largest difference between H1 and H10 occur at the most exposed point on the
Anasosopo line. At Aua and Leloaloa, H1 exceeds H10 by about 0.1 to 0.2 m
(0.4 to 0.7 ft). At Anua, H1 is very low, less than 0.1 ft (0.03 m) higher than H10.
Relative to the berthing area criterion of wave heights not exceeding 0.3 m
(1 ft) more than 10 percent of the time, the Aua and Anua sites and the protected
end of the Leloaloa line appear to be suitable locations for harbor docks. The
0.3-m (1-ft) criterion is exceeded at the Anasosopo site and the more exposed
part of the Leloaloa line, but these sites may also be suitable for some types of
harbor operation. All four sites satisfy the 0.6-m (2-ft) criterion. In comparing
sites with each other and with past experience in the Pago Pago Harbor
embayment, it is interesting to note that a severe event at Aua (H1 ) is
comparable, in general, to the more frequently occurring H10 at Leloaloa.
Similarly, a severe event at Leloaloa, H1, is comparable to H10 at Anasosopo.
Wave and Water Levels for Design
Wind wave and swell results from CGWAVE model runs in this study were
combined with extreme wave and water level modeling in a concurrent study of
hurricane-induced stage-frequency relationships along the coast of American
Samoa to provide information useful for design (Militello, Scheffner, and
Thompson 2002). That study is summarized in the following paragraphs and
results are presented for the four study sites.
Hurricane wind and wave growth models were run to produce deepwater
wave hindcasts for 31 hurricanes impacting American Samoa during the years
1959 through 93. For each hurricane, the time-history of hourly deepwater
significant wave height, peak spectral period, and peak direction was saved at
points around Tutuila, including a point representative of the seaward boundary
of the CGWAVE domain. The deepwater wave time-history was transformed to
a nearshore wave time-history at selected points along the 33-ft (10-m)-depth
contour in the Pago Pago Harbor embayment. The approach involved calculating
and applying spectral amplification factors, as discussed in the previous section.
This approach does not take into account wave breaking and other energy
dissipation processes that would affect extreme waves more than waves in the
general climate. Waves at 33-ft (10-m) depth are representative of waves
incident to the nearshore fringing reef.
For each hurricane, the time-history of water level at the shoreline was
calculated to include storm surge, ponding on the reef (due to incident waves
breaking on the reef face), and nearshore wave setup. Some hurricanes had no
appreciable impact on the study areas, while a few had a strong impact. The
most extreme wave and water level conditions were from Hurricane Val, a
powerful storm in December 1991 passing across Tutuila Island south of Pago
Pago Harbor.
The highest water level at the shoreline from each of the 31 storms provided
information for extreme water level analysis. The Empirical Simulation
Technique was used to calculate water level at the shoreline for various return
27
Chapter 4 Harbor Response to Wind Waves and Swell
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