Ports and harbours tend to be developed in a larger scale and in deeper seas. It is important to understand accurate and overall information on waves in each field of the plan, the design, and the construction of ports and harbors. Therefore, in addition to the parameters of the significant waves being used now, characteristics of the directional wave spectra will be necessary.
　In 1986,the Second District Port Construction Bureau of the Ministry of Transport started the field observation in deep seas, at Iwaki offshore wave observation station (h=-154m), 42km off the Pacific coast of Japan, as one of the stations of NOWPHAS (Natinwide Ocean Wave information network for Ports and HArbourS).
　This report introduces the characteristics of the directional wave spectra off Iwaki based on the observation result for about seven years.
　The major conclusions are shown as follows:
(1) Annual and monthly directional and frequential wave energy distribution art shown. Monthly average value of the energy distribution fluctuates largely accord
ing to the characteristics of the weather disturbance.
(2) When the wave comes from various directions, the correlation between the direction of the peak of the directional wave spectra and the average wave direction
obtained by the covariance method is extremely low. It is necessary to select an appropriate definition of the representative wave direction according to the purpose.
(3) Comparison of the observed spectra and theroretical directional spectra, which is shown by Bretschneider-Mitsuyasu's frequency spectrum and Mitsuyasu's angular spreading function shows good agreement at the peak of the wave hight during extremely severe sea states.
(4) The principal component analysis is applied to the observed directional wave spectra. The majority of the fluctuation of the spectra can be explained with the use of several principal components which has high-ranking contribution rate. The analysis is effective to understand the outline of the fluctuation of the time series of the directional wave spectra during extremely severe sea states.