Morphodynamic mechanisms for the formation of asymmetric barchans: improvement of the Bagnold and Tsoar models

Abstract

Asymmetric barchan dunes exhibit a marked asymmetry, this morphology develops from the combined effects of two dominant winds, with different directions and relative strengths, and can be described based on Bagnold’s and Tsoar’s models. In Bagnold’s model, the limb nearest to the strongest wind extends and is sustained and enhanced by a gentler wind nearly parallel to the barchan. Tsoar’s model expects that the limb farthest from the gentler wind extends in a manner similar to the evolution of seif dunes and that the limb closest to the gentler wind is eroded. To know which model will emerge with variations of the wind, we did some numerical simulations, and found that there exists a critical angle between orientations of the two winds, which decide the formation model of asymmetric barchans. When the angle is greater than this critical value, the effect of the bi-directional winds agrees with Tsoar’s model. When the angle is less than the critical value, the evolution of the two horns contradicts Tsoar’s model and agrees with Bagnold’s model. The critical value for this angle depends on the transport ratio of two winds.