Geotextiles are permeable fabrics used in various civil engineering and construction applications for the protection against erosion and scour. Geotextiles have predominantly been manufactured using synthetic materials due to their durability and precise design pore sizes. However, the environmental concerns associated with synthetic geotextiles have led to the exploration of sustainable alternatives, including natural fibers and biodegradable thermoplastic polymers.

This study addresses the problem of designing stable geotextile filters. The problem statement resolves around the assumption that geotextiles of natural fibers are considered "open" filters. Existing design criteria for open filters do not directly account for load conditions or the use of fibers in the fabric. Instead, they rely on the relationship of the geometrical properties of the base and filter material. To address this gap, a new approach is proposed based on hydraulic pressure gradients. This approach considers the actual gradient and critical gradient at the geotextile interface. To achieve this, the geotextile will be evaluated within the context of an open channel with uniform flow conditions.

The main research objective is to formulate a design method for geotextiles based on the principles of an open filter structures under a single granular filter layer. A comprehensive literature review has been conducted to investigate the hydrodynamic processes that define the actual gradient at the filter/base interface where the geotextile is positioned. The framework of linear wave theory proved to be a valuable tool for investigating and modeling these hydrodynamic processes. To define the actual gradient, the following hypothesis is proposed:

The actual gradient can be defined by adding the average gradient to a turbulent component derived from measured turbulent wall pressure spectra at the top of the filter layer. Based on spectral approach and linear damping, it is hypothesized that these turbulent components undergo damping within the filter layer, resulting in a diminished turbulent component at the filter/base interface.

From the literature review, it can be inferred that the dimensionless turbulent rough wall pressure spectra developed by Blake (1970) offer the most appropriate representation for the relevant hydraulic conditions (rough horizontal bed, uniform open channel flow). In a turbulent wall pressure spectrum, outer scaling variables and inner variables relate to different length scales within the flow, representing large-scale and small-scale fluctuations, respectively. In this hypothesis, it is assumed that the low-frequency region is scaled by the average velocity and water depth, while the scaling variables for the high-frequency region are presented in terms of the shear velocity and nominal diameter of the filter material. By combining both spectra, converting the pressure spectrum to the gradient spectrum, and transforming the spectral domain to the spatial record, the fluctuating gradient can be determined.

The hypothesis was verified using measured data from experiments conducted by Van de Sande (2012) and Wolters & Van Gent (2012). By incorporating Van de Sande's measured dataset into the scaling parameters of the dimensionless pressure spectra and determining the stability parameter for open filters, it was concluded that qualitatively the results align with Van de Sande's findings, indicating stability or instability of the filter layer. However, in some test cases, the hypothesis suggested instability while the experiments indicated stability.

This discrepancy may be due to the fact that while the stability parameter may indicate an unstable filter, it does not necessarily correlate with significant base material transport through the filter layer. In a quantitative analysis, the amount of dimensionless transport of base material was compared to the stability parameter. According to the data presented in figure 4.12, it can be observed that the stability parameter exhibits a value that is three times larger then the stability parameter defined by Schiereck (2012), while not inducing any significant transport of base material. This finding aligns with previous experimental investigations, such as the study conducted by Wolters & Van Gent (2012), which similarly demonstrated that the stability parameter could reach values that are two to three times larger, while still not resulting in notable base material transport.

A direct quantitative comparison was conducted between the measured dataset of Wolters & Van Gent (2012) and the calculated gradients obtained from the hypothesis. The results revealed that the calculated gradients exhibited a comparable magnitude, ranging from 0.25 smaller to 1.50 larger in comparison to the measured gradients reported by Wolters & Van Gent (2012). This indicates a reasonably close agreement between the calculated and measured gradients.

In conclusion, the research objective in this study was to formulate a design method for open geotextiles under a single granular filter layer. To achieve this, it was necessary to take some steps back and define the actual gradients, as well as identifying a suitable method for calculating these gradients. The actual gradient is calculated with the sum of the average gradient and the fluctuating gradient. The fluctuating gradient can be characterized by the turbulent wall pressures, which are derived from the pressure spectrum measured at the top of the filter layer, as described in Blake (1970). Assuming linear damping of the pressures within the filter layer, the actual gradient can be computed at the interface of the geotextile. Consequently, the assessment of stability entails a comparison between this calculated actual gradient and the critical gradient measured by Lemmens (1996).

