Deepwater Wave Climate

1_Samoa
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7

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

hindcast.

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