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.

Currently, there is no infrastructure between the two Indonesian islands Flores and Adonara. The islands are separated by the Larantuka Strait, which has a width varying between 600 and 1000 meters. The local government would like a bridge between the two islands. However, as the water in the 18 meter deep strait is heavily subjected to tidal forces - creating a tidal amplitude of about 1.5 m and tidal flow velocities ranging up to 4.5 m/s - structural design is challenging. As traditional bridges were deemed too expensive, a new type of bridge was introduced: the `Tidal Bridge'. The pendulum founded floating bridge, which is proposed to span the deepest 400 meters of the cross-section, is designed with tidal turbines attached to the bottom of the structure. The energy production mitigates the financial burden that the 225 million US dollar Palmerah Tidal Bridge will bring. At the time of writing, a pre-feasibility study has been performed by Antea, proposing initial structural dimensions. BAM took over the design process, which lead to questions regarding the dynamic stability of the design. The objective of this report is to answer the following two research questions: 1. How can the dynamic response due to two-dimensional forcing of a Tidal Bridge be determined? 2. What design choices can further optimise the dynamic behaviour of a Tidal Bridge? A numeric tool has been created to predict the dynamics of the proposed design as function of an input of wind, waves, and current. Based on (experimental) literature, hydrodynamic coefficients determining the fluid-structure interaction were determined. The complex shape of the floaters did not allow appropriate validation of the accompanying added mass and radiation damping coefficients. Comparison with a model constructed in Ansys Aqwa showed values of similar magnitude, but precise magnitudes could not be determined. In order to find these important coefficients, a set of experiments has been performed to determine the added mass (moment of inertia) and radiation damping (moment of inertia) for heave and roll motion. The experiments showed that the added mass equations for heave were well defined, while the added mass moment of inertia equations for roll motion deviated up to 250 per cent. The acquired data was used to find better relations between the added mass (moment of inertia) and the floater dimensions. Please note, the empirical relations are based on limited data and with little mathematical background. Hence, the relations should be used with care. The radiation damping coefficients that were also extracted from the experiments showed no clear trend, but did confirm that the hand-calculations were of correct magnitude. Using the constructed model, forcing characteristics of the Tidal Bridges are investigated. In these computations, the Palmerah Tidal Bridge dimensions are used as a case study. It was noted that the extreme non-linearity of different elements of the problem (changing pendulum angle, hydrodynamic pressure field, and particle velocity/acceleration field) do not allow for linear approximation. While a linear mass-spring system predicts that the natural period is about 6.7 seconds, the maximum dynamic amplitude is found for wave periods of 9 seconds. This coincides with the largest expected waves for the Palmerah Tidal Bridge location. Reducing the natural period of the design is recommended. Additionally, research was done into the contribution of the various types of forcing, where it was found that traffic weight has negligible effect on the dynamics. Wind forces add only a few percent to the pendulum forces, but do have a significant contribution to the displacements. Furthermore, research into the impact of an approaching wave field showed that accelerations during first impact may overshoot the maximum steady state acceleration by more than 200 per cent. A parametric study on the dynamics of Tidal Bridges was performed, which did research into the the forcing combinations that lead to most amplification of the dynamics. It shows that different loading combinations are governing for accelerations in the three different degrees of freedom present in a two-dimensional system. In here, difference was found for the dynamic behaviour induced by waves from the two different wave directions, leading to two sets of three forcing combinations. This data was used to investigate the effect of three design parameters: the angle of the pendulum, the hinge location of the pendulum and the depth of the strait. Moreover, a sensitivity study is performed on a set of parameters defining the Tidal Bridge. It shows that the mass of the segment, the added mass, the floater length, the design turbine force, and the pendulum angle are most important if it comes to design optimisation. Based on the parametric study and the forcing characteristics, conclusions and recommendations are made for improvements of Tidal Bridge designs in general and more specifically: the Palmerah Tidal Bridge.

