MIKE 21 BW, User Manual, Page: 73 / 196

2DH Boussinesq Wave Module - Examples
73
Figure 4.44 Map of absorbing sponge layers (left) and map of low-pass filter coeffi-
cients (right)
A constant Chezy number of C = 35 m 1/2 /s is used for the bottom friction dissi-
pation. This corresponds approximately to a constant wave friction factor of
f
m
= 0.03 used in Sørensen et al (1998).
As opposed to the shore-normal case steady solutions for the mean flow can
only exists when the forcing by radiation stress is balanced by bottom friction
and mixing processes. Mixing processes are usually modelled by means of
an eddy viscosity. In the breaking zone, the viscous shear is to some extent
take into account by the additional convective terms due to the surface roller.
Outside the breaking zone, the eddy viscosity is usually very small. Hence no
eddy viscosity is included in the simulation.
Model parameters
As in the other examples considering wave breaking the deep-water terms
are included.
Also in this example we use ´'Simple upwinding at steep gradients and near
land' for the space discretisation of the convective terms. The time extrapola-
tion factor is set to zero, which will not cause numerical damping of waves as
they propagate in the x-direction. If directional waves are considered a non-
zero value close to one should be used.
The time step is set to 0.1 as in the previous two examples.
The roller velocity type is set to 3 and the predominant wave direction is set to
270 ºN. Hence we assume the breaking waves (rollers) will mainly propagate
in the positive x-direction, which is a good approximation in this example. The
remaining parameters are set as recommended in the User Guide and Online
Help. The time scale for the roller is set to T/5= 1.58 s.
Except for the slot depth, which is set to 12 m and the slot width is set to 0.05
default parameters are used for modelling of the moving shoreline. The slot
width is fairly large compared to recommendations (0.001-0.01), which will