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Comptes Rendus Physique
Volume 16, n° 1
pages 118-138 (janvier 2015)
Doi : 10.1016/j.crhy.2015.02.002
Dune morphodynamics
Morphodynamique des dunes
 

Sylvain Courrech du Pont
 Laboratoire “Matière et systèmes complexes”, Sorbonne Paris Cité, Université Paris-Diderot, CNRS UMR 7057, 10, rue Alice-Domon-et-Léonie-Duquet, 75205 Paris cedex 13, France 

Abstract

The physics of dunes relies on the interaction between a wind flow and an erodible topography. Thus, if strong enough to transport grains, the wind shapes sandy areas into dune fields. These dunes are reminiscent of a wavy sea so that sandy deserts are called sand seas. However, the comparison stops there. Contrary to water waves, dunes propagate only under wind action and when the wind stops, they do not vanish but stand. Consequently, dunes are not only the result of the present winds, but can integrate the wind regimes over long periods. Thus, they exhibit a range of shapes and sizes with superimposed patterns. They are witnesses of past wind regimes and their shape and orientation are used to constraint climatic models on other planetary bodies where they are observed as well (e.g., Mars, Titan and Venus). Here, we discuss the morphodynamics of dunes and endeavor to identify and to explain the physical mechanisms at play in the selection of their shape, size and orientation, whilst focusing on Earth desert sand dunes.

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Résumé

La physique des dunes repose sur l'interaction entre le vent et une topographie érodable. Quand le vent souffle au-dessus d'une surface de sable, des dunes se forment dès lors que des grains sont transportés. Ces dunes offrent un paysage qui ressemble à une mer mouvementée et les déserts sableux sont appelés des mers de sable. Cependant, l'analogie entre dunes et vagues s'arrête là. Contrairement aux vagues, les dunes ne se propagent que sous l'action du vent, et si le vent s'arrête de souffler, elle ne disparaissent pas, mais persistent. Ces tas de sable ne sont pas seulement le fruit des vents présents, mais peuvent intégrer l'histoire des vents sur de longues périodes. Cette propriété explique la richesse des formes et des échelles observées, et fait des dunes des témoins des vents passés. Ainsi, on utilise leur forme et surtout leur orientation pour contraindre les modèles climatiques des corps célestes comme Titan, Mars ou Vénus, où elles sont observées. Dans cet article, nous expliquons la forme, la taille et l'orientation des dunes en passant en revue la littérature récente et en nous attachant à identifier et à expliquer les mécanismes physiques mis en jeu.

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Keywords : Dune, Geomorphology, Desert, Barchan, Sand transport, Collective motion

Mots-clés : Dune, Géomorphologie, Désert, Barchane, Transport de sable, Mouvements collectifs


1  Some may prefer the following explanation found in [[6], [31], [32]]. Well above the ground, the flow is inviscid and controlled by the balance between inertia and pressure gradient. As a result, the fluid velocity is in phase with the topography and maximum at the summit. In the inner layer, the turbulent stresses prevail on inertia and slow down the fluid. The inertia makes the fluid in the upper layers adapt with a delay. Hence, the bottom shear stress, as the fluid velocity in the inner layer, is maximum before the summit. This explanation is consistent, but the in-phase result of the inviscid, inertia-dominated flow ignores the inner layer.
2  This value is derived from the assimilation of wind data provided by the ERA-Interim reanalysis over the last 35 years [[58]]. Local measurements of the wind velocity over the year 1999 give   [[40]].
3  However, the secondary fluid flow on the lee side opposes the redirection imposed by gravity.


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