With a clearer understanding of how to calculate the actual gradients at the geotextile interface, the same approach employed in the hypothesis is applied to address the design of a stable open geotextile under a single granular filter layer. This has been demonstrated through a case study, where the thickness of the filter layer was determined to create a stable filter structure.

Several recommendations can be made to improve the understanding and design methodologies in relation to the hypothesis. Firstly, there is a need for a better understanding of the spectral density of wall pressure fluctuations in open channel flow. This understanding will enable more accurate estimations of fluctuating gradients. Secondly, further research is necessary to incorporate the distribution of pressure at the filter/base interface for non-linear damping. Thirdly, it is advised to conduct tests using modern natural geotextile materials to refine the critical gradient determination and explore alternative approaches, such as considering critical filter velocities. Lastly, it is recommended to extend the design method to encompass wave conditions. Verifying the effectiveness of the design approach under wave conditions through comparison with measured data will ensure its applicability and reliability in practical scenarios.

While further investigations are needed, it still offers a fresh perspective on evaluating the stability of open filter structures. If successfully refined, the design method could become a valuable tool in geotextile design and potentially find applications in other contexts involving "open" structures. ... Read more

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.
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Starting in the 1930’s, many research projects have been performed with the intent to establish rules, which can be used to design granular, geometrically closed filter structures for hydraulic structures. However, for the vast majority of these research projects, the filter structure was loaded by uniform flow, either in the parallel or perpendicular direction. This means that, while for filters under flow loading multiple design options and diagrams are available, the knowledge on design rules for sloped, closed filters under wave loading remained limited. These knowledge gaps mean that, in design practice, often the design criteria for closed filters under flow loading, for instance the original criterion by Terzaghi, are used for the design of closed filters under wave loading, while these have not yet been experimentally verified for this use case.

This thesis combines model test results from two historically performed experiment programs on sloped, closed granular filters under wave loading with new experiments, which were conducted especially for this thesis. By analyzing the results from these three research projects, it was found that commonly used interface stability criteria for closed granular filters under flow loading are not valid for use in the case of a sloped, closed granular filter under wave loading. Based on the test results from the three test programs that were analyzed, a new interface stability criterion for sloped, closed granular filters under wave loading is proposed. ... Read more

Sea level rise caused by climate change directly affects and threatens low-lying countries. In August of 2021, the report of the IPCC was published, and the findings were disturbing. The sea could rise by a few meters in the coming centuries, causing floods in large parts of West-Europe. Groeskamp and Kjellson proposed a solution to adapt to the effect of extreme sea-level rise, a Northern European Enclosure Dam (NEED). The dam stretched from Bergen in Norway to the north of Scotland and from Ploudalmézeau France to the Lizard Heritage Coast England. The main objective of this thesis is to investigate the technical feasibility of the NEED. To fulfill this objective, the following aspect were studied in more detail: the location and layout of this dam, the favorable cross-section of the dam, and the most likely closure scenarios.
The first step was to find out what the optimized location was. The proposed location from Groeskamp was compared to an optimized location based on the boundary conditions. The two alignments are distinguished by their length, depth, and location.
An engineering perspective was used to evaluate the alignments. The outcome of the multicriteria analysis (MCA) was that the initial alignment by Groeskamp was most favorable. However, a different perspective can result in a different conclusion, and it is therefore advised to look in further detail to come up with a well-founded decision.
For the favorable cross-section, two types of closure dams were investigated to determine the best dam design, the caisson dam, and the earthen dam. Both designs were dimensioned based on safety criteria. These safety criteria were derived from the Dutch guidelines for primary flood defenses, with the top event, inundation of west Europe, given a probability of failure of 1:10 000 per year. A Monte Carlo analysis was constructed to determine the dimensions of both dams. Two failure mechanisms are assessed fully probabilistically as these are considered to be the most relevant. The ULS (ultimate limit state) and SLS (serviceability limit state) conditions are considered per failure mechanisms. The main difference between the resulting designs is the sizes; the earthen dam needs a mild slope to be stable (1:5), requiring a tremendous amount of material. The earthen dam requires seven times more material than the caisson dam. This results in the caisson dam being a more technically feasible design for the NEED than the earthen dam. The caisson is made of concrete filled with sand. Rock protection and concrete foot protection blocks to protect the sill from erosion.
The closure procedure depends on the flow velocities inside the closure gap. The occurring flow velocities during the closure determine the feasibility. These velocities were calculated using the shallow water equation.
A critical boundary condition for the caissons closure is that the maximum flow velocity in the gap is less than 2.5 m/s. For higher flow velocities, placement of the caissons is impossible. Based on the model, a closure procedure was proposed in which this essential condition is met.
The most likely closure scenario is that first, the Southern dam is closed to ensure that the flow velocities do not exceed the critical value. The next step is to fulfill the Northern dam closure. The gap is closed until the maximum velocity is approximately 2m/s. The last part is then closed using sluice caissons. These caissons are placed openly in the gap and closed between 1 tidal cycle, completing the dam. The location of the final gap is at the Norwegian trench.
The results from this research are that the NEED is expected to be technically feasible. However, the enormous size of the NEED poses new challenges and effects. Further research is required to comprehensively analyze all impacts of the NEED in order to make it feasible.
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The erosion of the seabed in front of coastal structures induced by waves can lead to multiple types of failure instability or function of the structure. On this topic most research has been done into erosion in front of vertical structures. For sloping structures a knowledge gap exists in the understanding of the processes that lead to erosion of seabed material directly in front of the structure. In this report the structural and hydraulic paramters that influence the development of a scour-deposition pattern in front of sloping structures is tested. For this model tests have been done in the wave flume of the laboratory of Hydraulic Engineering at Delft University of Technology. This has resulted in a design formula that can be used for a first estimation of the maximum scour depth in front of sloping coastal structures where bed-load transport is dominant. Parameters that are used in determining this depth are the number of waves, the incident significant wave height, the relative water depth and the reflection coefficient of the structure. Performance of the formula with the model test results is good. ... Read more