In the Meuse seven weirs are located in the Dutch reaches, controlling the water level to enable inland navigation through the river. The weirs are being scheduled for replacement, where weir Grave is the first one in 2028. They are reaching their end-of-life time. One of the main issues that became of more importance in the recent years is ship collision. In the past 20 years two major ship collisions happened on two different existing weirs in the Meuse, one at Grave and one at Linne. The place of impact at the weirs was significantly damaged after those collisions. As a consequence, the water level dropped and inland navigation was not possible for one month. In this study an inflatable weir is proposed as replacement of the existing steel weirs in the Meuse. A conceptual inflatable weir design is made for location Grave, for replacement of the existing weir. The design is based on existing literature, such as the inflatable storm surge barrier Ramspol. The design for Grave is considered to be scalable to the overflow (Poirée) parts of the Meuse weirs. One of the aspects that has not yet been considered for those inflatable weirs is ship collision. The theory and formulas found for the existing collision analysis are not fully applicable to the inflatable weir, mainly due to large elastic deformation of the inflatable weir. In literature a standard expression has been found to quantify the ship force on the colliding structure. This expression forms the basis of the analytical model. The inflatable weir in the analytical model is schematized by a two-dimensional plate sheet. An effort was made to validate the strain found in the analytical model by a numerical model in Ansys. However numerical instabilities were found that lead in considerable modification of the desired model and so the results indicated no representative outcomes. To see what happens during the ship collision a physical scale model was made, with scale 1:25 for accurate representation of the physical phenomena. Sixteen experiments were done with four different draughts and four different velocities of the ship. For the experiment with the scaled maximum draught (0.14m) and velocity (1.1m/s). The full video experiments are uploaded to the 4TU-datacentrum (https://data.4tu.nl/portal). The two aspects uplift of the ship and gliding over the weir observed in the experiments are not yet included in the analytical model, therefore the analytical model is extended. The extended analytical model showed a 25% deviation with the uplift of the ship and a 15% deviation with the displacement of the weir from the experiments. With the extended model it was calculated that the limit strain is not exceeded and that the strain is maximum 5% on top of the static strain of 1.9%. Concluding, the first steps have been taken into research of ship collision on inflatable weirs. Further investigation on the ship with V-bow, the propeller of the ship and a more extensive numerical model is recommended for ship collision on inflatable weirs.

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

XblocPlus is a new uniformly placed single layer armour unit, developed by Delta Marine Consultants (BAM Infraconsult). This report focuses on the stability of the first row of armour units, as this row is interlocked less than other rows. When the first row starts to slide, this is a big threat to the stability of the entire armour layer. Furthermore, not much is known what exactly influences the stability of the first row. In this study physical modelling is used to determine the stability the first row of an XblocPlus armour layer and investigate what influences this stability. The results of the tests were analysed with a method called Digital Displacement Analysis. Two simplistic theoretical models were devised. The first model considers the destabilizing forces to consist only of the drag and uplift forces caused by the wave on the armour units of the first row itself, this is called direct hydraulic loading. The second model also considers destabilizing forces transferred by the armour layer to the first row, this is called indirect hydraulic loading. From the model tests, it was found that indirect hydraulic loading plays an important role, as the movement clearly occurred during down-rush. When waves started to plunge, the waves failed to destroy the breakwater. Furthermore, loose armour units placed in front of the first row moved at higher heights than the first row, indicating that the armour layer pushes out the first row. Water depth influences stability, as lower water depth gave a lower stability. It is feasible that this effect is caused by the down rush being closer to the first row. The second model is most appropriate when taking these findings into account. From the tests it was found that a toe berm increases stability as it adds weight to the first row and thus increases friction resistance. A foundation layer also increases stability as friction between the foundation layer and the first row is higher than between the first row and the concrete slab that was used in other tests. Both measures also increase the damage threshold. The damage threshold is the amount of damage that a first row can take before brittle failure occurs. Brittle failure is an increase in damage of 0.2Dn (Dn is the nominal armour unit diameter). When both measures are used together, the foundation layer makes the toe berm more stable and this is why the stability of the first row is also increased. Based on the tests in this study the stability of the first row can be stated as Ns =3.87 (Ns=Hs/(delta*Dn), the stability number is defined as the significant wave height divided by the nominal diameter of the armour units and the relative density of the armour units) as at the end of the tests the damage was only 0.13Dn. The test results were used to determine a working hypothesis for a design formula. More tests are necessary to validate this formula and determine its accuracy. A remarkable finding of this study is that two test series showed contradictory influences of wave steepness. The first test series showed that a lower wave steepness is detrimental for stability of the first row, while the second test series showed the opposite. More research should be conducted to investigate on this curiosity.

The XblocPlus 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 XblocPlus 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 XblocPlus. To prevent differences in stability due to the size of the under layer, the tolerances are expressed in the unit size (Dn) instead of the under layer grain size (Dn50). The resulting tolerances are maximum 0.25 Dn for the deviation from the design profile and maximum 0.1 Dn 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 Dn. 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.

