MIKE 21 BW, User Manual, Page: 44 / 196

Examples
44 MIKE 21 BW - © DHI
Sponge and Porosity Layer Maps' and algebraic manipulations in the Grid
Editor.
Figure 4.18 Porosity coefficient map
Model parameters
The deep-water terms are included in this example. It is recommended to
always include the deep-water terms when wave breaking (and run-up) is
modelled.
Figure 4.19 shows the numerical parameters used in this example. A similar
set of parameters is recommended for most applications including wave
breaking (and run-up). The use of simple upwinding at steep gradients and
near land and a time extrapolation factor of slightly less than one (0.9 in this
case) has successfully been used in a number of wave breaking applications.
Figure 4.20 shows the wave breaking parameters used in this application.
The roller velocity type is set to 3 and the predominant wave direction is set to
270ºN. Hence we assume the breaking waves (rollers) mainly propagate in
the positive x-direction, which is a good approximation in this case. The
remaining parameters are set as default.
The time step is set to 0.1 s, see Figure 4.21 which shows a snapshot of the
Java Script
MIKE 21 BW Model Setup Planner included in the Online Help.
Please note that the wave breaking option “exclude wave breaking” is ticked
in the MIKE 21 BW Model Setup Planner, see Figure 4.21. This requires an
explanation. As stated in the Java Script Note 4 the most important issue is to
make sure the breaking waves are well resolved at important areas in the
model domain. If you use the wave breaking option, the minimum spatial res-
olution (
dx ) will be based on hmin and T
min
( L/dx > 20) whereas the non-
breaking option is based on L/dx > 7. If h
min
is small in an area which is of
less importance the planner provides a very small dx (and dt). Hence, the