1. A Short Introduction to Barchan Dunes
 The shapes of aeolian sand dunes originate from their interaction with the wind. In fact, the wind can shape dunes by way of erosion and deposition processes, but on the other hand, dunes are large enough to modify the velocity profile of the wind. A century of field observations has demonstrated that the shape of aeolian dunes is mainly determined by the directionality of the wind and sand availability [Bagnold, 1941; Cooke et al., 1993]. When the wind is unidirectional and the sand supply is low, flat, crescentic dunes, called barchans, appear and migrate along the wind direction.
1.1. Aeolian Barchan Dunes: Basics
 A typical barchan (see Figure 1) has a flat windward side (∼10°) whereas the lee-side, usually called the slip-face and formed by successive avalanches, is steeper (∼34°). On its lateral sides, two horns extend along the wind direction, forming the typical crescentic shape and letting some sand escape from the barchan. The width and length of barchans range between 15 m and 150 m, while the height ranges from 1 m to 15 m. Furthermore, no mature barchan smaller than typically 15 m long and 1 m high develops in the desert [Bagnold, 1941; Andreotti et al., 2002]. For barchans large compared to this critical size, the height, H, the width, W, and the length, L, are almost linearly dependent and therefore, barchans can be considered as self-similar shapes [Sauermann et al., 2000].
 Barchans are also known for their high mobility. This comes from erosion and deposition processes: the wind carries sand grains from the bottom of the windward side up to the crest. There they are deposited because of the dramatic decrease of the velocity of the wind just after the crest. When the upper part of the slip-face becomes too steep, an avalanche spontaneously nucleates and redistributes the sand along its side, while the slope of the slip-face relaxes toward its equilibrium value. Thereby grains pass from the back of the dune to the slip-face and the dune moves. Field surveys have shown that the velocity of barchan dunes, c, varies with the size of dunes as c ≃ Q/(H + H0) [Cooke et al., 1993; Andreotti et al., 2002], where Q has the dimension of a volumic sand flux and varies with the strength of the wind [Hastenrath, 1967, 1987], and H0 is a cutoff length which takes into account the fact that very small dunes cannot move faster than the wind.
 However, hardly anything more is known concerning the effect of the flow on the shape and on the dynamics of barchan dunes. This is due to the difficulty of achieving precise measurements in the field, because of the large lengthscale and timescale involved and the lack of control on the wind. Therefore it is crucial to investigate this issue under controlled conditions in the laboratory. However, the existence of a minimal size for barchan dunes means that no barchan can be reproduced in air at the laboratory scale [Bagnold, 1941; Andreotti et al., 2002; Dauchot et al., 2002]. This critical size comes from the inertia of sand grains. Let us consider the simple situation in which a wind, that carries no grain, arrives on a flat sand bed. Grains are progressively set into motion by the wind, and the sand flux increases with the distance from the beginning of the sand bed. Simultaneously, the moving grains absorb momentum from the wind and accordingly the wind slows down until on average no new grains can be put into motion. Therefore, after some transient, the sand flux is saturated. This transient induces a space lag between the sand flux q and its value in the saturated regime, qs [Anderson et al., 1991; Sauermann, 2001; Kroy, 2002; Andreotti et al., 2002; Valance, 2005]. This saturation length, ls, is the characteristic lengthscale over which the sand flux adapts to a change of the wind velocity or the sand supply. For a sand pile smaller than a few times ls, the sand flux increases everywhere on its surface, and accordingly the sand pile is just swept away. On the contrary for a longer sand pile, the sand flux can become over-saturated and deposit sand in its lee side: the sand pile can transform into a propagating sand dune [Andreotti et al., 2002; Kroy, 2002]. It has been shown that this saturation length varies as the inertia length, ldrag [Andreotti et al., 2002; Valance and Langlois, 2005; Valance, 2005] which is the typical length needed for a sand grain to reach the velocity of a turbulent flow. This length can be expressed as
in which ρs ∼ 2500 kg m−3 is the sand density, d ∼ 200 μm is the typical diameter of sand grains and ρf ∼ 1 kg m−3 is the density of air. This strongly suggests that increasing the density of the fluid, ρf reduces the saturation length and accordingly the minimal size, leading to small-scale barchan dunes [Hersen et al., 2002].
1.2. Subaqueous Barchans Ripples as a Model of Aeolian Barchans
 In particular, crescentic ripples of tens of centimeters are often observed in rivers or marine environment [Allen, 1968; Mantz, 1978]. The origin of their formation lies in the same underlying physics as for aeolian dunes: a unidirectional current that shapes the sand bed and interacts with it. However, in water, ldrag is approximately one thousand times smaller than in air, and therefore crescentic ripples are expected to compare well to aeolian barchans first rescaled by ldrag. However, the subaqueous crescentic ripples usually appear in areas with a large sand supply. This is the reason why one generally observes many crescentic structures merging together and forming complex three dimensional ridges that propagate along the current direction. This has been nicely shown by the experiments of Mantz : A flat and uniform sand bed submitted to a uniform water current gives rise to many crescentic ripples that grow and merge as long as there is sand between the different structures. As a result, studies of subaqueous solitary barchans are very rare [Best, 2004]. Nevertheless, this does not preclude the possibility of observing solitary crescentic dunes under water providing that the sand supply is dramatically lowered. This is why we have designed a simple experiment to reproduce solitary underwater barchans from a single conical sand pile rather than a uniform sand bed. This study should be significant not only for the understanding of aeolian barchans, since it will provide us with new insights on their dynamics, but also for shedding light on marine sand wave dynamics since barchans can be seen as the elementary structure forming the more complex barchanoid ridges that are usually observed in river environment.