1. Laboratoire de Physique et Mécanique des Milieux Hétérogènes (LPMMH), UMR CNRS 7636, 10 rue Vauquelin 75005 Paris, France
2. Geology Department, Université Ibn Zohr, BP 28/5, Cité Dakhla, 80000 Agadir, Morocco

Correspondence to: Bruno Andreotti¹ Correspondence and requests for materials should be addressed to B.A. (Email: andreotti@pmmh.espci.fr).

Field studies of barchans—crescent-shaped dunes that propagate over solid ground under conditions of unidirectional wind¹—have long focused on the investigation of an equilibrium between sand transport by wind and the control of air flow by dune topography^{2, 3, 4}, which are thought to control dune morphology and kinematics^{5, 6, 7}. Because of the long timescale involved, however, the underlying dynamic processes responsible for the evolution of dune fields remain poorly understood⁸. Here we combine data from a three-year field study in the Moroccan Sahara with a model study to show that barchans are fundamentally unstable and do not necessarily behave like stable solitary waves, as suggested previously^{9, 10, 11, 12}. We find that dune collisions and changes in wind direction destabilize the dunes and generate surface waves on the barchans. Because the resulting surface waves propagate at a higher speed than the dunes themselves, they can produce a series of new barchans of elementary size by breaking the horns of large dunes. The creation of these new dunes provides a mechanism for sand loss that prevents dune fields from merging into a single giant dune and therefore plays a fundamental role in the control of size selection and the development of dune patterns.

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