure. Research is needed into a new design for a flat oyster brood stock structures, which is appropriate for the drop installation method. This leads to the following research objective for this master thesis; What is the design for a flat oyster brood stock structure, that can be installed at the scour protection at offshore wind farms, via dropping from a vessel, such that it will be stable and integer during deployment and operational lifetime? Based on literature study and consultation sessions with experts, a set of design criteria for the droppable brood stock structure are set. Behavioural predictions are made for the performance of the concepts during the lifetime of the droppable broodstock structure by performing calculations.. which are used to select the most suited concept parameters (e.g. volume, dimensions) for the six basic concepts by an iterative process. Ten concepts emerged. Physical model tests were performed to test the behaviour of the ten selected concepts during the relevant situations. Three types of tests are executed; fall test, land test and stability test. The physical model tests are performed in the wave flume in the Hydraulic Engineering Laboratory at TU Delft. The fall test investigated the fall of the structure from the vessel onto the scour protection, to define the dropping accuracy during the fall. The land test investigates the landing of the structure on the scour protection, by analysing the interaction between the prototypes and the stone layer after dropping them in the wave flume. The interaction observation is used to get insight in the amount of oyster damage encountered during the landing. The stability test investigates the stability of the concepts during extreme hydraulic conditions, by generating storm conditions in the wave flume. To select the design for a droppable flat oyster brood stock structure, the ten concepts are analysed and assessed, using two methods. The first method entails a requirement analysis and a multi-criteria analysis. Two concepts have been selected as suitable based on these methods, A Tetrapod concept and an Open Table concept.

Groynes are hydraulic structures commonly applied in the Dutch rivers. Depending on the water depth, groynes can be emerged or submerged. When a groyne is emerged, it has two main functions: maintaining the water depth in the main channel and preventing the river from eroding its bank. When the discharge in the river increases, the groynes become submerged. A submerged groyne acts as an obstacle to the flow, resulting in additional resistance and an increase in the upstream water level. To increase the flow capacity of a river during extreme conditions, a couple of measures can be considered: increasing the height of the dikes and embankments, reducing the overall resistance of the river cross-section and increasing the area of a river cross-section. This thesis focuses on reducing the overall resistance of the river cross-section, and more specifically, on investigating the effect that streamlining the downstream slope of a groyne has on the groyne-induced resistance. Hypothetically, streamlining the groyne by decreasing its downstream slope would decrease the intensity of flow processes like downstream flow separation, which could increase the discharge capacity of the groyne, and thus of the river cross-section, as a result of the decrease in groyne-induced resistance. An increase in discharge capacity would decrease the flood risk during extreme conditions. To investigate the effect of streamlining a groyne by decreasing its downstream slope, a physical model experiment was set up in the 5 x 40 m^2 flume at the Hydraulic Engineering Laboratory of Delft University of Technology. The physical model represents part of a river cross-section. From the total 5 m width, the main channel and groyne field both took up 2 m and the floodplain took up 1 m. In streamwise direction, the flume contained six groynes and five full groyne fields. The groyne fields had transverse bed slope of 1:25. Electromagnetic flow meters (EMF meters) were used to measure the flow velocity at many points in both streamwise and transverse direction. The water level was measured by laser altimeters. Decreasing the downstream slope of a groyne to 1:8 compared to a reference situation with a downstream slope of 1:3 resulted in a 4 % increase of the discharge capacity over the crest of the groyne. This increase of discharge capacity was not uniform over the transverse axis. Near the groyne tip, this increase of discharge capacity was more than 6 %, whereas at the end of the groyne, the discharge capacity decreased slightly compared to the reference situation. A more detailed analysis of the data gave insight in why decreasing the downstream slope of a groyne increases the discharge capacity of a groyne. Streamlining the groynes resulted in a significant decrease of the relative turbulence intensity along the downstream slope of the groyne. This was consistently around 50 % lower for the streamlined groynes compared to the reference situation, indicating less intense turbulent structures. Moreover, auto-correlation functions of the flow velocity signals showed a shorter correlation length in conjunction with lower amplitudes and smaller periods of the fluctuations for the streamlined groynes. This indicates a signal with less correlation and higher frequencies. These results explain why the discharge capacity of a streamlined groyne is larger than of a reference groyne.

