Sea waves have such characteristics that they consists of component waves propagating with directional randomness. The component waves, thus, propagate in different directions with random amplitudes, periods and phases. Therefore, it is very important to recognize how much discrepancy will occur when the directional randomness is not properly taken into account in the estimation of wave overtopping rate at offshore seawalls.
　The report examines experimentally the overtopping rate of directional waves at the seawalls in deep sea. A numerical model to estimate the overtopping rate of directional random waves is also proposed and validated on the basis of the experimental results. The effects of wave directionality on the overtopping at seawalls located in shallow water area are also discussed.
　For the cases that verticall seawalls are installed in the water about 40m deep, the following major conclusions are derived from the model test and numerical simulation on wave overtopping.
(1)The overtopping rate of directional random waves becomes about 70% of that in uni-directional waves.
(2)The proposed numerical model well estimates the wave overtopping rate measured in the experiments of oblique uni-directional and multi-directional waves.
(3)The wave overtopping rates measured in the experiments under the action of multi-directional waves turned out to be almost equal to those observed in uni-directional waves when the deviation of the incoming wave direction is 30°from the normal direction to the seawall face.
　The following are results from the observations of waves in shallow water area:
(1)The wave overtopping rate at vertical seawalls turned out to be smaller in the multi-directional wave condition than in the uni-directional one. This is the same phenomenon as was observed at the offshore seawalls.
(2)The numerical model mentioned above well estimates the wave overtopping rate of seawalls when the wave condition at the sea area in front of seawalls is calculated properly. The energy balance equation is one of the appropriate methods to analyze wave transformation as the waves propagate into shallow water area.