Several data sources were accessed for development of the computational
grid. Initially, shoreline and bathymetry data were obtained from the
U.S. Department of Defense Digital Nautical Chart database (National Imagery
and Mapping Agency 1999). The digital database was supplemented in offshore
areas by digitizing points and contours from National Oceanic and Atmospheric
Administration (NOAA) Chart #81004 and DMA Chart #81025 as needed to get
a complete representation. Bathymetry around Guam and detailed bathymetry
around Apra Harbor were supplemented using NOAA Charts #81048 and
#81054. The Rota Island boundary and coastal bathymetry, which were absent
from the NIMA database, were digitized from NOAA Chart #81063. Grid depths
are referenced to mean sea level (msl).
Tidal elevations specified at the open-water boundary were calculated from
tidal amplitudes and phases contained in the LeProvost World Tidal Constituent
Database, which provides constituent data at 1-deg increments in latitude and
longitude. A bilinear interpolation algorithm was applied to calculate tidal
amplitudes and phases at 118 open boundary nodes. The six tidal constituents
Wave and Wave Transformation Models
Deepwater wave fields were calculated by application of the Wave Informa-
tion Studies Wave (WISWAVE) model (Hubertz 1992; Resio and Perrie 1989).
This model is a second-generation discrete directional spectral wave model in
which the spectral wave computations are based on the integration of energy
over the discrete frequency spectrum. Model output includes time series of sig-
nificant wave height, peak (dominant) or mean wave period, and mean wave
direction. Peak or dominant wave periods are not integral quantities in that they
are not derived by summation over the spectrum. Peak period is defined as the
period associated with the midband frequency, or that frequency band containing
the largest spectral energy density. Mean wave period is an energy-weighted
quantity integrated over all user-specified frequencies of interest. Model input
includes a rectilinear computational grid, with water depths specified at each
node, and wind speed and direction over the grid domain.
Application of the wave model required a grid resolution such that calcula-
tion points could be distributed around and near to the coasts of Guam and Rota
so that representative wave conditions would be captured for all sides of the
islands needed in the studies. To meet this requirement, a grid with constant
spacing of 0.083 deg was developed. For wave modeling at this scale, deepwater
can be applied over the grid. The islands of Guam and Rota were specified as
land in the grid for accurate calculation of wave sheltering and refraction. At this
grid scale, Guam is represented by 22 land points and Rota by 6 land points.
Details of the grid are given in Table 4.
17
Chapter 3
Modeling Approach
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