On the seabed in front of coastal structures sediment movement takes place due to incoming waves. The erosion of sediments creates a scour hole at the toe of structures such as embankments, breakwaters and dikes. This scour hole may lead to instabilities in the main structure due to material sliding into this hole. A common method of protecting against scour is the placement of a rubble mound toe structure and bed protection. The goal of this study is to better understand the background and main parameters that leads to scour, to predict the (location of the) maximum scour depth and how this is influenced by the bed protection. Consequently, the main research question of this thesis is: What is the expected, wave induced, scour depth over time in front of a sloped embankment, and what is the effect of a scour protection on the (location of the) maximum scour depth? ... Read more

The Xbloc^Plus is a new type of single layer armour unit for breakwaters. The key difference between earlier single layer armour units is that the unit is not only placed in a regular grid, but also with a regular orientation. The main benefit of this is that it can directly be seen whether the unit is placed correctly. Additionally, the placement always takes place with the same repetitive movement, as is preferred by crane drivers. The benefits to the placement procedure would however be reduced when the requirements for the under layer become very tight. Since the time that is saved during placement of the armour units, then would be lost during the profiling of the under layer. This has led this study to focus on the influence of irregularities in the under layer on the stability of the Xbloc^Plus and the allowed tolerances for the placement of the under layer.
In the process of obtaining a stable armour unit, multiple researches have been conducted. The final shape has been further optimised by adding a hole in the middle of the unit and increasing the interlocking capacity. Better interlocking increased the connection between the units and thereby improved the resistance. The hole increased the porosity and thereby reduced the overpressure underneath the armour layer. Which is a result of a difference in water level inside and outside of the breakwater. The increase in stability due to the enlargement of porosity, validated earlier findings that the acting failure mechanism is extraction due to the overpressure.
The stability was expected to be mainly effected by the amount of interlocking. Subsequently, the amount of interlocking is determined by the relative angle between the armour units. For this research physical model tests have been performed to check that hypothesis. Multiple tests have been conducted with different radii of convex and concave shapes in both long shore and cross shore direction. The goal was to find the critical relative angles of each combination of shape and direction. The convex shape in cross shore direction was the only irregularity that caused failure and proved to be critical. The relation between the relative angle and the stability number resulted to be linear, instead of the expected drop of stability at a certain critical value. The relation showed signicant spreading, for which tests with small deviations from the design profile seem to form the upper bound and large deviations the lower bound. Thereby indicating that the level of stability is not only determined by the relative angle but also by the deviation from the design profile.
In succession of the tests with specific shapes and directions, more realistic tests have been performed with micro irregularities and S-proles. The convex shape in cross shore direction proved to be critical for these congurations as well. Furthermore it was found that for large S-proles, sudden failure could occur without previous indication of loss of interlocking. Further analysis of the divergent failure behaviour of large S-profiles indicated that the arched shape of the profile enabled the lifting of the armour layer. A mechanism similar to the buckling of beams. The pressure inside the breakwater, the drag of the down-rush and the weight of the upper part of the slope, cause the initial distortion of the profile to be enlarged.
