Abstract
Directional spectrum represents the energy distribution of irregular sea waves as a function of frequency and propagation direction. It is one of the most fundamental physical quantities of sea waves. Therefore, it is very important to know the characteristics of the directional wave spectra, not only for the research purposes but also for the plan, design, and construction of ports and their related facilities. The spreading parameter Smax is an important parameter to describe the degree of the energy concentration in propagation direction. The investigation of the spreading parameter has not been conducted well, whereas the accuracy of wave period and wave height which obtained from wave prediction model is sufficient. Although the spreading parameter can be calculated based on wave observation, but response characteristics to disturbance are not clarlfied since the survey sites are limited. In this study, we analyzed the wave data measured with Doppler-type directional spectra wave meters to estimate directional spectra and to investigate the occurrence characteristics of Smax, an energy concentration parameter in direction, around the Japanese coastal area. Also, a fundamental study about the appearance characteristics of spreading parameter was done by using wave prediction model. We investigate the response characteristics of Smax with one direction wind, model typhoon, and GPV wind data. As a result of the statistical analyses on the directional spectrum, the calculated Smax show rapid change in every 2 hours and the clear correlation was not confirmed between the wave period or the significant wave heights and Smax. At Naoetsu, the value of Smax calculated from the mean directional spectrum during one violate wave term is about 14 which is a little higher than the value generally used for the wind waves. On condition with one direction wind, Smax is increased with the increase of the decrement distance. Smax is low at the center of typhoon, Smax is increased with the increase of the distance from the center of typhoon. In wave prediction model, Smax is decreased with the increase of the wave steepness. Comparison of wave observation data and wave prediction data shows that Smax which calculated using wave prediction model is bigger than those calculated using wave observation data.
