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.

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.

Recent research identified a novel scour-enhancing flow mechanism related to 3-D flow behaviour. Under specific conditions, vertical flow separation at the upstream crest of a scour hole can be suppressed as a result of lateral nonuniformities in the main flow. This so-called vertically attached flow state is associated with increased scour potential. The objective of this study was to evaluate the effects of lateral nonuniformities on scour-driving flow behaviour in setups with realistic width-to-depth ratios. Laboratory experiments form the backbone of this study. A measurement technique was developed that enables efficient and reliable determination of near-bed flow directions. The technique is based on tufts: small threads fixed to the bed that follow the local flow direction. An algorithm was created to convert video footage of tuft movement to numerical data on flow directions. Next, flow behaviour was studied through a set of experiments conducted in a simple laboratory setup that simulates a common scour situation in the field. Lateral nonuniformities were generated in a mixing layer downstream a lateral expansion of the inflow channel. Two distinct horizontal flow states were observed across the tested configurations: horizontal divergence and horizontal contraction. Horizontal contracting flow was more likely to occur in geometries with a short bed protection, a large horizontal expansion and a narrow inflow channel. The transition point between both flow states was narrow. Only a slight change in geometry could entirely turn around the flow state. The horizontally diverging flow state is accompanied with constant vertical flow separation at the slope's crest. The corresponding return current entails a near-bed flow in upward direction. The horizontally contracting flow state is related to an alternating vertical flow pattern in which vertical flow separation and vertical flow attachment take turns. An alternating vertical flow regime was never identified before in this context. The alternating flow state is not present in the jet's centre. Instead, vertical separation takes place here. Additionally, a consistent lateral near-bed flow from the mixing layer to the centre of the jet is present. The frequencies in which vertical attachment and separation alternate match the frequencies of two-dimensional coherent turbulence structures (2DCS) in the mixing layer. This similarity indicates a relation between mixing layer instabilities and suppression of vertical flow separation, which is a valuable new insight into the process of understanding nonuniformity-induced flow attachment. The presence of the horizontally contracting and vertically alternating flow state likely increases scour dimensions. As a scour hole develops, unidirectional lateral near-bed flow velocities stay relatively high, maintaining scour potential. Scour potential is largest in and around the mixing layer. This most likely results in a strongly 3-D scour hole with maximum scour depths in the areas directly affected by the mixing layer. Scour hole development in previous experiments on scour in similar setups confirms this scouring behaviour. The premise that excessive scour is caused by an alternative flow state provides the opportunity to further improve and extend the abilities of empirical scour prediction.

A combination of the growth of the offshore wind energy sector and the decline in both species richness and species abundance in the North Sea has inspired the goal to use the scour protection in wind farms as a means to enhance marine life. Present research has focused on general changes which are to be applied to benefit marine life. However, it lacks practical applications of changes to the design of the scour protection and fails to quantify the expected effects of these changes. In this study the extend to which marine life in the North Sea can be enhanced through improvements to the scour protection design for monopiles in wind farms is researched by comparing the habitat suitability in three situations (North Sea in absence of wind farms, North Sea with wind farms and scour protections as currently designed, and wind farms with enhanced scour protections) for four indicator species (Atlantic cod, European lobster, flat oyster, and the Ross worm). For the third situation (the enhanced scour protections), several habitat enhancements as well as some stock enhancements have been proposed and their expected effects and costs have been studied. The availability of hard substrate and the absence of (seabed disturbing) fisheries is what makes wind farms suitable compared to a sandy sea bed. These habitats can be further improved by using a large and narrow grading to provide shelter and stable attachment material and by placement of additional elements such as piles of rock, (concrete) tubes, and shell filled nets. Species which are not likely to successfully colonize the wind farm, such as the European lobster and flat oyster, are to be introduced by humans through stock enhancements.

