Field evidence for surface-wave-induced instability of sand dunes

Abstract

Field studies of barchans--crescent-shaped dunes that propagate over solid ground under conditions of unidirectional wind--have long focused on the investigation of an equilibrium between sand transport by wind and the control of air flow by dune topography, which are thought to control dune morphology and kinematics. Because of the long timescale involved, however, the underlying dynamic processes responsible for the evolution of dune fields remain poorly understood. 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. 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.

Transverse Instability of Dunes

“…Transverse dunes are in fact also unstable under variations in wind direction [5,17] or collisions with other dunes [19]. While field observations are plagued with uncontrolled weather conditions and large time scales, it would be interesting to perform systematic laboratory experiments of the instability growth in order to confirm the results of our calculations.…”

“…By assuming z 0 ≈ 10µm [2,16] and c ≈ 1.5 [2,3,8,17], a transverse dune with H ≈ 4 m and L 0 ≈ 29 m should fully break into barchans within a distance shorter than L ∞ ≈ 19L 0 ≈ 550 m. Since all information on wind speed and on the attributes of sediments and atmosphere are encoded in T m and in the dune's morphological relations, Eq. (6) is universal, i.e.…”

“…(6)). Moreover, varying wind directions are also a destabilizing factor and can further accelerate the decay process of transverse dunes [5,17].…”

The dune size distribution and scaling relations of barchan dune fields

“…The influence of the geographical constrains and the external physical conditions [2][3][4], of the dune-dune interactions [5][6][7] and even of the emergence of vegetation covers [8] in the dynamics and morphology of single dunes were quite well-established with the help of dune models [1,8,9]. There are also a few studies of entire dune fields [10][11][12][13], but a simple theoretical understanding of the size selection process within dune fields has still not been achieved.…”

“…First, we show that, while a single dune is suitably characterized by its width w [2,3,13], an entire dune field contains dunes with different sizes following a unique distribution (Fig. 1f, g).…”