Wave Grouping and Spectral Shape Effects on the Stability of Rubble Mound Breakwaters

The influences of wave groups and spectral shape on the stability of rubble mound breakwaters have been investigated by several researchers up to now. But, results were not conclusive in these researches, where different wave grouping and spectral shape parameters were used. This study aims to find an answer to that question by means of hydraulic model experiment. According to the result of the experiments, the damage to breakwater armour layer is almost same for different spectrum shapes and pronounced wave grouping, under the condition of similar wave statistics. Experiments also indicate that the wave trains with same significant wave height, H_1/3, but with different extreme waves whose heights are higher than H_1/3 cause different damage levels. The higher the extreme waves are, the more destructive the wave train is. Therefore it is concluded that occurrence of high extreme waves in a wave train is more important point than grouping and spectral shape in the stability of rubble mound breakwaters. It is also noted that the spectral shape indirectly affects the stability not due to the wave grouping but due to the extreme waves in a wave train since the occurrence probability of the high extreme waves becomes higher as the spectral shape becomes narrower under same significant wave height condition.