Bidirectional winds, barchan dune asymmetry and formation of seif dunes from barchans: a discussion

Barchan dunes-which have a crescent shape with two horns pointing downwind-may undergo a transition to a longitudinal (seif) dune under a bimodal wind regime. Understanding the barchan-seif dune transition is important for the research of dune field evolution and for the investigation of planetary climate and wind regimes. Two models have been proposed to explain the barchan-seif dune transition: Bagnold (The physics of blown sand and desert dunes. Methuen, London, 1941) and Tsoar (Z Geomorphol 28:99-103, 1984). The significance of both models has been investigated through much field and modeling works over the last few decades. However, the conditions for the barchan-seif dune transition as well as the models proposed to explain it are still poorly understood. To correct this situation, here we present and discuss some examples of asymmetric barchans and barchan-seif transitional dune morphologies occurring in nature and show how to characterize wind regimes and identify the relevance of different factors leading to the observed patterns (in addition to wind directionality). Bagnold's and Tsoar's models were conceived to explain the barchan-seif dune transition under asymmetric bimodal winds. They were not conceived to explain all types of barchan asymmetry. However, these models must be evaluated in the light of an insight that has been gained more recently, from field investigations, experiments and numerical simulations: The seif dune forms only if the divergence angle between the two main wind directions is >= 90 degrees.