In this research OpenFOAM is used to model and determine the complex hydrodynamic behaviour of a Homogeneous low-crested structure (HLCS) consisting of cubipod artificial concrete elements. The validated model is used to gain insight in the design sensitivities of a two dimensional cross sectional layout to reduce sea wall overtopping. HLCS and Low-crested structure (LCS) in general dissipate energy from the incoming wave field by wave breaking over the crest of the structure and porous flow through the structure. By energy dissipation milder wave conditions are created inside the basin between the HLCS and the sea wall. Milder wave conditions result in reduced hydrodynamic loads on the sea wall and reduced flood risk. However not only the milder wave conditions determine the amount of sea wall overtopping. Additionally wave-induced water level set-up and basin hydrodynamics (i.e. seiching and resonance) contribute to the amount of overtopping. The relative importance of these different hydrodynamic interactions on sea wall overtopping depend on the main geometrical layout parameters of the system. The main geometrical layout parameters that are analysed in this research are the crest height of the HLCS (푅c ), the crest width of the HLCS (퐵) and the basin length between the HLCS and the sea wall (퐿pool). These geometrical layout parameters can be used by the engineer to steer the hydrodynamic behaviour towards the most cost effective design to reduce sea wall overtopping. However, due to the complexity of physical processes involved and their interactions, theoretical analysis is cumbersome. Therefore advanced OpenFOAM Computational Fluid Dynamics (CFD) simulations are performed in this research. These simulations are used to capture the complex hydrodynamics and gain more insight in the design sensitivities of these types of hydrodynamic systems. A coupled numerical model using both OceanWave3D and OpenFOAM has been set-up in this research. Model dimensions are based on conducted physical model experiments to assess the amount of wave transmission over HLCS as described in J. Medina et al. (2019). No raw data was available from these physical model experiments. Therefore the wave flume hydrodynamics (i.e. irregular wave characteristics) have been calibrated using a grid resolution study. In this study also simulations with varying courant numbers have been performed. Both extracted statistical wave parameters of the coupled model and a standalone OceanWave3D model have been compared. Additionally the separate output of OceanWave3D and OpenFOAM within the coupled model have been compared. Grid convergence has been found for increasing OpenFOAM grid resolution. The measured mean overtopping discharges from the OpenFOAM model are validated against Eurotop 2018 prediction guidelines. The grid which showed the most accurate results in comparison to the required computational time has been selected for the remainder of the study. This OpenFOAM grid is characterized a grid resolution of Δx = Δy= 퐻s/10. In order to assess the hydrodynamic behaviour related to wave transmission for HLCS, the van Gent (1995) parameterization of the extended Darcy Forchheimer equation has been used to model the amount of flow resistance that is exerted on the flow by the HLCS. Based on the differences between conventional rubble mound low-crested structures and HLCS a detailed analysis on the input parameters of the van Gent (1995) parameterization (i.e. the 퐷n , 퐾퐶, np and the closure coefficients 훼 and 훽) has been conducted. Large porosity gradients are found near the boundaries of the artificial cubipod elements. This effect has been implemented in the numerical model using two numerical layers with different porosity values resulting from the derived porosity distribution for artificial cubipod concrete elements. Additionally the effect of numerical outer layer schematization has been addressed. Both numerical additions only show to have minor effect on the modelled wave transmission behaviour (i.e. < 2% on 퐾t ). The closure coefficients 훼 and 훽 have been calibrated and validated based on the conducted physical model experiments J. Medina et al. (2019). Additionally a sensitivity analysis is included which can be used for the calibration of different artificial concrete elements. The best agreement between the modelled transmission coefficient and the experimental transmission coefficient is v vi Summary found for 훼 = 500 and 훽 = 1.0. A parametric study is performed using the validated OpenFOAM model including the HLCS and sea wall to describe the complex hydrodynamic interactions within the hydrodynamic system and assess the design sensitivities. For this parametric study multiple simulations on 6 parallel processors were performed with a duration of 500 waves. Each simulation took approximately 48 hours to complete. The most important findings and implications of the parametric study are in summary: • For all varying geometrical parameters a decay in the form of an exponential function of the mean overtopping discharge is found. The most influence on the overtopping reduction is found for varying crest height of the HLCS. • Wave transmission is found to be the dominant over the water level set-up, seiching and resonance inside the basin for the overtopping assessment of varying crest height and crest width of the HLCS. The most striking result for wave transmission over HLCS is that HLCS shows a constant trend in wave transmission for emergent structures (i.e. 푅c > 0). A further increase of crest height does not result in reduced wave transmission, contrary to conventional rubble mound LCS where a further reduction is observed. • Furthermore a large dependency is found for varying basin length. The propagation of broken waves (i.e. hydraulic bores) due to wave breaking over the crest of the HLCS result in a significant increase in mean overtopping discharge. It is observed that these hydraulic bores die out for larger basin length. Additionally low-frequency wave motion (i.e. seiching and resonance) is observed for varying basin length. However this effect is smaller compared to the dissipating bores. • By comparing the estimated overtopping discharge using the Eurotop guidelines solely based on the amount of wave transmission and the obtained overtopping discharge from the OpenFOAM model a mean underestimation of 69% is found by only using the transmission coefficient for the assessment of the amount of mean overtopping discharge. This concludes that the water level set-up cannot be neglected for overtopping assessments. Furthermore the use of advanced CFD modelling (e.g. using OpenFOAM) or physical modelling is of added value for the assessment of the amount of overtopping for these complex hydrodynamic systems due to the influence of dissipating bores, seiching and resonance inside the basin. The ability of OpenFOAM to gain insight and to study the interactions for complex hydrodynamic systems has been demonstrated. Moreover design sensitivities of the hydrodynamic system under consideration are presented. These results can be used during early design stages for the assessment of the most cost effective design for these types of hydrodynamic systems. Additionally the sensitivity of the geometrical layout parameters can be used by the engineer to make targeted adjustments. Furthermore for comparable hydraulic boundary conditions (i.e. the same order of 퐻s , 퐻s/ℎ, ℎ/퐿p) and the use of low-crested structures (i.e. 푅c/퐻s,i ≈ 0) this OpenFOAM model can be used during design stages without further calibration.

