Deepwater Wave Climate
The 5-year global hindcast includes the parameters significant wave height,
Hs , peak period, Tp, and peak direction, θp, (dominant direction for wave energy
within the frequency band of peak energy). All three parameters are necessary
for an adequate description of the wave climate incident to the south side of
Tutuila. Wave direction, which is not available from the buoy gage, has a key
impact on how much incident wave energy propagates to different locations
inside the Pago Pago Harbor embayment.
Fortunately, the hindcasts overlap the time period of the first buoy gage
deployment. Kinhill, Riedel & Byrne (1992) included a comparison of hindcast
and measured Hs and Tp during the months of overlap. They concluded that the
hindcast wave climate was sufficiently validated by buoy data, despite their
differences in location and exposure. Even for Hurricane Ofa, the extreme event
during the overlap period, the hindcast compared reasonably well with the buoy.
The hindcast peak Hs was 9.0 m (30 ft) versus 7.3 m (24 ft) measured at the
buoy. The corresponding Tp was 9.0 sec from the buoy and 12 sec from the
Wave climate summaries of the 5-year hindcast are given by Kinhill, Riedel
& Byrne (1992). Percent occurrence information for Hs versus θp shows that Hs
is generally less than 3 m (9.8 ft) and most waves come from the general direc-
tion of east (i.e., ENE-ESE) (Figure 4). Waves from the south, southwest and
west occur infrequently. The figure indicates that the higher waves generally
come from directions between west and north. From southerly and southeasterly
directions, the directions of greatest exposure for the Pago Pago Harbor embay-
ment, wave heights are relatively mild, with most Hs less than 2.5 m (8.2 ft) and
no occurrences of Hs greater than 3 m.
Percent occurrence information for Tp versus θp shows that nearly 80 percent
of the Tp values are 9 sec or less (Figure 5). Longer Tp conditions are also
present in the wave climate, with periods up to 25 sec and longer. For the
directions of greatest exposure for Pago Pago Harbor embayment, nearly all Tp
occurrences are 13 sec or less and the great majority are 9 sec or less.
Percent occurrence information for Hs versus Tp shows little relationship
between the two parameters for the lower, more common wave conditions (Hs
less than or equal to 2 m (6.6 ft) (Figure 6). For higher wave conditions, the
shorter Tp values become less common and a preference for Tp values in the 8- to
9-sec band or longer is indicated.
Percent occurrence information for wind speed versus wind direction is also
available from the hindcasts (Figure 7). The summary indicates that winds are
mild much of the time, with speeds of 5 m/sec (10 kts) or less occurring over
70 percent of the time. Wind speeds over 10 m/sec (20 kts) are rare. Winds are
generally from east through southeast.
Chapter 2 Wind Wave and Swell Climate