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.

In this thesis, it is studied if the stability of a stone in a granular bed protection, can be predicted by the local output of a three-dimensional (3D) eddy resolving simulation technique.

In earlier studies regarding stone stability, Reynolds-Averaged Navier-Stokes (RANS) models are used to determine the loads on the bed. In the resulting stability formulas, depth-averaged flow parameters are used, and the loads caused by turbulent fluctuations are taken into account by the modelled turbulent kinetic energy k. A load caused by turbulent wall pressures is never explicitly taken into account before. With the use of a 3D eddy resolving modelling technique, turbulence can be resolved to a certain extent, by which local parameters can be used to determine the load on the bed. This may result in a more accurate prediction of stone stability, and a more economical design method for granular bed protections.

Due to the computational requirements needed for the most detailed eddy resolving modelling techniques, it is concluded that for the aim of assessing stone stability, Improved Delayed Detached Eddy Simulation (IDDES) is the most appropriate 3D eddy resolving modelling technique for now and the nearby future. This modelling technique is also applied in a study regarding the influence of tidal energy turbines in one of the gates of the Eastern Scheldt barrier. In this thesis, special attention is paid to develop a stability formula, which can be used to assess the stone stability in the highly turbulent flow region behind the Eastern Scheldt barrier, based on the output of these simulations (hereafter ”Eastern Scheldt case”).

In order to derive a new stability formula, IDDESs are made of the two long sill experiments of Jongeling et al. (2003). In these experiments, an accelerating flow region is present above the sill. At its downstream end, the flow is separating, causing a highly turbulent flow region behind the sill. Thereby, the dominant flow characteristics are similar to those at the Eastern Scheldt barrier. In both regions of the experiments, on top of the sill and in the area downstream of the sill, damages to the granular bed protection are measured.

A new stability formula is proposed. To avoid the new stability formula to be grid dependent, the wall shear stress and the pressure gradient are used to
represent the loads by drag and inertia respectively.

It appeared, that the proposed stability formula does not predict the number of measured stone movements well, for the entire modelled domain of the long sill experiments. Nevertheless, it is hypothesised, that the assumed pre-dominant load terms are right, but that the ratio between those load terms on top of the sill differs from the ratio between the load terms in the downstream area. Two different entrainment mechanisms are described, that may not be predicted accurately by the same stability formula.

With regard to the Eastern Scheldt case, the choice is made to derive a stability formula that is only valid for the entrainment mechanism in a highly turbulent flow region behind a sill of backward-facing step. The data, behind the point of separation in the long sill simulations, is used to derive this stability relation. It appeared that the best results are obtained for a stability formula that is similar to the stability relation proposed earlier, with a Cm:b-value of 1. This is in agreement with the hypothesised entrainment mechanisms for this region.

Finally, the proposed stability formula is applied to the Eastern Scheldt case. A firm conclusion about the exact influence of the tidal energy turbines on the granular bed protection, cannot be drawn based on this study. However, it can be concluded, that the influence on the stability of the stones seems to be insignificant. At the analysed locations, the loads on the bed even seem to be slightly reduced in the simulation with turbines, compared to the simulation without turbines.

At least as important, is the conclusion that IDDES potentially is an appropriate modelling technique to assess the stability of stones in a granular bed protection. For the long sill experiments, the measured flow characteristics are clearly reproduced more accurately when using IDDES, than by applying a RANS model with the same boundary conditions. The computational effort needed for the Eastern Scheldt case is comparable to the computational requirements of the long sill simulations. Nevertheless, in both cases, the effective grid resolution was not yet sufficient to resolve all fluctuations towards the size of 1dn50. Despite the given that the desired resolution is not yet reached in this thesis, the simulated velocity signals of the long sill experiments are in good agreement with the measured ones. The choice between the use of IDDES or a RANS model should depend on the available computational power, time and required accuracy.

In this research the hydrodynamic stability of a ballast filled mattress under waves is investigated. There is checked if this can be applied as scour protection around a suction bucket jacket: an offshore wind turbine foundation that uses a vacuum to suck itself in position. A ballast filled mattress is a mattress made from polyester with so called spacer threads that take up tensile forces when the mattress is trying to deform. The combination of stiffness and combined weight make the ballast filled mattress a possible new scour protection.

Coastal areas around the world have attracted settlements and human activities since the early stages of the history until nowadays. This has introduced continuous modifications to the natural characteristics of these coastal regions by means of coastal structures and engineering interventions. The design of such coastal structures has evolved significantly since the first quarter of the XXth Century, when more scientific design methods and formulae were developed. Nevertheless, further research is required given the stochastic nature of the environmental loads involved, the remaining uncertainties regarding the response of these structures to the applied loads and the growing impacts of climate change and sea level rise. Four knowledge gaps are identified regarding the “Damage assessment of coastal structures in climate change adaptation”. Based on these, the objectives of this thesis are summarized as follows. First, climate change adaptation: demand for validated upgrading alternatives. Second, damage characterization concepts: demand for unified damage characterization concepts. Third, damage characterization parameters: demand for universal and more accurate damage characterization parameters. Fourth, damage characterization measuring techniques: demand for validating the suitability of innovative survey methods. These knowledge gaps are addressed using physical modelling results from two test campaigns (UPorto deep water and Deltares shallow water tests. In consequence, this study includes the validation of the damage criteria required for a precise assessment of a coastal structure (second knowledge gap), the validation of an universal damage parameter for rubble mound structures (third knowledge gap), the validation of the benefits of innovative measuring techniques when carrying out physical modelling tests (fourth knowledge gap) and the validation of upgrading alternatives for climate change scenarios (first knowledge gap). Thus, it can be stated that with these definitions, parameters and measuring techniques, a complete method for damage characterization of coastal structures is presented. It was also defined how this damage characterization method can be used to precisely and accurately describe the damage to conventional and non-conventional coastal structures. Furthermore, this method was also used to describe the effects of not only current environmental forces acting on the structures but also future and more energetic scenarios. For such future scenarios, adaptation alternatives for coastal structures were evaluated and berm configurations are recommended for their upgrading. Future research is needed in order to evaluate, adjust and generalize the conclusions made in this thesis, considering additional structure configurations and environmental loading conditions.