The ability to predict the fatigue of flood gates due to dynamic wave loading is becoming increasingly important as coasts and waterways worldwide are being reinforced to suit a changing climate. There are few comprehensive ways to do so however, which often leads to conservative estimates. This thesis presents an integral framework with which the fatigue of flood gates due to dynamic waveloading can be described in much more detail, while also being efficient and adaptable to different circumstances. First, the fatigue experienced by a single load event is evaluated based on readily available data. This was done by creating a phase-amplitude model from a wave spectrum based on the site properties and environmental conditions. These wave loads can then be transformed to pressure spectra with linear wave theory and pressure impulse theory, which will dynamically excite the fluid-structure system. The gate is then parametrically modelled in a FEM software package in order to export the in-vacuo response modes. These are combined with a fluid-structure interaction model to derive the response of the system to external pressures. The fatigue was evaluated with the standard Eurocode method to find a fatigue damage factor. Next, the load events imposed on the gate over its lifetime were probabilistically defined. This was done by fitting two independent probability distributions to historical wind- and water level data, the latter of which was also adjusted for climate change. These probability distributions are then split into segments which are integrated to obtain discrete load cases with representative values for the average wind velocity, water level, and probability of occurrence. Two filters were also applied to remove the load cases which cause a negligible amount of fatigue. Computing the fatigue damage caused by the average values of each of these load case bins then gives a set of fatigue damage factors with an associated probability of occurrence, which can be used to perform a Monte Carlo analysis. A case study for a hypothetical parametrically defined gate with an overhang located in the Afsluitdijk was performed, where the response for different gate configurations and environmental conditions was evaluated. The fatigue response was evaluated for fatigue across the gate, at the critical coordinate, and per mode. A ULS check was also done for the chosen design. Results following from the model were also compared to those of methods commonly used in practice.The method gives insight into the relative importance of different modes and load events, can compute fatigue for the entire gate or just a critical coordinate, and does so in a much more time-efficient manner than current numerical models. It gives a more comprehensive view of the fatigue over the lifetime of the gate by modelling its entire lifespan rather than a set of design load events. The modular structure of the integral framework allows for easy adaptation to other use cases in hydraulic engineering where fatigue due to hydrodynamic loading is of interest.

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.

