MIKE 21 BW - Boussinesq Simulations with Wave Breaking

Reference Manual
118 MIKE 21 BW - © DHI
In deep water the enhanced Boussinesq equations become effectively linear.
In more shallow water, the enhanced equations converge towards the classi-
cal equations, and the non-linear properties will be similar to the ones
obtained by the classical equations. These are known to provide good results
for finite amplitude waves up to at least 75% of the breaking height assuming
wave breaking is not included in your model setup
For most practical applications where wave breaking is not included you
should thus make sure that the ratio between the significant wave height and
water depth is less than 0.3-0.4.
Boussinesq simulations with wave breaking
Wave breaking and moving shoreline have been implemented in a both mod-
ules of MIKE 21 BW (1DH and 2DH module). The inclusion of wave breaking
is based on the surface roller concept for spilling breakers following the for-
mulation by Schäffer et al (1993), Madsen et al (1997a,b) and Sørensen et al
(2004), see references and scientific documentation.
Successful modelling of the wave breaking process requires an adequate
resolution of the surface rollers. In most 2DH applications where wave break-
ing is included a grid spacing of 1-2 is used for wave events having a spectral
peak period larger than, say, 7 s. It is recommend to resolved the most ener-
getic waves with minimum 20-40 grid points per wave length in the surf zone.
In 1DH applications you would typically use an unstructured grid/profile in
such way that the relative resolution is constant, say, 40 grid points per wave
length. This means a coarse spatial resolution offshore (e.g. 10 m) and a fin
resolution at the shoreline (say, 0.5 m).
Independent of the choice of module, the deep-water terms should always be
included, i.e. the enhanced model equations are solved. Otherwise the non-
linear interactions between harmonics in shallow water may be underesti-
mated.
Figure 5.10 Instantaneous surface elevation simulated with the 2DH module includ-
ing wave breaking (the surface rollers are shown in white)