ERDC/CHL CHETN-I-66
June 2002
Grid Nesting with STWAVE
by Jane McKee Smith and S. Jarrell Smith
to describe new grid nesting capabilities in Steady-State Spectral WAVE Model (STWAVE).
Background information about advantages of grid nesting and the previous nesting capabilities are
given first. Then, the improved nesting capability is described, and an example application is
discussed. Last, input and output for the upgraded STWAVE (Version 4.0) is described, and the
Web site to download the STWAVE executable and user manual is given.
BACKGROUND: Large wave model grid domains are needed in many applications to minimize
boundary impacts on the area of interest, collocate input wave boundary information with the grid
boundary, or to model regional processes. Fine model grid spacing is required to resolve complex
bathymetry and current fields. Wave model execution time is linearly related to the number of
model grid cells, so a large domain together with fine grid resolution leads to long computational
times even for a relatively efficient model like STWAVE. To reduce computational time, grid
nesting can be applied. A coarse grid can be used in an offshore region where the bathymetry is less
complex and wave-bottom interactions are smaller, and a fine grid can be used in the nearshore
where complex bathymetry and current fields may cause significant modifications to the wave field
over short distances. High model resolution provides a more accurate description of the bathymetry
(given that sufficient bathymetry data is available) and a more accurate numerical solution as the
finite differencing interval (∆x) is decreased. Another advantage of grid nesting is that simulations
with the coarse offshore grid can often be used to drive multiple nearshore grid simulations, e.g.,
modifications of nearshore bathymetry, application of nearshore storm surge or tides, and application
of tidal currents. The advantage of grid nesting is to minimize computational requirements and
maximize accuracy.
Grid nesting has been applied in the past with STWAVE by saving wave spectra from one location
on a coarse offshore grid and then feeding that wave spectrum as input along the entire offshore
boundary of the nearshore grid. This approach is sufficient if the bathymetry at the offshore
boundary of the nearshore grid is well behaved (depth is approximately constant on the boundary)
and the longshore extent of the nearshore grid is relatively small (so that the wave conditions are
approximately constant along the boundary). For larger regional applications or applications with
variation in wave characteristics along the nested grid boundary, a more sophisticated grid nesting
capability is required. For these applications, output spectra from the coarse grid must be saved at
several locations and interpolated onto the nearshore grid boundary. This technical note describes
the new grid nesting implementation in STWAVE.
NESTING METHOD: A coarse offshore STWAVE grid is used to transform offshore waves to the
boundary of a nearshore grid. Wave spectra from the coarse grid are saved at user-selected points
that coincide with the offshore boundary of the nearshore grid. A separate STWAVE model run is
then initiated to interpolate the coarse grid spectra onto the nearshore grid and model the nearshore
wave transformation. Accomplishing the nesting in two steps provides extra flexibility: a) multiple
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