In the present study the stability of crown wall elements on top of a rubble mound breakwaters is investigated. The first step was conducting a literature review, in order to identify knowledge gaps. It was found that current design methods do not take the freeboard of the crown wall into account when calculating the vertical force acting on it. Recent research has established that when the freeboard is increased, this force is reduced while the portion of the crown wall base that is wet is reduced as well. However, there have been different approaches to increasing the base freeboard, that could lead to different effects on the loading. Moreover, parametric investigations regarding the vertical force have been very limited. Furthermore, the current design formulas assume that the maximum horizontal and vertical forces occur simultaneously but research suggests that there is a time lag between the two.
Based on these knowledge gaps, the study goals were defined. In order to achieve these goals numerical model simulations were prepared and ran, where the freeboard was varied with two different approaches, as well as simulations with varied breakwater slope. The selected CFD numerical model is OpenFOAM® making use of the waves2Foam toolbox, implementing the volume of fluid (VOF) method.
It is found that the currently used empirical methods fail to predict the changes in loading for increasing freeboard. For an increasing base freeboard, less part of the base becomes wet and that the vertical force, as well as the critical weight are reduced. On the other hand, the horizontal force increased. It is concluded that when increasing the base freeboard by means of lowering the water level results in lower loading compared to an increase of freeboard by elevating the crown wall element. Additionally, for the latter approach, a larger portion of the base slab is wet. Also, it was confirmed that a recently proposed reduction coefficient by (Bekker et al., 2018) for calculating uplift pressures can provides more accurate results in the case of freeboard increase.
Examining the uplift pressure distributions, it was found that for a zero base freeboard the pressure distribution follows an S-shaped profile, which with increasing base freeboard reverses. The peak pressure is located slightly inwards instead of the seaward end of the base, followed by a reverse peak. In order to propose a generally applicable profile shape more data are required.
The results indicated the presence of a time lag between the maximum horizontal and vertical forces. This time lag results in lower critical loading on the structure than when assuming simultaneous maxima. Nonetheless, further research is necessary, as these findings are a result of only one wave condition.
A finding which contradicts the predictions made with empirical methods is that gentler breakwater slopes resulted in higher loading. This is considered to be a result of an increased internal set-up for gentler slopes.
Further research is recommended, especially on tests with varying wave conditions and geometries, which should make these conclusions more generally applicable.

The design of offshore and coastal infrastructures, sand nourishment and other ’soft’ coastal interventions require the analysis of environmental variables (e.g. wind, waves, rainfall) that can potentially cause the failure of such structures. Processes such as overtopping, beach erosion, and coastal flooding can result from a combined action of two or more physical processes. Traditional infrastructure design practices assume the highest load previously experienced as the design load, regardless of possible interactions between variables (or processes). This may lead to a misrepresentation of critical design loads. This thesis presents a methodology for defining infrastructure design loads accounting for their interdependence. The methodology is general and is based on regular vines. Vines are graphical tools for defining high dimensional distribution functions through pair-copula construction. With this premise in mind, the main effort was concentrated in formulating a series of steps to integrate several stages of the design: from the processing of raw data up to the choice of design loads for any specific design purpose. The vine-based methodology was applied to the design of a breakwater in Galveston Bay, Texas. This application showed that accounting for the interdependence between design variables provides a more comprehensive description of the physical system acting on the infrastructure. However, the vine-based method is computationally demanding. Hence, the applicability of this methodology should be evaluated on a case by case basis. In parallel, the possibility to define goodness of fit test for vine-copula based on the concept of tree-equivalent classes is explored. The focus is on model selection strategies based on graphical and statistical properties of the vines. The main motivation to investigate model selection strategies for vines is the considerably large computational time needed to fit all regular vines in more than 6 nodes to the data. In this thesis, a novel algorithm is developed to facilitate the implementation of vines in higher dimensions (vines with more than 6 nodes). This algorithm significantly reduces the computational effort to select a regular vine by allowing the user to test only a subgroup of vines in n-nodes constructed on specific characteristics of the vines in (n-1)-nodes.

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.