The differences between the measurements of the profile before and after the tests, showed that the same movements occur for smaller S-proles and the tested convex shapes in cross shore direction. The movement could even be seen in tests that have not failed. Which implies that the arching mechanism that causes sudden failure for large S-proles, also induces the loss of interlocking for other convex shapes. A simplied model of the forces on the convex sections in the profile indicates that the enforcing of the arching mechanism mainly depends on the length and steepness of the downward part of the convex section and the  atness of the upward part of the convex section. These findings correspond with the test results and explain why the stability performance is the highest for the short length scale micro irregularities and lowest for the large scale S-proles with large deviations from the design profile.
Evaluation of the effect of irregularities is performed with the tolerances of the regular Xbloc, which are also intended to be used on the Xbloc^Plus. To prevent differences in stability due to the size of the under layer, the tolerances are expressed in the unit size (D_n) instead of the under layer grain size (D_n50). The resulting tolerances are maximum 0.25 D_n for the deviation from the design profile and maximum 0.1 D_n deviation between subsequent measurements along the profile. The profile is measured every 10m of the breakwater and subsequent measurements along the profile have a distance of 0.3 D_n. The requirements are a little more liberal than the tolerances of the regular Xbloc since the tolerances of the largest under layer grain size are taken as a starting point. The test results indicate that the tolerances are sufficiently safe, with potential for more liberal requirements.
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Breakwaters are hydraulic structures used to create calm and steady environments for moored ships in harbors. They dampen the wave by dissipating the energy through wave breaking. Most studies of breakwaters have only been carried out on the seaward slope of the structure. However, few have conducted research on the landward-, inner-, slope of breakwaters. The existing accounts
re all empirical design formulae, commonly denoted as applicable for conceptual design. Improvement of the design of the armour layer of the inner slope leads to economical advantages.This research focused on three main goals: (1) Presenting a theoretical methodology for the waves tested, (2) It veried the Wave Overtopping Simulator as built by Rietmeijer (2017) and (3) Performed stability tests for the inner slope by conducting physical model tests in the Fluid Mechanics Laboratory of Delft University of Technology.

The first goal of this research has been to develop a theoretical approach to overtopping waves in combination with a simulator. This has been done by linking certain storm characteristics, being the significant wave height Hs and peak period Tp to wave characteristics; water volume, water layer thickness and the front velocity of the water body. These three combined lead to an impinging wave on the rear slope causing damage. Subsequently the storms have been altered to only select the highest percentage of waves, since these were marked as extreme storm conditions. This resulted in dierent signicant wave heights which were tested.

To verify the Wave Overtopping Simulator tests have been performed and specic elements have been measured and analyzed. This has resulted in relationships between theoretical wave characteristics and simulator settings. The relationships are the required water level in the reservoir to obtain a certain front velocity of the water body. To adjust the water layer thickness the valve settings have been changed. For a specic volume the overtopping period has been altered. These three elements have been measured using wave gauges and a high speed camera after which the data was carefully analyzed.

The stability tests have been executed using a model of a breakwater in the Fluid Mechanics Laboratory of Delft University of Technology. Results of the preparatory calculations together with results obtained from former research have led to the design of the test set-up. A single armour layer of 20 mm cubes with an porosity of nearly 40% was built with an fixed toe. This configuration was tested with increasing storm intensities until failure. Subsequently the damage has been measured using photo camera's and laser technique.

