ERDC/CHL CHETN-I-66
June 2002
output points do not need to be evenly spaced, so they can be more closely spaced in regions of more
complex bathymetry. Presently, the nesting locations must be specified directly by the user, but in
the future, the Surface-Water Modeling System (SMS) (Brigham Young University Environmental
Modeling Research Laboratory 1997) will automate the process of selecting nesting locations. For a
nearshore nested grid, parameter IBND is set equal to 1 or 2, and the last section (starting with
NEST) is omitted. If NEST = 1, then nesting is based on a single input point on the boundary and
no interpolation is required. For NEST = 1 cases, the output nesting file from the coarse grid is the
input spectral file to the nested run (with no modification required). All input in the parameter file is
free format, i.e., at least one space separates parameters on a line.
Simulation File. Simula-
STWAVE
0.0000
0.0000
0.0000
tion files specify input and DEP
coarse_l.dep
output file names for an OPTS
coarse.std
STWAVE run. In the past, SPEC
test.eng
simulation files have been WAVE
coarse_l.wav
optional (then STWAVE OBSE
coarse_l.obs
NEST
coarse_l.nst
reverts to default file
names). To run nested
grids, the grid origin and
Figure 10. Sample coarse grid simulation file for grid nesting
orientation are provided in
the simulation file, so the simulation file is required. A sample simulation file is shown in Figure 10.
The file is the same as in previous versions of STWAVE with the addition of the NEST file. This is
the file where output spectra from coarse grid simulations are saved for nesting. For the fine grid
simulation, the nesting output file from the coarse grid simulation becomes the spectral input file,
and the NEST line is not required. For the example in Figure 10, the corresponding fine grid
spectral input would be specified in the simulation file as "SPEC coarse_l.nst". The first line of
the simulation file must begin with "STWAVE" to specify that the run is for STWAVE and provide
the grid origin (x and y coordinates) and grid orientation (degrees counter clockwise from east). In
the sample file shown in Figure 10, the origin is set as (0,0) and the orientation is zero. This input
line has a format (A6, 3f15.5). Remaining input lines of the simulation file are free format. The
origin and orientation are used in STWAVE only to determine the relative locations of the coarse
and nested grids. The coarse grid values are saved into the NEST file to bring into the nested grid
application.
The origins and orientations are used to determine the geometry for interpolation, so it is critical that
the origin and orientation values be specified correctly for the coarse and nested grids. For grids
generated in SMS, these parameters are set automatically, based on the coordinate system specified
for the bathymetry. In typical applications, orientations for the two grids may be the same, but
origins will differ. STWAVE defines bathymetry and wave parameters at the center of each grid
cell. The STWAVE origin is located on the offshore grid boundary as shown in Figure 11. Note
that the grid origin is located along the outside grid cell faces and not in the cell centers. For the
nested grid simulations shown in this technical note, the coarse grid origin was (0, 0) and the
orientation was 0 (grid spacing of 250 m). The nested grid origin was (2,112.5 m, 112.5 m), and the
orientation was 0 (grid spacing of 25 m). The nested grid origin was selected to align cell centers of
the coarse grid and every tenth nested grid cell for direct comparisons (e.g., the center of cell (9,1)
on the coarse grid is located at (2,125.0 m, 125.0 m), so to match cell centers with the nested grid
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