The concrete armour unit industry exist for more than 70 years, in this period various units were developed. The first units derived their hydraulic stability on their own weight, later the units got more complex shapes and got interlocking capabilities. The first interlocking units were placed in a two-layer configuration, like Tetrapod and Dolos. Later the industry shifted to a one-layer design where concrete use became less since fewer units were needed. The industry standard of one-layer gave rise to various new armour units, like Core-Loc and Xbloc. Up to the present, this industry is mainly studied from a technological perspective where the hydraulic capabilities and design formulae are extensively researched (so called 'hard factors'). However, some units became very commercially successful in terms of units placed but others did not. In this highly technological industry, no market research is done in order to study which factors led to commercial success. In this study, an effort is made to investigate which factors are responsible for becoming successful in this branch. To do so, the concept of technological dominance is linked to this industry. Technological dominance is a technology management concept wherein a market similar technologies are ‘battling’ for the allegiance of the market. Eventually, the market selects a particular design and becomes dominant. Over the years scholars identified factors that contribute to obtaining technological dominance and collected those in a framework. In this study, the framework by Van de Kaa et al. (2011) is chosen. This framework consists of 29 factors that influence the outcome of a technology battle. The methodological approach for this study is to select relevant factors regarding this industry by interviewing 4 key experts. The identified relevant factors are then carried into the next round of interviews, where a broader group of 14 experts were asked to rank these factors according to the Best Worst Method. The Best Worst Method (BWM) is a multi-criteria decision-making tool, designed by Rezaei (2015), where the decision-maker expresses their preference by making pairwise comparisons. This method gives more reliable weights and requires fewer pairwise comparisons in respect to other multi-criteria decision-making tools, like AHP. The results yielded by this method suggest that the factor ‘brand reputation and credibility' are among experts considered as the most important factor for technological dominance regarding this industry. Furthermore, ‘hydraulic stability’ is in this branch considered second most important for obtaining technological dominance. Also, factors like 'marketing communications' and having a large network of stakeholders are considered very important. Based on the results, there can be concluded that in this technological branch soft factors can be more important than hard factors. This implies that in order to be successful in this industry actors should focus more on building a reliable and well-known brand reputation. In this branch, actors have proved that a strong brand name can help to extend the commercial lifespan of an armour unit with expired patent and can help to speed up the adoption of new armour units.

Over the past centuries, the Dutch rivers have been extensively engineered with the construction of hydraulic training works. A changing climate and increasing demands of river functionalities require innovations to the system to ensure its resilience. Exploring potential applications of Xstream elements might contribute to the development of sustainable river management strategies. The 33 cm high concrete three-dimensional cross is the little version of the Xbloc, a widely used breakwater element for coastal protection. Random arrangement of Xstream elements creates 60% porous spaces and the ability to build slopes at a 45∘ angle. Alterations to the riverbed morphology induced in the near-field of homogeneous Xstream hydraulic structures are assessed in the present study. A physical scale model featuring a movable bed of lightweight sediment was employed to simulate a river section. Minimising any compromises on Froude scaling, this material was used to properly scale the Shields parameter, striving dynamic similarity between the model and reality. Bed load transport processes are reasonably well represented by this material, providing well matched equilibrium scour depths and good qualitative comparisons of the influence of structural variations on these processes. Multiple setups differing in material and geometry were tested in a 12 m long and 2.6 m wide sediment recirculating flume. The primary comparisons of this study include: • the flexible groyne head in the model against field data; • two abutments with a 1:2.5 slope, varying in material between stone and Xstream elements; • two abutments made of Xstream elements, differing in slope between 1:1 and 1:2.5. Water levels were gauged and flow velocities were measured with acoustic Doppler velocimeters during the experiments. High-resolution bed elevation data was collected by means of a laser scanner. Generally, results of the model show an overestimation of bedform dimensions. This is attributed to the concessions necessitated by the limitations of the test facilities due to the low length scale factor. The effect of contraction was exaggerated in the model as the structure width to flow width ratio was 3 times greater and the influence of any upstream river training works was neglected. The flexible groyne head blocked 43% of the cross sectional flow area in the model. Furthermore, due to the laminar flow behaviour, the water encountered increased resistance within the structure. The deepest scour is found along the leading edge of the structures. This can be explained by the intricate flow patterns created by the primary vortex entering the flow acceleration. Inspired by the longitudinal training wall as built in the river Waal, a less porous stone abutment was constructed to compare the effect of porosity and roughness of Xstream elements on local sediment displacement under high water conditions. The increase in water level in front of the Xstream abutment was half that of the stone abutment and streamwise velocities in the main flow were 5% lower. These findings indicate better dissipation of energy by the Xstream abutment. Nevertheless, the relative turbulence level was 17% higher close to the Xstream abutment due to the higher roughness, resulting in equivalent peak velocities in both experiments. Along the Xstream abutment, however, the scour depth was twice as large. Taking into account the live-bed conditions, where sediment is supplied from upstream causing the scour depth to fluctuate around its equilibrium, a significant difference of at least 20% remained. The principle of a falling apron, a mechanism where individual units at the toe of a structure tumble down, covering one slope of the scour hole and thereby increasing the roughness, could be the reason for this. The erratic shaped Xstream elements enhance complex turbulence patterns very locally. Due to the absence of a supporting filter construction or a bed protection layer, the Xstream abutment was undermined, leading to individual elements decaying at the base of the structure. The interlocking ability of Xstream seems to be stronger when the structure is built with a steeper slope. This insight can be taken into account in the design of Xstream hydraulic structures. The substantial discrepancy in density between the Xstream model units and the angular polystyrene particles must be considered as the geotechnical properties may differ from the real world. The present study has shown the kinetic energy absorbing capacity of Xstream. Turbulence levels increase due to higher roughness. Exposing a uniform structure of Xstream elements to high water conditions might lead to instability which alters both positive and negative effects. Future research must give valuable insight into how the design of Xstream structures could look like for a durable implementation in the Dutch river system.

