New Stability Formula for Rubble Mound Armor Units of Composite Breakwaters

The composite breakwater is one of the typical structures of breakwaters. In composite breakwaters, the stability of the rubble mound foundation plays quite an important role as well as that of a caisson. The rubble mound is usually protected by armor units, large stones or concrete blocks. This is to prevent dispersion of the rubble stones and deformation of the mound due to the action of the waves. The stable mass of the rubble mound armor units of composite breakwaters is generally calculated using the stability formula based on Ns. As to the stability number Ns, Tanimoto et al. proposed a formula for the stability of rubble mound armor units based on the standing wave theory and Kimura et al. modified Tanimoto's formula especially for concrete armor units. In recent years, there is more attention paid to maritime structures with high rubble mounds from the viewpoint of harmony with the sea environment. In high rubble mounds, there is a high possibility that impact breaking wave action occurs on the mound. Tanimoto's formula can not be applied in this case because it is based on cases where the water depth on the mound is comparatively large. Although Kimura's formula covers a high rubble mound, it does not take impact breaking waves into consideration. In our study, the stability of concrete armor blocks covering the rubble mound was carefully examined in a two-dimensional model test. From these results, a method to calculate the necessary mass of armor blocks for composite breakwaters is newly proposed, in which the impact breaking waves is taken into account.