Investigation of crown wall stability on top of rubble mound structures with OpenFOAM®

Abstract

In the present study the stability of crown wall elements on top of a rubble mound breakwaters is investigated. The first step was conducting a literature review, in order to identify knowledge gaps. It was found that current design methods do not take the freeboard of the crown wall into account when calculating the vertical force acting on it. Recent research has established that when the freeboard is increased, this force is reduced while the portion of the crown wall base that is wet is reduced as well. However, there have been different approaches to increasing the base freeboard, that could lead to different effects on the loading. Moreover, parametric investigations regarding the vertical force have been very limited. Furthermore, the current design formulas assume that the maximum horizontal and vertical forces occur simultaneously but research suggests that there is a time lag between the two.
Based on these knowledge gaps, the study goals were defined. In order to achieve these goals numerical model simulations were prepared and ran, where the freeboard was varied with two different approaches, as well as simulations with varied breakwater slope. The selected CFD numerical model is OpenFOAM® making use of the waves2Foam toolbox, implementing the volume of fluid (VOF) method.
It is found that the currently used empirical methods fail to predict the changes in loading for increasing freeboard. For an increasing base freeboard, less part of the base becomes wet and that the vertical force, as well as the critical weight are reduced. On the other hand, the horizontal force increased. It is concluded that when increasing the base freeboard by means of lowering the water level results in lower loading compared to an increase of freeboard by elevating the crown wall element. Additionally, for the latter approach, a larger portion of the base slab is wet. Also, it was confirmed that a recently proposed reduction coefficient by (Bekker et al., 2018) for calculating uplift pressures can provides more accurate results in the case of freeboard increase.
Examining the uplift pressure distributions, it was found that for a zero base freeboard the pressure distribution follows an S-shaped profile, which with increasing base freeboard reverses. The peak pressure is located slightly inwards instead of the seaward end of the base, followed by a reverse peak. In order to propose a generally applicable profile shape more data are required.
The results indicated the presence of a time lag between the maximum horizontal and vertical forces. This time lag results in lower critical loading on the structure than when assuming simultaneous maxima. Nonetheless, further research is necessary, as these findings are a result of only one wave condition.
A finding which contradicts the predictions made with empirical methods is that gentler breakwater slopes resulted in higher loading. This is considered to be a result of an increased internal set-up for gentler slopes.
Further research is recommended, especially on tests with varying wave conditions and geometries, which should make these conclusions more generally applicable.