XblocPlus is a single layer armour unit that is recently developed by BAM Infraconsult. It is the successor of the Xbloc. An important advantage of the XblocPlus is the relatively high construction speed compared to the Xbloc. The increased construction speed is caused by the possibility to place XblocPlus in a uniform pattern and the relatively large surface area that can be covered with a small number of blocks. A disadvantage of XblocPlus is the limited block stability at the top armour rows of a low crested structure. The limited stability is caused by the reduced interlocking capacity of the blocks on the top armour row. In previously performed studies, a back support is placed behind the blocks to stabilise the top armour row on the front slope. The back support of a concrete element significantly improves the stability of these armour blocks. However, at normative wave conditions, the stability of the top armour row on the front slope is still insufficient to meet the target stability (Ns,c = 3.0). The stability of the top row on the rear slope is not investigated in other studies. Consequently, no conclusions can be drawn about the stability of the crest blocks on the rear slope. In this study, physical model tests in a wave flume were performed to assess the effect of crest modifications on the stability of the top armour rows on the front and rear slope. The main objective of this study is to design a stable breakwater crest that has the best hydraulic performance with respect to the economically most feasible breakwater dimensions. After an exploratory study into the meaning of the main objective, it can be concluded that a relative crest width of 1 is desirable. The relative crest width is determined as the ratio between the crest width (B) and the significant wave height Hs. The principle of previous studies is used to set a reference situation. This crest configuration contains a concrete element between the top armour rows that functions as a support. This is a rather wide structure with a relative crest width equal to 2.7. After testing the reference situation, the top row on the front slope turned out to be normative for the crest stability. The found stability at failure coincides with the stability found in previous studies. The blocks on the top row of the rear slope remained stable. Also after reduction of the crest width to a relative crest width of 1.9, the crest blocks on the rear slope remained stable while the crest blocks on the front slope failed. It can be concluded that the blocks on the top row of the front slope are normative for the crest stability. The weight of these blocks is too limited to meet the target stability. By application of modified XblocPlus blocks is attempted to obtain stable blocks on both top rows. The adjusted blocks are much bulkier and consequently heavier than the original XblocPlus blocks. The blocks shape is modified in such a way that the crest blocks on the front and rear slope can be placed back to back without the support of a crest element. As a result, the relative crest width is reduced to 1.1. Several variants of modified crest blocks are tested to acquire information about the stabilising effect of the applied modifications. Modified blocks with an increased permeability, an increased interlocking capacity and a relatively large resistance against rotational instability were tested. Overall, the bulky block shape, the block's permeability, and the interlocking generated by the almost vertical back of a block, appear to be very important for the crest stability. The acquired information of this first test series is used as a foundation for further optimisation of the permeable block. An attempt to optimise the modified block is done by reducing the volume at places that have the least impact on the block stability. Application of measures that streamline the block shape in flow direction and further increase the block’s permeability were applied in this optimisation phase. Finally, a block with a 44% larger volume than the original XblocPlus block is obtained that meets the target stability of Ns = 3.0.

