Wave Climate at Pago Pago Harbor
The incident wave climate needed for modeling wave response in the Pago
Pago Harbor embayment is a deepwater climate which takes into account
sheltering by the Tutuila Island land mass. The 5-year hindcast can be modified
to provide the needed wave climate. The study area is exposed to waves coming
from east clockwise to southwest. Other wave approach directions are blocked
by Tutuila Island.
Available hindcast information is limited in that the summaries do not give
percent occurrences of three-parameter, Hs, Tp , and θp, combinations. A three-
parameter incident wave climate was developed, based primarily on percent
occurrence information for Tp versus θp (Figure 5). Only wave directions of open
exposure were included, which accounted for nearly 50 percent of the hindcast
wave climate. Percent occurrences for these directions were increased
proportionately so they would total 100 percent. This approach essentially
assumes that hindcast events with θp from sheltered directions also include
energy from exposed directions and should not be considered as calms along the
south coast of Tutuila. A presence of multiple wave trains with different
propagation directions has been common at exposed ocean locations and would
be expected at the Samoa Islands (Thompson 1980). This approach was judged
to be the best for developing wave climate for the south side of Tutuila because:
(a) details of the wave climate are not available and a definitive analysis is not
possible; (b) the sheltered directions indicate waves as energetic or more
energetic than the exposed directions, and (c) the buoy at Western Samoa, which
was mostly sheltered from the north, still compared reasonably well with
hindcast Hs values over a wide variety of wave conditions.
Within each bin of the Tp versus θp percent occurrence table, values of Hs
were distributed as described in the Hs versus Tp percent occurrence table (Fig-
ure 6). For example, 3.89 percent of the occurrences fell in the bin for θp = E and
Tp = "to 6 sec" (Figure 5). For Tp = "to 6 sec," Figure 6 shows that 0.09 percent
of the cases have Hs = 0.5 to 1.0 m, 11.37 percent have Hs = 1.0 to 1.5 m, etc.
Proportionately, 0.2 percent of the cases with Tp = "to 6 sec" have Hs = 0.5 to
1.0 m, 29.7 percent have Hs = 1.0 to 1.5 m, etc.
The wave climate developed as described above was reviewed to ensure that
a reasonable representation of the hindcast information was preserved. A rose of
Hs shows the percentage of cases coming from different directions (Figure 8).
The length of the radial bar in each direction shows the percentage from that
direction. As in the previous hindcast figures, the most common wave direction
is from the east southeast (37 percent of the cases). The width of the radial bars
indicates Hs band. The lowest Hs bands are shown nearest the center of the rose.
The radial bars become more narrow toward the outer end of each bar, indicating
increasing wave heights. A similar rose for Tp is shown in Figure 9.
Chapter 2 Wind Wave and Swell Climate