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Studies on the stability of the amour layer (d’Angremond et. al. [1999] revealed the importance of density of placement. The current research focuses on the influence of the density of placement on the stability of cubes in a double armour layer and tetrapods and rocks in a single armour layer. The experiments were performed in the Laboratory of Fluid Mechanics of the Faculty of Civil Engineering and Geosciences at the Technical University Delft. A model of a breakwater was constructed in a wave flume. An increase in density of placement resulted in all cases in an increase of stability, except in the case of cubes. Cubes have the tendency to start behaving like a placed block revetment including the characteristic failure mechanisms like uplift and sliding. Tetrapods seem to be unsuitable for single layer armour layers due to the fact that the filter layer is easily attacked by the waves even when no tetrapod has been removed. Experiments on rock showed that vertically placed elements lead to a much more stable construction due to their self-repairing ability. Characteristic for rock is the piling up of elements under the waterline caused by the impact of collapsing waves. This lead to very low densities of placement higher on the slope. Existing damage criteria are not suitable for density of placement. In this research an effort has been done to create a damage criteria, which considers area of attack, density of placement and different failure mechanisms.

Physical model tests were done on an armour of Tetrapods, placed in a single layer. The objective of the investigations was to study the stability of the secondary layer, and to see if the material of this secondary layer could be washed out through the single layer of Tetrapods. It was concluded that secondary armour is not washed out through an undamaged layer of Tetrapods, and that all damage to the secondary layer is related to damage in the primary layer. To prevent damage to the Tetrapod layer, a high placing density is needed. In case of low placing density, the area around the waterline will slide down, creating gaps in the main armour and exposing the secondary layer. Because of this process the placing density in lower sections of the breakwater increases, and consequently also the strength increases at those places.

In a classical design approach to breakwaters a design wave height is determined, and filled in into a design formula. Some undefined safety is added. In the method using partial safety coefficients (as developed by PIANC [1992] and recently also adopted by the Coastal Engineering Manual of the US Army Corps of Engineers [CEM [2003]) the degree of safety is formalised, and safety factors are given. However, because this method is still rather complicated, a Monte Carlo probabilistic approach allows the designer more flexibility and is also able to use predefined safety values.

In case of coasts with steep foreshores coastal structures suffer more from damage than normally could be expected from given boundary conditions at deep water. For that reason in many guidelines it is recommended to apply a heavier class of rock in those cases; manufacturers of single layer units (like Accropode, Core-loc or Xbloc) recommend a lower Kd value in case of a steep foreshore. Unfortunately until recently there was no insight in the physical processes leading to this extra load. In the stability formula of VAN DER MEER [1988] shallow water and steep foreshores are not considered.

Experience has shown that stability parameters for breakwater armour are different for steep and shallow foreshores, as well as for deep and shallow water. A change from a deep water spectral information (Tm0) to shallow water spectral information (Tm-1,0) does not completely explains this difference. Tests have shown that stability also depends on parameters not described by the spectrum at the toe of the breakwater. It is suggested that these parameters include for example the skewness of the waves.

In de Van der Meer formulas for armour stability the Notional Permeability is used as a parameter. Unfortunately the physical basis of this parameter is weak. It is therefore suggested to use a relation between the Notional Permeability P and the reduction of wave run-up due to infiltration into the breakwater. The advantage is that the latter can be computed with VOF models. This makes is possible to estimate the value of P from mathematical models. Also the run-up reduction can be measured in a physical model, which has the advantage that physical tests for run-up are much faster to execute than models for armour stability.