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Reference Manual
150 MIKE 21 BW - © DHI

for 2D studies involving longer waves (e.g. harbour resonance studies)
sponge layers may need to be 50 or even more grid lines wide

in most wave transformation applications (say, mesh size <1 m) the num-
ber of sponge layers is usually within the range 50-200 grid lines

the sponge layers should normally be backed by a closed boundary (i.e.
land grid points)

the sponge layer coefficients at open water grid points should always be
set to unity (i.e. sponge coefficient = 1.0)

to minimise reflections, the values of the sponge layer coefficients should
be close to unity along the first grid line, and should increase smoothly
towards the boundary/land
Given the guidelines above, you are free to select your own values for the
sponge layer (damping) coefficients.
Tool for generation of files including sponge layers
Most often you will use the MIKE 21 Toolbox program Generate Sponge and
Porosity Layer Maps for generation of sponge layers:
Recommended sponge values
When selecting the sponge layer coefficients, the following formula has been
found to work well:
(5.8)
where a and r are assigned constant values, and N
sponge
is the number of
sponge lines (counting from the closed boundary towards the first grid line).
The implemented method is based on the sponge layer technique introduced
by Larsen and Dancy (1983).
For twenty grid lines, it is recommended to use a = 7, and r = 0.7. The sponge
layer coefficients are then obtained by setting the value at the line closest to
the closed boundary equal to a (the base value) and successively taking the
C
sponge
a
r
i 1 –
 
, i 1 N
sponge

= =