but only a normal incident direction identifies possible long-period resonant
responses.
Four alternative harbor sites were studied. Results for wind waves and swell
are presented in Chapter 4. Long-period oscillation results are presented in
Chapter 5. The presentation focuses on wave conditions in the vicinity of
proposed dock facilities, but some results over the full model area are also given.
Model Description
Model formulation
The numerical wave model CGWAVE is a steady state finite element model
used in the calculation of wave response in harbors of varying size and depth. It
may also be applied along open coastal regions, at coastal inlets, around islands,
and around fixed or floating structures. CGWAVE simulates the combined
effects of wave refraction and diffraction included in the extended mild-slope
equation, also known as the steep-slope equation. It can also include effects of
wave dissipation by friction, breaking, nonlinear amplitude dispersion, and
harbor entrance losses. The basic model deals with regular waves, but irregular
(spectral) wave conditions can be simulated by combining regular wave results.
Several fundamental theoretical and computational advances are included in
the model. The open boundary condition (seaward boundary of the model
domain) is treated with a new parabolic approximation method along with the
classical super-element technique. An efficient iterative procedure (conjugate
gradient method) is used to solve the discretized model equations, enabling the
model to be used practically for larger domains.
The CGWAVE model is interfaced with commercially available Corps of
Engineers-supported software to assist in preparing model grids and other inputs
and in displaying model results. This software-assisted pre- and postprocessing
is needed in any practical application.
More information on CGWAVE is available from Demirbilek and Panchang
(1998) and from the model internet web site (http://chl.wes.army.mil/research/
wave/wavesprg/numeric/wentrances/cgwave.htp). The software package for pre-
and postprocessing is part of the Surface Water Modeling System (SMS). The
SMS software is also described through the model web site.
Finite element grids
Bathymetric data for Tutuila are available on National Oceanic and
Atmospheric Administration (NOAA), National Ocean Service (NOS)
hydrographic chart 83484. In addition, the U.S. Army Engineer District,
Honolulu (HED), provided detailed bathymetric contours from a recent
hydrographic survey of the entire Pago Pago Harbor embayment. Bathymetry
15
Chapter 3 Numerical Model
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