The shape of the barchan dunes of Southern Morocco

We present detailed shape measurements of several barchan dunes in southern Morocco, near Laâyoune. Using these data, we disprove the concept of shape invariance of barchan dunes of different sizes. Nevertheless, some parts of the barchan dune scale and we try to distinguish these from non-scaling ones. Furthermore, we point out the importance of the exact position of the brink for the overall dune shape. Finally, we propose a simple model for dune shape, using our data to determine the model parameters. Despite its simplicity, this model is able to explain the difference between barchans where the crest and the brink coincide and those where they are separated. Typical side slopes can be obtained and the basic properties of the evolution of a barchan dune can be described.