This thesis is the third study of a research line that focuses on the interface stability of a rubble mound sand retaining structure. The type of interface considered is a geometrically open granular filter because of its economical and constructional benefits. The stability of an open filter structure in which the base layer is located on top of the filter layer was investigated by the use of physical modeling. The model used was the same waterproof elongated container as used by Van de Ven (2019), after some adjustments were made. Three compartments were allocated in this model. In Compartment A, the water level was controlled by the use of a plunger. This caused water level differences between the compartments and therefore induced a hydraulic gradient (i) in the filter structure that was installed in Compartment B. The hydraulic gradient was considered as the main load parameter for erosion. The main strength parameter was the stability ratio (SR), defined as the sieve diameter for which 15% of themass of the filter material is smaller divided by the sieve diameter for which 85% of the mass of the base material is smaller. During testing, this hydraulic gradient was increased by the plunger until erosion was noted, defining the critical hydraulic gradient (icr) for the filter-base combination tested. Two loading conditions were distinguished, namely the parallel and combined set-up. Besides the SR and these loading conditions, the influence of the characteristic grain size of the base material and the filter layer height on icr was studied. During this thesis, a new phenomenon influencing the icr was observed, which was not included in the research line yet. This was the initial sand saturation of the filter layer. Therefore, the physical model results were split up into unsaturated and partly saturated tests. For the unsaturated parallel configuration, SR = 5.7, 6.7, 12.9 and 13.1 were tested. Results were found for the upper two values. For the lower SR, no erosion was detected resulting in a bottom limit of icr . The partly saturated parallel tests resulted in a reduction of icr up to a minimumof 73%. Projecting this effect into reality could have a significant influence on the stability of an inversed open granular filter in land reclamation structures. However, further research is advised. All found icr in the parallel tests were higher compared to Van de Ven (2019). Besides, it was concluded that a larger characteristic grain size of the base layer and the filter layer thickness did not influence the icr significantly. Furthermore, three unsaturated combined tests were conducted with SR = 6.7 and 12.9. No erosion was noted for SR = 6.7. Erosion was detected twice for SR = 12.9, once in the first test segment – resulting in an upper limit for (icr ) – and once in the second test segment. A reduction of 93% for icr in the combined set-up compared to the unsaturated parallel set-up was noted, implying that an additional load decreases the critical load for the considered filter structure. However, this could also be the results of model installation differences. Lastly, a new physical model was designed and constructed to facilitate the study on the influence of the superimposed load and the filter inclination on the interface stability. A newly developed numerical model has been made to plot design graphs for this new model.

Including vegetation in flood defense systems has the potential to be a more cost-effective solution than conventional dike reinforcement measures, as vegetation contains wave damping properties. However, more insight is required on the complex physical processes due to wave-vegetation interaction in order to predict the amount of wave damping. Past research found that these processes are dependent on both flow conditions and vegetation characteristics. Therefore, reliable quantification of these vegetation characteristics is of importance to understand these processes and thus to improve predictions for wave dissipation due to vegetation. This study aims to gain insight on practical methods for quantifying relevant vegetation characteristics for wave damping. Data is used from full scale physical experiments conducted in the Delta Flume at Deltares, with 40 meters of willow forest. The vegetation characteristics are quantified in this study both by manual measurements on the willow trees and by executing Terrestrial 3D laser scans (TLS) of the whole forest. The frontal area of vegetation is found in literature to be a relevant parameter for determining the wave attenuation, therefore the focus in this study lies on schematizing this parameter over the vertical, which is seen as representations of the average willow tree. This schematization is referred to in this report as “tree model”. In this study, four tree models are obtained. The four tree models include: tree model 1a (manual measurements on the primary branches, excluding side branches), tree model 1b (branching method based on Strahlers ordering scheme, including side branches), tree model 1c (adjusted branching method) and tree model 2 (from terrestrial laser scanner). The reconstructed area from the TLS point cloud is determined by using Matlab built-in alpha shape function. The tree models are compared in terms of frontal area distribution over the vertical and of their effect on the corresponding wave attenuation. The latter comparison is achieved by using the numerical wave model SWAN and confronting the results with the measured wave attenuation from the physical experiments. With regards to the manual measuring methods, the frontal area of the average tree from tree model 1a serves as a lower limit, while tree model 1b gives the upper limit. The TLS outcome (tree model 2) underestimates the frontal area as computed by the other tree models. In particular, the underestimation is 70% when compared with tree model 1b, and 30 % when compared to tree model 1a. Adjustments on tree model 1b leads to tree model 1c. This tree model accounts for the tapering form of the branches and results in a total frontal area in between tree model 1a and tree model 1b. In this study, the representative area for the willow trees can be best captured with tree model 1c. This tree model results in a drag coefficient (CD) of 1.15 averaged over the tests with leafless willows and showed a negative correlation with the Keulegan-Carpenter number (KC). However, the capabilities of the TLS should be analyzed further and this study encourages to use a larger data set of trees in order to find a relation between the laser penetration and corresponding mismatch. Gathering of vegetation parameters by hand is in fact not economically attractive forwoody riparian vegetation, as these trees are characterized by complex canopy structures and high elevations. The TLS can serve as a practical tool for obtaining these relevant vegetation parameters for applying willows in hybrid solutions.

Wave impact on vertical structures with overhangs is studied. 3 research questions are addressed: 1. How does the entrapped air change during the process of wave impacts ? 2. How is the wave loading influenced by the presence of horizontal overhang ? 3. How is the wave loading influenced by the presence of air pocket?