In the past humans used to protect their shores mainly with rocks. In the past decades the shore protections did gently shift to concrete element protections. An example of an often applied concrete armour unit is the Xbloc. This element is quite strong and well investigated. BAM Infraconsult has developed a uniformly placed armor unit, the XblocPlus. The new XblocPlus is placed in a regular pattern, which is easier for contractors to construct. A design detail of a XblocPlus armor layer which still needs some attention is the transition from the slope to the crest. The elements are placed in horizontal rows, locked in place by two elements in the row above and two elements in the row below. However, the upper element is not supported by any element above it, leading to less interlocking. This study focusses on the physical processes which lead to (in)stability of the upper element and tries to increase this stability. First, physical laboratory tests were performed with the main aim to visually observe the failure methods of the upper XblocPlus element. After the initial tests, a computational fluid dynamic model was built with the aim to get a better insight in the load distribution on a single XblocPlus element under wave impact. To increase the stability of the element as much as possible, several increments to the stability of the upper element are proposed. The first method which increases the stability of the element is to cover the back of the element with rock, this changes the rotation point of the element backwards which increases the wave load required to initiate rocking. A second possibility is to face the top of the upper element downward, this does decrease the drag on the element. A third possibility is to face the top of the upper element upward, this leads to a higher drag on the upper element. However, this orientation makes it possible to bury the element in a rock backfill, increasing the weight of the element. This thesis did slightly research the applicability of CFD models to concrete armor design. The main conclusion for this trial is that it is possible to apply numerical models for the design of breakwaters, however physical model tests are still required to validate the obtained data from the numerical model, since the flow around a coastal structure is that complex, it only estimates the loads in the right order of magnitude when not validated. The validity of the model can be increased by calibrating the soil parameters using measured pressures in the different layers of the breakwater. To increase the accuracy of a numerical model, one could construct a three dimensional structure, which requires much computational grid cells, making the computational demand of the system quite high.

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 (Sop) 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 (Hm0) and wave period (Tm-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. 

In recent years, the use of Xbloc units has increased exponentially. However, the placement of this unit is not always done as randomly as it should be and consequently, the stability of the armor is affected. In order to overcome this problem, Delta Marine Consultants is developing a new armor unit called Xbloc+ that has a regular placement. In this research, the hydraulic performance of version 1 and 2 of this block are analyzed. Small scale tests were performed in a 2D wave flume in order to analyze the damage, rocking and the (partially and fully) displacement of units. In total, 1 series of tests were performed with Xbloc+v1 and 6 series with Xbloc+v2. To analyze the influence of the wave steepness and the slope angle, three wave steepness were tested (Sop = 2%, 4% and 6%) and tests were conducted 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 armor slope was reached in order to completely define the failure mechanism. Furthermore, tests after failure where also executed to further investigate the stability of the armor after the damage has started. Results obtained from the laboratory tests provided an overall understanding of how the Xbloc+ performs under certain conditions. It was perceived that the permeability of the armor layer is low as it happens often with single layer units. Thus, the pressure gradient between the underlayer and armor layer is significantly high creating an uplift pressure that leads to a revetment-like failure mechanism. Although the failure mechanism can be related to both slopes used during the laboratory tests, (3:4 and 1:2), the behavior of the armor layer differed completely between slopes. On a steeper slope, the armor layer remained undamaged for wave heights significantly higher than the design wave. However, once one unit was fully displaced, the damage was quite destructive. In contrast, on a milder slope, failure occurred much faster but the damage was not as aggressive. Moreover, after the failure was reached, the structure gained a new level of stability in which remained to provide shelter without reflecting significant damage. Furthermore, the wave height variation did not have much influence as the wave steepness. There was a noticeable difference between the performance of the structure during swell and wind waves. During swell waves, it could be seen that not only failure was achieved faster but it caused much more damage to the structure, while during wind waves the structure had a higher stability.