A description is given of a model to evaluate the fraction of waves which, in a sea state, break directly onto the vertical face structures, or sfightly earlier, generating severe hydrodynamic actions over the walls. The model moves from the breaking criterion proposed by Calabrese in 1998, which allows to estimate the percentage of breaking waves, as the number of waves which, according to the Rayleigh distribution, would have been larger than a threshold wave height Hb,- This portion of waves represents an upper limit for the number of impacts, including the waves which, collapsing before reaching the wall, arrive already broken at the structure. In this work a second wave height threshold Hbro is proposed, which appear effective in describing the transition from the impact to the broken mode. This permits to derive the impact waves percentage, simply removing the broken waves from the number predicted by the Calabrese's breaking criterion.
Vertical and composite breakwaters are frequently employed for the protection of harbours areas around the world and, in Europe, most of them have been constructed along the Italian coasts. Although these structures have been used for many decades, some of them suffered damages by storms, that can be considered catastrophic as the major failures of vertical breakwaters cost 2–3 times more to rebuild than the original construction costs. The reasons leading to those failures are very complex, related to different aspects of hydrodynamic, geotechnieal, morphological and structural nature, but undoubtedly the impulsive actions due to breaking waves represented one of the most relevant damage source even in the cases where depth - induced breaking was not expected (Oumeraci, 1994). A better understanding of the mechanisms governing the impulsive wave breaking pressures occurrence, appears then as an urgent research need for the improvement of vertical-face breakwaters design practice.
