The purpose of this MSc thesis is to do an experimental study into the spatial and temporal variation of rocking armour units. Armour units on a breakwater slope under wave loading can sometimes start to move back and forth, this phenomenon is known as rocking. Rocking can lead to significant impacts between armour units, which can result in breakage. This is especially important for single layer armour units, like the Xbloc. The development of the smart Xbloc makes it possible to measure accelerations and angular velocity with a stand alone sensors at a sampling frequency of around 100 Hz. The current literature does not provide the spatial distribution of the number of impacts and the impact velocities due to rocking. Furthermore, only limited knowledge is available on the distribution in time. The research aim of this thesis is: Determining the spatial and temporal distribution of the number of moving armour units, the number of impacts and the impact velocity of rocking armour units. To achieve this aim a physical scale model was set up and model tests have been performed with 10 smart Xbloc units to measure rocking. The collected data, analysis and results provide a unique look into the behaviour of single layer armour units. The results can be used to validate rocking models and provide valuable statistical information on the number of impacts and the impact velocities.

The static stability of stones on mild slopes under wave attack is investigated by this research. First it was checked whether there are any numerical models that could be used to describe the stability of mild slopes. As such, there is no existing model at this moment that can accurately model the stability of mild slopes. To model this correctly, the model has to be able to model plunging and spilling waves, include bed sediment transport and to model the velocity and acceleration due to a breaking wave over multiple layers in depth. The existing models do comply mostly with one of these specifications. To get accurate results, the stability should be modelled with all of the specifications and not one model complied with this. Therefore, it was decided to investigate the stability of stones on mild slopes under wave attack with physical modelling tests at the research institute Deltares. The physical tests were executed with a constant slope angle of tan α = 1:10 and a constant nominal median stone diameter. The wave steepness (that determines the type of breaking for a constant slope angle), the significant wave height, the number of waves and the layer thickness are varied in between the tests to determine the influences of each parameter on the stability. Of each test an erosion profile is made by photogrammetry from which the profile-related damage parameters can be determined. In the bed strips with coloured stones are laid down to follow the transport of the stones. According to these erosion profiles three damage parameters were determined. The damage level, S, as used by Van der Meer (1988), the erosion depth, de, and the damage depth, E3. The damage depth, E3, is used to describe the damage for mild slopes. The range of the damage depth for start of damage to failure is from 0.5 to 2.3. These values are only applicable for a layer thickness of 2.5dn50 and a slope angle of tan α = 1:10. The development of the damage depth is investigated for multiple parameters. The breaker type gives a change in development of the damage depth. For a wave condition with plunging breakers the damage depth development is higher per increasing wave height than for a wave condition that includes both plunging and spilling breakers. For the transition zone from plunging to spilling breakers the damage depth increase is very small if the wave height increases. The damage depth is influenced by the number of waves, because after about 11,000 waves the development of the damage depth is still linearly increasing with the number of waves. The damage depth was not stabilized towards a maximum value after this number of waves. It can be concluded that the transport of the stones is mainly all below the still water level. This also followed from the damage zones. These indicated that the total area of the damage is in between the -2Hs and SWL. The maximum area location is around -0.8Hs w.r.t. the SWL. The transport is mostly upslope, which is attributed to the mechanism whereby wave forces are strong in upslope direction. The location where the most stones are picked up depend on the breaker type, the wave height and the layer thickness. Plunging breakers cause damage more downslope than spilling breakers. Plunging breakers cause more local damage and have a much smaller runup than the spilling breakers. Per increasing wave height, the location of damage is shifted more downslope due to the increase in breaker depth.

Until now, an extensive design method for mild slopes has not been available. The aim of this thesis is to understand the stability of rock on mild slopes under wave attack for impermeable cores in order to optimize designs. Physical model tests have been executed to study this stability for a 1:8 slope. Mossinkoff (2019) executed physical model tests for a 1:10 slope and a re-analysis of these tests is included in this thesis as well. Damage caused by entrained rocks is quantified by damage parameters using stereophotogrammetry and coloured rocks in strips. In this study, the influence of several hydraulic and structural parameters on damage parameters has been investigated for mild slopes. A positive correlation has been found between the significant wave height and the damage parameters. Besides this, based on the analysis it can be concluded that the wave steepness and damage parameters are negatively correlated. An increase in layer thickness of the rocks does not seem to increase the stability of rock on mild slopes. However, the slope angle does have an effect on the stability as a milder slope is associated with less damage. Another conclusion is that more damage continues to be observed even after 15 000 waves. Based on the results of the physical model tests, a design formula is developed for mild slopes to be able to increase efficiency in designs of coastal structures for these mild slopes. This study also provides evidence that rocks on mild slopes have different characteristics of damage and damage development compared to steep slopes. The largest share of entrained rocks transport in upward direction and rocks on mild slopes seem to be more mobile compared to steep slopes. This suggested that it might be more efficient to study the moment when the filter layer or the core becomes visible instead of the static stability of rock within the armour layer itself.