The Influence of the Wave Height Distribution on the Stability of Interlocking Single Layer Armour Units

This paper presents the results of a physical model study on the influence of the wave height distribution on the stability of single layer armour units according to rocking. A 2-dimensional scale model of a rubble mound breakwater with an armour layer consisting of Xbloc armour units was tested. The tests resulted in a rocking probability – wave height relation. Based on this relation and the acceptance criterion for rocking a stability formula is proposed for Xbloc.

The stability of rubble mound breakwaters is commonly deduced to the significant wave height (reference is made for example to the stability formula of van der Meer, 1988) although damage is mostly induced by the largest waves in a storm. In coastal engineering applications it is commonly assumed that the largest waves in a storm are about 1.8 times larger than the significant wave height (Goda, 2000). In the breaker zone the ratio of maximum wave height and significant wave height is further reduced (Battjes and Groenendijk, 2000). In offshore engineering design waves that are 2 times larger than the significant wave height are commonly applied (see for example DNV-OS-J101, 2007); freak waves (H_max/H_s > 2) may be also considered (DNV-RP-C205, 2007). Such exceptional large waves may have an adverse effect on the stability of rubble mound breakwaters in deeper water, that is not addressed in common design formulae.