MIKE 21 BW, User Manual, Page: 117 / 196

Basic Parameters
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Figure 5.8 Sign convention of bed levels
Simulation start type
In most cases you will start the simulation as a cold start, meaning the simu-
lation will start “from scratch”, and not as a continuation of an earlier simula-
tion, i.e. a hot start simulation.
Breaking waves
As a wave shoals it steepens, increasing in height until the crest becomes
unstable and the wave breaks. Once the wave breaks, excess wave energy
is dissipated through turbulence. The onset of the physical instability occurs
when the water particle velocity at the crest of the wave begins to exceed the
speed of wave propagation.
Figure 5.9 Wave breaking
Boussinesq simulations without wave breaking
By comparison, the Boussinesq equations in MIKE 21 BW use the depth-inte-
grated velocity as the main velocity variable. This can never exceed the
speed of wave propagation. As a consequence, shoaling waves in MIKE 21
BW will continue to shorten and steepen until they can no longer be resolved
adequately by the computational grid. Ultimately this leads to numerical insta-
bility. Depending on the size of the grid, the onset of the numerical instability
may occur well beyond the point at which a real wave would have broken.