Barchan dune corridors: Field characterization and investigation of control parameters

Abstract

[1] The structure of the barchan field located between Tarfaya and Laayoune (Atlantic Sahara, Morocco) is quantitatively investigated and compared to that in La Pampa de la Joya (Arequipa, Peru). On the basis of field measurements, we show how the volume, the velocity, and the output sand flux of a dune can be computed from the value of its body and horn widths. The dune size distribution is obtained from the analysis of aerial photographs. It shows that these fields are in a statistically homogeneous state along the wind direction and present a ''corridor'' structure in the transverse direction, in which the dunes have a rather well selected size. Investigating the possible external parameters controlling these corridors, we demonstrate that none among topography, granulometry, wind, and sand flux is relevant. We finally discuss the dynamical processes at work in these fields (collisions and wind fluctuations) and investigate the way they could regulate the size of the dunes. Furthermore, we show that the overall sand flux transported by a dune field is smaller than the maximum transport that could be reached in the absence of dunes, i.e., in saltation over the solid ground.

The dune size distribution and scaling relations of barchan dune fields

“…Which mechanisms are behind the local selection of the specific size scale of a dune field remains an open question, since it involves not only binary collisions and flux balance under a stationary wind, but detailed processes in real changing wind conditions, i.e. calving [13,14], that can locally change the dune size and are not included in the analysis we have presented.…”

“…The constant value of the relative standard deviation is consequence of the scale invariance of our collision dynamics that depends on the relative dune size instead of the absolute size. On the contrary, with changing wind conditions, the dune scale may influence the collision outcome and also several dunes may emerge from a single collision [14].…”

“…Therefore, the dune size distribution should be determined by the competition between the balance of sand flux on a single dune and the collisions between neighboring dunes [6,7,11]. Recently a third size selection mechanism was discovered, that involves the calving of large dunes due to wind fluctuations [13,14]. This is a complex scale dependent process, relevant for fields with large dunes and should not be responsible for the distributions here addressed.…”

“…This empirical result is a clear sign of a richer internal dynamics in dune fields where collisions play an important role. Indeed, due to collisions, small dunes are continuously emerging from larger ones [13,14], destroying any simple correlation between dune size and inter-dune spacing, and therefore leading to the observed spatial uniformity.…”

Out-of-equilibrium stationary states, percolation, and subcritical instabilities in a fully nonconservative system

“…Previously [13,14], geophysical matters have lead us to build a model for barchan fields in order to understand the peculiar characteristics of such structures. Our studies were based on experiments [15,16] and on field observations [17][18][19]. However, we will now ignore the natural background of the model to study its whole phase diagram.…”