MIKE 21 BW, User Manual, Page: 136 / 196

Reference Manual
136 MIKE 21 BW - © DHI
still water shoreline it is recommended to use a filter coefficient between 0
and 1, e.g. 0.25 or 0.5. Outside this area the filter coefficient should be zero
(no filtering).
Figure 5.22 shows the position of a non-zero filter applied in one of the slop-
ing beach examples in the installation.
Figure 5.22 Application of a spatial varying filter coefficient in the uprush/downrush
zone.
How to prepare a filter file?
The data file including the filter coefficients (1DH applications: dfs1-file and
2DH applications: dfs2-file) can easily be prepared by making a copy of the
bathymetry/profile file: replace values within a certain depth range (say, -3 m
to +1 m) with a constant filter value (say, 0.25) and change remaining depth
values to zero. This manipulation can be undertaken in the Grid Editor (dfs2)
or in the Profile Editor (dfs1). To obtained a smooth transition between “0” and
“0.25” it is recommended to delete some “0”s around the area with filter coef-
ficient 0.25 and subsequently make an interpolation.
5.3.8 Wave Breaking
The incorporation of wave breaking in MIKE 21 BW is based on the concept
of surface rollers.
Surface roller concept
The wave breaking is assumed initiated if the slope of the local water surface
exceeds a certain angle, in which case the geometry of the surface roller is
determined. The roller is considered as a passive bulk of water isolated from
the rest of the wave motion, while being transported with the wave celerity.
The influence of the roller is taken into account through an additional convec-
tive momentum term arising from the non-uniform vertical distribution of the
horizontal velocity. The approach is illustrated in the figure below. For a thor-
ough description of the determination of the surface rollers, reference is