  Beautiful dune patterns can be found in deserts and along coasts due to the
instability of a plain sheet of sand under the action of the wind. Barchan
dunes are highly mobile aeolian dunes found in areas of low sand availability
and unidirectional wind fields. Up to now modelization mainly focussed on
single dunes or dune patterns without regarding the mechanisms of dune
interactions. We study the case when a small dune bumps into a bigger one.
Recently Schwammle et al. and Katsuki et al. have shown that under certain
circumstances dunes can behave like solitary waves. This means that they can
``cross'' each other which has been questioned by many researchers before. In
other cases we observe coalescence, i.e. both dune merge into one, breeding,
i.e. the creation of three baby dunes at the center and horns of a Barchan, or
budding, i.e. the small dune, after ``crossing'' the big one, is unstable and
splits into two new dunes.
