Evaluation against Operational Criteria for Long
Waves
Procedures for evaluating the operational acceptability of different harbor
plans subjected to long waves were reviewed by Thompson, Boc, and Nunes
(1998). Two guidelines are applied in this study, as discussed in the following
paragraphs. Each guideline provides a different and useful perspective on the
long wave response of the various harbor sites.
One operational guideline is based on the value of Aamp,l for the higher
resonant peaks. Experience with Los Angeles and Long Beach harbors has
indicated that if Aamp,l is greater than about 5, some operational difficulties may
be encountered. If Aamp,l is greater than 10, major operational problems can be
expected (Seabergh, personal communication). This guideline may be applied to
be the most suitable site, but major operatonal problems might be expected at all
four sites and at existing docks.
Amplification factors computed in the Pago Pago Harbor embayment are
significantly higher than what has been reported in past studies of successful
harbors in other geographic locations. Despite the high amplification factors, the
existing harbor is not known to experience operational problems because of long-
wave motions. To cause problems, harbor resonance periods would need to
couple with a natural period of moored vessel motion, but vessels and mooring
configurations in Pago Pago Harbor are not known to differ significantly from
other commercial harbors.
Several possible factors can be considered to reconcile model results with
experience in the existing harbor. First, the presence of fringing reefs over nearly
the entire embayment is unusual relative to most harbors. The reefs may reduce
reflectivity of harbor boundaries for long waves. Even a small reduction in
reflection coefficient can have a major impact on resonant amplification factors.
No data are available to assess this effect in the Pago Pago Harbor embayment.
Second, the strength of resonant oscillations depends on both amplification
factors and incident longwave heights. Since no long wave data are available in
this geographic area, incident longwave heights are unknown. They may well be
quite small for this south-facing coast exposed to relatively short-period waves.
Past experience at locations in Hawaii and California exposed to swell from the
north Pacific has shown that energy level in the 30 to 500-sec period part of the
long wave spectrum is directly correlated with energy level in the longer-period
sea and swell wave conditions (Okihiro and Guza 1996; Briggs et al. 1994).
Energy at periods longer than 500 sec is attributed to forcing mechanisms other
than swell, such as meteorological or atmospheric disturbances, internal waves,
and tsunamis.
The second operational guideline relates to longwave velocity along the
docks. PIANC (1995) gives criteria for maximum horizontal translational
motions of moored vessels in terms of distance and velocity. Since horizontal
motions are highly constrained by mooring lines, the velocity criteria seem more
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Chapter 5 Harbor Oscillations
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