This research has successfully verified the wave overtopping simulator by measuring individual waves and their specific discharge, water layer thickness and velocity. These measurements have been compared to an other method, using a high speed camera. The found values have been checked with found literature and it has been concluded that the Wave Overtopping Simulator together with the test set-up as built in the Fluid Mechanics Laboratory is useful for this research. By measuring the displacement of cubes by means of video camera and laser technique, the research has been able to identify damage and failure cases. The horizontal displacement has been divided into several classes ranging from settlement of the armour layer, to instant failure. In addition, this research has presented a possible link between the stability of a single cubic armour layer and the number of rows beneath the waterline. ... Read more

Climate change is likely to affect our infrastructures in the Netherlands and seems to accelerate: sooner, faster, stronger. This stresses the need for adaptable infrastructure. Asset owners cannot assume a certain final situation of the infrastructure and the impacts on it due to climate change. This report answers the following research question: How can existing infrastructural asset designs in the Netherlands be adapted to anticipated current and future expected climate change effects? A method is developed to explore what the effects of climate change are on infrastructure and what further steps can be taken to make the infrastructure adaptable. The Afsluitdijk is used as a case study and has five embedded sub-assets: the dam, motorway, outlet sluices, ship locks and bridges. Nine climate change effects are distinguished for the Netherlands in the scenario WH of the KNMI in 2085. The climate change effects are combined with the asset parts and all combinations are assessed separately. An expert reflects on the combination of climate change impacts and asset part and elaborates on how the climate change effect impacts the reliability, safety or maintenance of the asset. This resulted in 21 identified possible problems for the 2020 design of the Afsluitdijk. In this report is looked at foreign situations that are comparable to the Afsluitdijk’s circumstances. Some of the 21 identified problems can easily be solved. A structured approach on how to assess the impact of climate change on infrastructure is lacking so far. The method makes sure all aspects are considered and helps the expert to think of all theoretical problems that have not occurred yet but are in the future a possible problem. Assets that are an integral system are preferred. The Afsluitdijk is interconnected to its environment and hard boundaries are difficult to determine. This makes the Afsluitdijk not an ideal case study, but this interconnectedness is typical for large scale infrastructural projects and those projects are often most interesting. Rijkswaterstaat recognizes the surplus of water problems in the new design. However, drought, heat waves and a low water level are three climate impacts that are not taken into consideration in the new design of 2020. Summer related problems are explored by Rijkswaterstaat but have not yet received the attention it deserves. Rijkswaterstaat is unprepared to a situation where climate change effects are more severe than was estimated in their assumptions. On top of that, Rijkswaterstaat is inclined to address problems of big infrastructural problems by cutting them of in smaller projects and try to solve them. However, in large projects it is better to have an integral solution since all aspects are interconnected and influence one another. The method provides this integral viewpoint and allows the user to make long-term decisions while taking the uncertainties of climate change into account. By using the method, choices in design and management of the infrastructural asset can be made with confidence. This does not guarantee that the outcome of a risky decision will be optimal. But the decision will be rational in the face of uncertainty and that the repeated application of the method will on the long run be more beneficial. ... Read more

Wind energy from the North Sea is one of the solutions to achieve the climate goals of the Paris agreement. The Dogger Bank, a North Sea sandbank, has a lot of potential for wind energy due to the relatively shallow location and good wind conditions. An artificial island creates even more opportunities, because alternating energy can be converted here into direct current. An island will also make the construction and maintenance of the windmills cost-effective. The Dogger Bank, however, is part of Natura2000 area, for which strict legislation applies. To design a stable island coastal defence, which contributes to the North Sea ecology, it is researched how natural value could be added to make the island licensable.

The flat oyster (Ostrea edulis) has been chosen as an umbrella species, a species that creates an environment in which a large group of other species can benefit. The disappearance of the present ecology by building an island can be counteracted by restoring an oyster habitat. To obtain the boundary conditions for a successful oyster bed, research was done into the relationship between the bed shear stress and the historic presence of oyster beds. The shear stress boundary condition for the highest significant waves was 119.8 N/m2 while the shear stress due to the average significant wave climate must stay between 2.5 and 10.3 N/m2. The orientation of the coastal protection, the depth, the profiles (as the presence of a reef or foreshore) and the chosen material are decisive factors for success of oyster beds. Concluded is that opportunities are available in the coastal defence. In typical cross-section are only few areas suitable. For the most exposed side (330-360ºN), it is opportune for oyster beds to grow around the toe and foreshore when located in water deeper than -25.0 m MSL. At the least exposed side (120-150ºN) this is -18.0 m MSL. Reefs and prolonged foreshores are expected to be efficient for creating larger nature enhancing surfaces.