The consortium Projectorganisatie Uitbreiding Maasvlakte (PUMA), consisting out of Van Oord and Boskalis, constructed a hard sea defence for the protection of Maasvlakte 2, which is the expansion of the Port of Rotterdam. This hard sea defence consists of a Dynamic Rock Slope (DRS) with a Block Dam (BD) situated in front of it. The BD is build up out of large concrete blocks which were recycled from Maasvlakte 1. The DRS behind it protects the hinterland through an approximate three to four meter thick layer of quarry rock. When waves travel obliquely through this Block Dam and interact with the DRS, stones are transported in alongshore direction. This process is called Longshore Transport (LT) and will have erosion and accretion of stones to effect along the DRS. In order to maintain the layer thickness of this stone layer, PUMA made a prognosis on LT for the coming 50 years after construction. From this prognosis a nourishment campaign was set-up, consisting of volumes of stone PUMA expected to nourish every 2.5 years. After a maintenance period of five years it was concluded by PUMA that the nourishment volumes were lower than expected. Therefore this thesis assessed where differences in the prognosis of PUMA on LT and the observed LT rates over the period 2013 to 2018 come from. Within their prognosis, PUMA made use of a dimensional LT relation. As this specifically calibrated Longshore Transport relation is only applicable for the computation of LT rates at a structure containing the characteristics of the DRS at Maasvlakte 2, this research looked into the possibilities of using a non-dimensional Longshore Transport relation for the computation of Longshore Transport rates at the DRS of Maasvlakte 2.

Currently, sand retaining rubble mound structures are often constructed with geotextiles, lining the interface between the core material and the sandfill. These geotextiles are placed to make sure the sand from the sandfill is not flushed out through the core by incoming hydraulic forces from the surrounding water. It is proposed that difficulties faced during placement, or uncertainties regarding the correct installation of these geotextiles can be overcome by curtailing the geotextiles. The potential for this abbreviations is stressed by Polidoro et al. (2015) as this author concluded, that at the lower inner corner of rubble mound structures with a closed inner slope, pressures are dampened below a certain estimated critical value. However, no proof was found for the applicability of this critical value. Despite several researches predicting hydraulic loading in rubble mound breakwaters, and studies assessing the stability of sand to stone interfaces, insight in the behaviour of sediments in this particular interface configuration of a sand retaining breakwaters was lacking. Therefore, the aim of this research was to study the behaviour of sediments at the interface to a core in a sandfill. Under supervision of VAN OORD and DELFT UNIVERSITY OF TECHNOLOGY (TU Delft), a physical model is designed to study specifically this interface. The model tests took place in the Fluid mechanics Laboratory at TU Delft, where a relatively small setup (1 * 0.5 * 0.15m) was used to model on a relatively large scale (lambda= 15). An exact scaled representation of a nominal breakwater by Polidoro et al. (2015) was used. In the model both the Development of the interface during placement of the sandfill as the behaviour of the sediments when subjected to hydraulic loading is studied. The research concluded that a stable initial interface was found with a slope of approximately 35degrees. In what extent the infill migrated inward through the core varied depending on the installation
method. When subjected to hydraulic loading, a critical hydraulic gradient was found of 0.05m/m on average and 0.04m/m in the most conservative case. These results were established with a measurement accuracy of » 5% and the consistency over different series of tests was¸ 80%. Sometimes segregation of the stones was observed. The main attribution to the deviations in the measured hydraulic gradients and sediment transport were concluded due to this variation in the positioning of the stones. It is concluded that the current existing literature is able to give reasonable initial approximations of the critical gradient in the system(5¡40% accurate), however, the deviation can be significant and further research by varying more geometrical parameters should conclude if the obtained approximations are constant. Concluding, the results obtained in this research suggest that the critical loading conditions for the interface stability of to a rubble mound in a sandfill are of comparable order to conventional filter criteria and are higher than the currently calculated and measured appearing gradients by for instance Vanneste and Troch (2012) and Polidoro et al. (2015). These results justify the further exploration towards the potential of the abbreviation of geotextiles at the considered interface. In order to guide further research a list of recommendations is given as well as additional model improvements.