The XblocPlus (referred to as Xbloc+) is a new, uniformly placed single layer armour unit, developed by Delta Marine Consultants (BAM Infraconsult). Although a breakwater armour layer with XblocPlus combines material saving and easy placement with increased stability, the transition between the armour layer and the crest is not adequately stable. The XblocPlus units of the top armour row (crest units) become easily displaced. In this research, several solutions to this problem are investigated through physical modelling, in order to determine the best one. Firstly, tests are performed on a breakwater with single, XblocPlus crest elements without rear support, in order to determine the most important parameters and mechanisms contributing to failure. The criterion for initiation of failure for the top armour row is 10%, so when 1 of the 10 crest units fails. Failure of a unit is defined as the condition where contact with the units of the row underneath is lost under at least one of the two wings. During wave run-up, under the forces resulting from the wave velocities, the XblocPlus crest units initially rotate and, subsequently, make a combined motion consisting of rotation, vertical and horizontal translation. The main parameters of influence to this movement are the crest freeboard (Rc/Dn) and wave steepness (sop): for Rc/Dn≥1.7, stability increases for increasing Rc/Dn and for increasing sop (from 2% to 4%). For zero freeboard, wave impact forces at breaking proved to be an important mechanism in the case of sop=4%, where collapsing breakers occurred. Failure and partial displacement were initiated at stability numbers (Ns=Hs/(ΔDn)) of 1.39 and 0.74, respectively, for Rc/Dn≤1.7, which are much lower than the stability number for displacement of the armour layer (Ns>3.88). A second set of tests was conducted, in order to investigate a way of optimising the stability of the transition area between the armour layer and the crest. Under the most critical conditions of the first tests, 7 crest configurations, based on their potential ability to resist the failure mechanisms, were investigated: 2 orientations of the Xbloc+ crest units (with the tail or nose tilted upwards), 2 different ways of placement of Xbloc at the crest, placement of Xbloc with a concrete crown wall element, underlayer rock material at the crest and underlayer rock material with a concrete crown wall element. From the above “trial and error” approach, it was concluded that the most effective way of increasing stability (optimised configuration) is the provision of a backwards support, which fulfils the criteria of no erosion and no uplift and resists rotation by bringing the rotation point of the Xbloc+ crest elements further to the back. The form of this concept tested was the placement of underlayer material between the Xbloc+ crest units and under their tails and a concrete element behind, which functions as backwards support to prevent erosion of the rock fill. No displacement of the Xbloc+ crest elements occurred for Rc/Dn≤1.7, 2%≤sop≤4% and no failure happened for Rc/Dn=0, sop=4% for a stability number up to 3.55. Rocking was decreased to zero. Failure and rocking of 10% happened only at the case of Rc/Dn=1.7, sop=2% for Ns≥2.78, due to the erosion of the fill, resulting from the uplift of the supporting crest element, with the final damage (at Ns=3.49) being repairable and limited at the 4 upper rows of the breakwater.

Twenty years after the introduction of Xbloc, DMC, which is a member of Royal BAM Group, introduced a uniformly placed and interlocking concrete armour unit named XblocPlus, which is suitable for straight to mildly curved breakwater sections, whereas Xbloc is currently applied on roundheads. Therefore, this study investigates modifications to the design or placement configuration of XblocPlus to enable its application on roundheads. Two concepts were tested on a H/V=4/3 roundhead slope for hydraulic stability in DMC’s wave flume. The first configuration consisted of regular XblocPlus combined with XP-Curve, a variant adjusted for narrowing radii, and XP-Base, a legged XblocPlus originally devised for bottom row applications. Tests with normative 0.02 wave steepness yielded a stability number (Hs/ΔDn) ranging from 2.72 to 3.09 due to uplift-related damage. The second configuration, featuring solely XP-Base and XP-Curve, resulted in a hydraulic stability number of 1.96 due to significantly less interlocking. No improvements were observed with respect to hydraulic stability compared to previous research. However, these designs notably reduced rotation-induced failure and enhanced applicability by enabling transition to the trunk and increasing maximum construction height. Additionally, this thesis investigated the validity of testing a three-dimensional structure in a 2D configuration, a method employed to maintain consistency with previous research. Wave heights in front of the structure were measured using parallel gauge sets, allowing the determination of the average disparity. This disparity was found to be within the acceptable margins for the principal test configuration. However, further research is needed to assess test conservativeness. In conclusion, this thesis provides recommendations aimed at improving the applicability and stability of XblocPlus on breakwater roundheads.