Armour layer stability on a bermed slope breakwater

Abstract

In coastal regions where the land is sensitive to erosion, but also for harbours and ports, numerous types of defensive structures have been developed to protect it from the effects of incident waves. More recently breakwaters have been constructed which combine the stability characteristics of a conventional two-layered uniform sloped breakwater with the wave energy dispersive character of a berm breakwater. This type is referred to as a 'breakwater with a bermed slope' or a 'bermed sloped breakwater' In order to develop more insight in the development of damage on bermed sloped breakwaters, small-scale experiments have been performed in which the two most important parameters related to a berm were tested. These governing parameters were the relative berm length and the relative water level with a range of respectively 0.00 < B/Lm-1,0 < 0.35 and -0.8 < Rc/Hm0 < 0.7. It is concluded that the development of damage on a bermed slope has a similar, but more stable, trajectory as predicted for a uniform slope with the stability formula of Van Gent (2003). The increase in stability can be indicated with a constant factor rD. Also the damage level parameters for 'start of damage' and 'failure' on a bermed slope are independent of the governing parameters and correspond to the values of a uniform sloped breakwater. The influence of the governing parameters on the stability of the lower slope for 'start of damage' is shown in Figure 1. Conceivable trendlines are drawn for constant values of Rc/Hm0 and B/Lm-1,0. It can be concluded that when regarding constant values of Rc/Hm0 the test results show, for initial values of the relative berm length, an increase in stability as B/Lm-1,0 increases. As the relative berm length gets increasingly larger, the increase in stability indicates a horizontal limit. Also the range for which the relative berm length has a positive contribution on the increase in stability is strongly related to the relative water level. For Rc/Hm0 < 0 this range is small but it widens quickly as the water level approaches the berm level. As the water level on the berm increases, it gradually becomes smaller again. A research by WL|Delft Hydraulics (Vermeer (1986)), which performed similar tests but for Rc/Hm0 > 0.9, showed different results. The development of the increase in stability for constant values of the relative berm length showed a peak at B/L0 = 0.15. This peak seemed to flatten out as the water level on the berm became smaller. This process could possible link the findings of this study to the findings of Vermeer (1986), however, more research has to be done to confirm this hypothesis. Finally the two design principles, which in practice are used as indication of the increase in stability, were validated with the results of this study. The first principle applies stability formulae for uniform slopes on the average slope of a bermed profile. The second principle adopts the characteristics of low-crested structures on the bermed profile. The correlation between the predicted increase in stability and the results of the test series was very low. Apparently the complexity of the processes related to the (in)stability of armour layers on a bermed slope can not be overcome by means of the design principles. This is most probably caused by the influence of the return current, which has large impact on the stability, is not accounted for. Therefore both principles are not well suited to predict the increase in stability of armour layers on bermed slopes.