MIKE 21 BW, User Manual, Page: 123 / 196

Basic Parameters
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Figure 5.11 Example of time series of simulated surface elevation
Time step interval
The selection of the time step depends on selected module and the form of
the Boussinesq equations. The guidance and recommendations listed below
is mainly related to the 2DH Boussinesq wave module. The time step con-
straint for the 1DH Boussinesq wave module is that the Courant number is
less than about 0.5.

Excluding the deep water terms:
The main requirement is that the time step should be sufficiently small to
resolve the shortest individual wave periods. For most practical applica-
tions 8-10 time steps per wave period will be OK, applied to the minimum
wave period to be considered. Further, it is also required that the time
step (in combination with the grid spacing) is selected so that the Cou-
rant number is less than or equal to 1.

Including the deep water terms:
To avoid numerical damping and instability, the minimum wave period
should be resolved by 25-35 time steps. Again it should be checked if the
Courant criterion is fulfilled.
It is recommended to use the MIKE 21 BW Model Setup Planner for
determination of the optimal time step.

Including wave breaking and moving shoreline:
In practical applications involving wave breaking and moving shoreline
the grid space is typically 1-2 m and the time step is typically 0.05-0.1 s
depending on the time scale of the waves. The lower values are required
when a moving shoreline is included.
Warm-up Period
The warm-up period is a period where the forcing functions (boundary condi-
tions) gradually are built up to their prescribed value. Often a warm-up period
can be used to avoid blow-ups during the first time steps.