Abstract
Barchans are crescentic dunes propagating on a solid ground. They form
dune fields in the shape of
elongated corridors in which the size and spacing between dunes are rather
well selected. We show that even
very realistic models for solitary dunes do not reproduce these corridors.
Instead, two instabilities take place.
First, barchans receive a sand flux at their back proportional to their width
while the sand escapes only from
their horns. Large dunes proportionally capture more sand than they lose,
while the situation is reversed for
small ones: therefore, solitary dunes cannot remain in a steady state. Second,
the propagation speed of dunes
decreases with the size of the dune: this leads, through the collision process,
to a coarsening of barchan fields.
We show that these phenomena are not specific to the model, but result from
general and robust mechanisms.
The length scales needed for these instabilities to develop are derived and
discussed. They turn out to be much
smaller than the dune field length. As a conclusion, there should exist further,
yet unknown, mechanisms
regulating and selecting the size of dunes.