An accurate estimation of wave forces and pressures is of great importance to design breakwaters protecting harbors from large sea waves. Uni-directional waves are employed for the estimation in the present design standard manuals so far mainly because the difficulty of reproduction of directional random waves in experimenetal basins. Sea waves, however, have characteristics of multi-directional random waves especially in deep sea area. The characteristics of directional wave forces and pressures on a verticall wall which represents a simple wave barrier in deep sea area are experimentally investigated. The experimental results demonstrate that the wave pressures acting on the wall become smaller in multi-directional sea conditions. A simple numerical model has been proposed to evaluate the reduction rate of wave pressures by wave directionality.
Real sea waves have characteristics of directional random waves offshore. The directionality of sea waves becomes weak as waves propagate in shallow water area because the component wave direction is changed to the normal to shoreline by refraction in shallow water. Nowadays, the construction site of breakwater has tendency to become deep as the human activity area is spreading. Therefore, some breakwaters have been constructed in the deep water area with more than 60m depth (Tanimoto, etc., 1988). For the design of a breakwater, uni-directional waves are mainly employed to estimate the maximum wave pressures and breakwater stability. In order to carry out an accurate estimation of wave pressures and forces, the evaluation in directional sea conditions are inevitable. Recent development of directional random wave generators in several hydraulic laboratories (Mansard, etc.,1995) has made it possible to carry out the hydraulic experiment in directional sea conditions. Already, several items concerning the wave actions have been experimentally studied in the directional wave basins. For example, Hiraishi, et al. (1998) demonstrates that the wave overtopping rates become smaller in directional waves than in uni-directional waves.