DIRECTIONAL WAVE OBSERVATIONS DURING HIGH SEA STATES FROM AN NDBC DISCUS BUOY

Estimates of the directional wave spectrum were obtained from an NDBC 3- meter discus buoy moored outside Monterey Bay, California. Three high sea states with significant wave heights greater than 9 meters were measured by the buoy and were used to examine the characteristics of directional waves in high seas. The data showed that the high frequency end of a frequency spectrum was better described by an f-4 law when the mean wave direction was close to the wind direction. When normalised by either the friction velocity or the significant wave height, the nondimensional wave spectra from the three cases matched well with each other and were consistent with those obtained by Forristall (1981) from different sea states and locations. It was observed that waves with higher frequencies aligned with the wind direction rapidly, and a strong swell tended to align other component waves into its direction. The directional spreading, based on the cos2s(one halfalpha) distribution, for a high sea state due to a swell was much narrower than that from wind-generated waves. When the difference between the wind direction and the peak wave direction was larger, the directional spreading near the peak was much wider. The directional spreading parameters (S1), which depend on the first-order harmonics of the above distribution, at the high frequency end for the three high sea state cases were different from those presented by Mitsuyasu et al. (1975) and Hasselmann et al. (1980).