MIKE 21 BW, User Manual, Page: 57 / 196

2DH Boussinesq Wave Module - Examples
57
level) at the top point and decreasing to a constant depth of approximately16
m. The bathymetry is shown in Figure 4.31.
The spatial resolution is 2 m. The most energetic waves have a period around
8 s and are resolved by the 17-42 points per wave length at depths 2-16 m.
As the primary part of the waves is expected to break where H
m0
~ 0.5
.
h
(experience rule), the wave breaking is initiated at approximately 4 m water
depth. At this depth the most energetic waves are resolved by 24 grid points
per wave length. In most practical 2DH applications including wave breaking
and moving shoreline a grid spacing of 1-2 m is used for peak wave periods
larger than, say, 7s.
As illustrated in Figure 4.32 a 50 points wide sponge layer is used for efficient
wave absorption at the four model boundaries. The thickness of the sponge
layer is corresponding to one wave length or more. The sponge layer map is
generated by using the MIKE 21 Toolbox 'Generate Sponge and Porosity
Layer Maps' and algebraic manipulations in the Grid Editor. Please note the
sponge layer map used in the regular wave case is modified compared to the
sponge layer map shown in Figure 4.32.
Figure 4.32 Sponge coefficient map (Directional.Sponge.dfs2)
A lowpass filter is included to remove high-frequency waves generated during
uprush and downrush at the shoreline and to dissipate the wave energy in
areas where the surface roller is not properly resolved. The filter is used at
water depths less than approximately 0.5 m.
Model parameters
The deep-water terms are included in both wave situations. It is recom-
mended to always include the deep-water terms when wave breaking and
run-up is modelled.