The design and three-dimensional consequences of the suggested measures are recommended to study into detail. In addition, the biotic factors and the method of initiation of oyster beds will have to be further investigated to obtain more information about successfully creating oyster beds around an island on the Dogger Bank.
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Nowadays, the use of concrete armour units on rubble mound breakwaters designs has become a common practice. Concrete armour units can be placed in a single layer system or in a double layer system and can its placement can be either random or uniform. Recently, Delta Marine Consultants have designed a new armour unit called Xbloc+. This new block is applied as a one layer system and has a regular placement. As Xbloc+ is still under development, preliminary guidance on the performance of this new armour unit is required. This led this research to investigate how Xbloc+ behaves concerning wave overtopping. To analyse wave overtopping, small scale tests were performed in a 2D wave flume. Wave overtopping was measured at a 3Dn distance from the seaward edge of the crest, which is made of rock. In front of the slope there is a flat transition of 0.3m followed by a sloping foreshore of 1/30. In total, 10 series of tests were conducted. In this research, three wave steepness (S_op) were tested (0.02, 0.04 and 0.06) to see the effect of wind waves and swell conditions on the armour layer and tests were performed in two different slope angles (1/2 and 3/4). Each series is formed by several sub tests conducted with increasing wave heights (and wave period in order to maintain a constant wave steepness). Tests were carried out until the failure of the armour slope was reached. Besides, a smooth slope was tested in a 1/2 slope angle to be able to compare smooth and rough results and determine the roughness coefficient of the armour unit more accurately. The test results showed that wave overtopping rates increase exponentially with wave height (H_m0) and wave period (T_m-1,0). Moreover, waves with lower wave steepness (swell conditions) induce higher overtopping discharges as compared to larger wave steepness. Steeper slopes also lead to higher overtopping rates. Therefore, the larger the breaker parameter is, the larger the overtopping discharge results. The influence of the armour roughness and armour permeability is also analysed and it is observed that swell conditions are less affected by these properties. This can be explained by means of the breaker type. Swell conditions are characterized by large surging waves. These waves have a thicker water tongue as compared with other type of breaking (e.g. spilling or plunging) and therefore, they tend to feel the top layer “smoother”. This fact is supported by the analysis of the influence of the roughness coefficient since higher values are obtained for surging waves. In fact, the roughness coefficient ranges between 0.34 and 0.68 showing that, although empirical prediction consider this parameter as a constant, it is dependent on the wave conditions.
Concerning the comparison of dimensionless overtopping rate over Xbloc+ armour between test results and empirical prediction, it was found that the present formulae does not give a good estimation due its simplicity. In order to improve the representation of some parameters such as slope angle and wave steepness, correction factors are introduced in the empirical formulae leading to a better fit between prediction and measured data. 
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The static stability of stones on mild slopes under wave attack is investigated in this research. The first part of the research is focused on reproducing the physical scale model tests regarding profile change of Kramer (2016) numerically with the model XBeach-G. The erosion profiles modelled with the bed-load transport formulas of Nielsen (2006) and Van Rijn (2007) in XBeach-G do not match the erosion profiles of the profile change experiments of Kramer (2016). The bed-load transport formulas of Nielsen (2006) and Van Rijn (2007) are not able to model the sediment transport in XBeach-G accurately. Furthermore, XBeach-G cannot determine the velocity and acceleration near the bed, because the model solves the flow due to currents and waves for a single layer. Therefore, it can be concluded that XBeach-G should not be used to describe static stability of stones on mild slopes under wave attack. For the application of dynamically stable structures (which is not investigated in this research), XBeach-G functions satisfactorily (Postma, 2016). For further research, a model that solves the hydrodynamics for multiple layers should be applied. In this way, the hydrodynamics near the bed can be used to describe the static stability of stones.

The aim of the second part of the research is to develop a design method that describes the static stability of stones on mild slopes under wave attack. The basis of this design method is the initiation of motion of a stone and the hydrodynamic forces that initiate this movement. The hydrodynamic forces and corresponding mobility parameters are determined with the velocity and the acceleration near the bottom. Using Bubble Image Velocimetry (BIV), the velocity and the acceleration are derived from the videos of the BIV experiments of Kramer (2016) with regular waves breaking on a slope. It is found from the results of the BIV analysis that the effective, adapted Shields parameter θ’McCall can be used to describe movements of stones on mild slopes under wave attack. This mobility parameter has been determined with the bed shear stress of McCall (2015), which added an inertia term to include the influence of accelerations. For initiation of motion of stones, it appears that the stability parameter θcr could be a value of 0.024 (in case no slope correction factor has been applied). To substantiate a design method that describes the static stability of stones on mild slopes under wave attack, the value of 0.024 could be used to define a threshold for initiation of motion of stones. More experiments need to be executed to optimize this value of the stability parameter. Moreover, a statistical value for the stability parameter could be used (like θcr,1%) to describe the static stability of stones by means of a certain number of stones that are allowed to move for a certain number of waves. ... Read more

