  Barchan dunes are found where sand availability is low and wind direction
quite constant. The two dimensional shear stress of the wind field and the sand
movement by saltation and avalanches over a barchan dune are simulated. The
resulting final shape is compared to the results of a model with a one
dimensional shear stress. A characteristic edge at the center of the windward
side is discovered which is also observed for big barchans. Diffusion effects
reduce this effect for small dunes.
