MIKE 21 BW, User Manual, Page: 23 / 196

A Quick Guide for MIKE 21 BW Model Simulation Setup
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x = 0.1-10 m for the 1DH module (unstructured mesh) with wave
breaking and moving shoreline
The next step is to generate a sponge layer map. Sponge (or absorbing) lay-
ers are used as wave absorbers. These may be set up along model bounda-
ries to provide radiation boundary conditions, which absorb wave energy
propagating out of the model area. Sponge layers may also be used along
shorelines (see e.g. the Rønne Harbour example and Torsminde Harbour
example in this User Guide). The sponge layer map is easily created using
the
MIKE 21 Toolbox Generate Sponge and Porosity Layer Map tool.
2. For simulation of partial wave reflection and/or wave transmission
through various types of structures you would need to create a porosity
layer map. The porosity layer map includes a porosity, which is set to
unity at open water points (no dissipation) and between 0.2 and 1 along
structures where you want to include the dissipation effect of porous flow.
If a porosity value is backed up a land value (> 0), partial reflection will
take place. Conversely, (partial) transmission will also take place if land
points do not back up the porosity values.
An efficient step-by-step procedure for establishment of a porosity map is
described in this User Guide, see Section 5.3.10. The porosity map is
easily created using the
MIKE 21 Toolbox Generate Sponge and
Porosity Layer Map tool. The porosity values are calculated using the
MIKE 21 Toolbox Calculation of Reflection Coefficient .
3. If wave breaking and moving shoreline is included you would most often
also have to specify a map (2DH, dfs2 data file) or profile (1DH, dfs1 data
file) including filter coefficients. This lowpass filter is introduced to
remove high-frequency instabilities during uprush/downrush and to dissi-
pate the wave energy in the area where the surface rollers can not be
resolved properly.
4. Next step is to prepare wave boundary conditions for the model. In most
applications you will force the model by waves generated inside the
model domain, i.e. using internal wave generation. Internal wave genera-
tion is performed by adding the discharge of an incident wave field along
one or more generation lines. One of the advantages of using internal
generation is that sponge layers can be placed behind the generation
line, to absorb waves leaving the model domain (radiation type bound-
ary).
5. The format of the internal wave generation data depends on type of
waves; regular, irregular or directional. For generation of regular wave
data you should use the
MIKE 21 Toolbox Regular Wave Generation
tool and for irregular and directional waves MIKE 21 Toolbox Random