The European energy system is significantly changing to restrict the rise in global average warming. There is a shift from fossil fuels to renewable energy sources, which resulted in an exponential growth of offshore wind energy capacity in the North Sea. However, the North Sea is already one of the most intensively used sea basins in the world and space is limited. Offshore wind farms are therefore increasingly constructed under less favourable conditions. On the one hand the industry is rapidly developing and the cost of offshore wind energy goes down, but on the other hand offshore wind farms are constructed at greater water depths and further from shore. This latter development counteracts the cost reduction achieved by the industry, but countries are forced to construct offshore wind farms under less favourable conditions in order to achieve the climate agreements.

In June 2016, Dutch transmission system operator TenneT proposed the Hub and Spoke concept. It is an alternative method to connect offshore wind farms with the onshore grid and makes the parameter distance to shore far less important. Basically, the concept combines offshore wind farms with interconnector cables. An artificial island is created somewhere in the centre of the North Sea which is surrounded by offshore wind farms, thereby being far away from the crowded coastlines while profiting from near shore conditions. The interconnector cables subsequently transport the generated electricity to shore.

Dogger Bank is the envisioned location for the Hub and Spoke concept. The area contains strong winds, shallow water conditions and is centrally located. However, the unique characteristics of the shallow sandbank make it also very favourable to a variety of species. The area is therefore appointed Natura 2000 territory and could cause major resistance from environmental organisations. The general consensus amongst environmental organisations is not yet known, but industry experts indicate this as the major reason which could stop the Hub and Spoke concept from being realised on Dogger Bank.

The objective of this research is to determine the optimal location for the Hub and Spoke concept in the North Sea. The concept feasibility has never been determined and it is not even certain if Dogger Bank is the most beneficial location, while the location contains a very large risk. Therefore, the site selection model is created. The model contains input data from metocean conditions and electricity markets, which makes it possible to calculate the feasibility of the Hub and Spoke concept for every location in the North Sea. The result is a contour plot with the net present value of the Hub and Spoke concept, which shows the optimal location in the North Sea.

The Hub and Spoke concept is divided into five components to determine the influence of certain conditions on cost or revenue and to compare the components between each other. The artificial island and offshore wind turbines are primarily influenced by water depth, while the subsea interconnector cost is mainly influenced by the cable capacity and length of the cable. The optimal location for the island and offshore wind farms with respect to cost is thus along the shallow coastlines or at Dogger Bank, while the optimal location for the subsea interconnector cables is determined by the minimum total cable length. This point is equal to the centre of gravity of the countries surrounding the North Sea and is located just above the Netherlands and Germany.

In terms of revenue, the wind conditions are normative for the production and revenue of offshore wind farms and interconnectors derive their congestion rents from electricity price differences between countries. Offshore wind farms generate most electricity in the northeastern part of the North Sea where the wind conditions are strongest. This negatively impacts the generated revenue from subsea interconnectors as the cable is used more often for offshore wind energy. Interconnector revenue of the Hub and Spoke concept is therefore lowest at locations with the strongest wind speed, but in total the highest revenue is generated. A comparison between these components for four various locations is presented in Figure 1.

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Offshore wind farms are both in cost and revenue the major component in the Hub and Spoke concept. The optimal location is thus expected to be found in areas with shallow water depths and high wind speeds. Finally, each component with its cost and revenue has been determined and can be merged in the site selection model to determine the feasibility of the Hub and Spoke concept in the North Sea. See Figure 2.

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It can be concluded that the highest net present value for the Hub and Spoke concept is situated along the coastline of Denmark and Germany. However, these locations are known to be very crowded and the reason to investigate alternative locations in the first place. Dogger Bank and the northwest of Denmark are two very interesting alternatives. Dogger Bank is very shallow with depths of 10 m, while the northwest of Denmark is slightly deeper with 20 m but contains roughly 10% higher wind speeds. Both locations are primarily free from other marine uses, although Dogger Bank is protected Natura 2000 territory and therefore contains a very large risk. It is therefore advised to construct the Hub and Spoke concept at the northwest side of Denmark. ... Read more