OUTLINE OF OBSERVATIONS

Wave data to be analyzed here were obtained at a location 3km off the Yura fishery harbor facing the Sea of Japan (1). The observations were carried out during the period from September on 1986 to July on 1990 by Ship Research Institute, the Ministry of Transport of Japan.

Figure 1: A location of wave observation point 3km off the Yura harbor.

Figure 2: A location of wave gages at the observation point.

Temporal sea surface elevations were measured with ultrasonic-type wave gages installed at three points (2) in 43 m water depth. The measurements were made normally in 20min at intervals of two hours, and further continuous recordings over 20 hours were made temporarily when sea conditions became stormy. The continuous data were divided into 30min record unit, from which wave statistics were calculated at intervals of 30min. General wave statistics of the observed waves were presented by Yoshimoto and Kato (1992) and wind field statistics including the weather conditions were reported by Yasuda et al. (1997) and Mori et al. (2000).

Table 1: Wave and wind statistics at observed date of the maximum waves with $H_{max}\ge 10$m.

Observed date	$H_{max}$(m)	$T_{1/3}$(s)	$H_{max}/H_{1/3}$	$U_{19.5}$(m/s)	$\bar{\theta}$
1987/01/01 05:35	11.5	9.1	2.17	18.1	NW
1987/01/04*12:18	10.8	9.2	2.04	20.9	WNW
1987/01/18*10:28	10.0	9.6	2.08	20.0	W
1987/02/16 11:35	11.2	9.5	2.11	15.8	WNW
1987/02/26*23:39	11.3	10.1	1.98	19.6	W
1987/03/25*17:58	11.1	10.4	1.88	23.8	SW
1987/09/01*02:18	13.1	11.7	1.96	--	--
1987/11/24*11:42	13.6	10.8	2.67	13.6	WNW
1987/12/17*10:13	14.6	10.6	2.06	20.8	WNW
1988/01/09*22:48	10.9	9.2	2.06	16.4	WNW
1988/02/03*11:13	14.9	11.2	2.22	17.6	WNW
1988/12/15*09:03	12.8	10.9	2.13	20.9	WSW
1989/11/19*23:42	12.4	10.9	1.82	15.7	WSW
1990/01/25*18:34	10.7	9.5	2.10	17.8	WNW

Table 1 shows wave and wind statistics at all the observation times when waves with the maximum wave height $H_{max}$ exceeded 10m were recorded during the four years the observations were in place. The statistics shown in the table refer to those of (Yoshimoto and Kato, 1992) who analyzed wave data by using zero-up-crossing method. The asterisk * denotes the data obtained at the times when the continuous observations were carried out. It is of interest to note that most of the maximum waves listed in Table 1 can be considered as giant freak waves. This aspect of the findings demonstrates that giant freak waves may occasionally occur without particular regard to wave focusing or directionality. While they are basically during strong wave cases, but in general the cases only examplify normal wind conditions in winter season.