MIKE 21 BW, User Manual, Page: 126 / 196

Reference Manual
126 MIKE 21 BW - © DHI
Open boundary
At an open boundary the incident wave energy is given as a time series of
surface elevations or flux densities perpendicular to the boundary. If the deep
water terms are included, the time series of the surface curvature or the sur-
face slope must also be available.
With respect to wave modelling it is important to notice that an open boundary
is fully reflective. This is the main reason that internal wave generation is
often preferred as opposed to 'open boundary wave generation'.
To prepare the boundary input files the wave tools in the MIKE 21 Toolbox
may be applied.
Closed boundary
A closed boundary (land points) does not allow for flow across the boundary.
A closed boundary is fully reflective when not combined with sponge or
porosity layers. For a closed boundary, boundary conditions (such as surface
elevations) can not be specified.
Absorbing boundary
An absorbing boundary allows wave energy to pass out of the model area
without reflections propagating back into the model area.
In the model an absorbing boundary consists of a number of sponge layers
typically backed up by a closed boundary (land values), see section 5.3.11
Sponge layers.
Radiating boundary
When applying internal wave generation the wave energy propagating out of
the model area has to be absorbed. This is done by placing sponge layers
behind the generation line.
Remarks and Hints
Open Boundary or Internal Generation?
In most cases internal wave generation should be applied for wave genera-
tion, as open boundaries are fully reflective and will therefore reflect any
wave energy radiating out of the model area back into the model.
5.3.3 Surface elevation
The initial surface elevation for your simulation can be specified either as a
constant over the entire model area, or as a two-dimensional map read in
from a data file.