A description is given of a model for the prediction, in a sea state, of the onset and the fraction of impact waves on simple vertical and composite breakwaters. The starting of impacts is estimated by defining a local maximum wave height, Hbc. The percentage of breaking waves is based on the number of waves which, according to the Rayleigh distribution, would have been larger than Hbc. For the approximations introduced, the predicted fraction of breaking waves should be considered as an upper limit for the percentage of impacts.
Recent study research and case-history of the collapse of vertical breakwaters have revealed the destructive potential of impact loads due to breaking waves and confirmed that in a sea state a large number of impacts can be generated of such severity as to cause the failure of the breakwater-foundation system (Oumeraci, 1994, Takahashi et al., 1994, Oumeraci et al., 1995). This gave rise to studies aimed at improving the reliability of the design procedures of vertical breakwaters and further research work is in progress under the MAST 111 project on "Probabilistic design tools for vertical breakwaters" (PROVERBS). Result analyses of hydraulic model tests have stressed the importance of particular combinations of wave conditions, bottom slope, berm and vertical wall profile on the generation of impact loads and within PROVERBS a parametric decision map has been developed to provide an easy guidance to identify wave loading on the structure (Allsop et al., 1995, 1996 a, b; Calabrese et al., 1996; Kortenhaus and Oumeraci, 1998). Additionally Calabrese (1998) has proposed a breaking criterion, based on extensive hydraulic model tests with random waves and previous theoretical works by Oumeraci et al. (1993), which permit the identification of the loading case (quasi-static or impact) which is most likely to occur at the wall.
