Phase diagrams of dune shape and orientation depending on sand availability.

Xin Gao,Clément Narteau,Sylvain Courrech Du Pont,Olivier Rozier

doi:10.1038/srep14677

Abstract

New evidence indicates that sand availability does not only control dune type but also the underlying dune growth mechanism and the subsequent dune orientation. Here we numerically investigate the development of bedforms in bidirectional wind regimes for two different conditions of sand availability: an erodible sand bed or a localized sand source on a non-erodible ground. These two conditions of sand availability are associated with two independent dune growth mechanisms and, for both of them, we present the complete phase diagrams of dune shape and orientation. On an erodible sand bed, linear dunes are observed over the entire parameter space. Then, the divergence angle and the transport ratio between the two winds control dune orientation and dynamics. For a localized sand source, different dune morphologies are observed depending on the wind regime. There are systematic transitions in dune shape from barchans to linear dunes extending away from the localized sand source, and vice-versa. These transitions are captured fairly by a new dimensionless parameter, which compares the ability of winds to build the dune topography in the two modes of dune orientation.

Highlights

Flows[6,7]
In a first set of experiments dedicated to conditions of high sand availability, a flat sand bed with a thickness larger than the flow depth ensures that bedforms never reach the non-erodible ground at the bottom of the cellular space
Boundary conditions are open to remove all sedimentary cells that reach the downstream border of the cellular space. For these two conditions of sand availability, we systematically investigate the dune morphology and orientation for periodic bidirectional wind regimes

Summary

Introduction

Using this rule together with the dune type classification, a given wind regime should be associated with a specific dune shape and a single orientation[8,9] This single line of reasoning has been recently challenged on both experimental and field observations by Courrech du Pont et al.[10]. Over the entire parameter space of the bidirectional wind regime, the numerical results are compared to the prediction of the model of Courrech du Pont et al.[10] using only one fitting parameter This parameter is the wind speed-up, a key parameter in the physics of dunes, which introduces the effect of dune topography on the flow and on the subsequent sand flux over dune flanks. We show a field example where this parameter map and the dune morphology map correspond

Methods

Results

Conclusion