Abstract
Barchan dunes - crescent-shaped dunes that form in areas of
unidirectional winds and low sand availability - commonly display an
asymmetric shape, with one limb extended downwind. Several factors have
been identified as potential causes for barchan dune asymmetry on Earth
and Mars: asymmetric bimodal wind regime, topography, influx asymmetry
and dune collision. However, the dynamics and potential range of barchan
morphologies emerging under each specific scenario that leads to dune
asymmetry are far from being understood. In the present work, we use
dune modeling in order to investigate the formation and evolution of
asymmetric barchans. We find that a bimodal wind regime causes limb
extension when the divergence angle between primary and secondary winds
is larger than 90 degrees, whereas the extended limb evolves into a seif
dune if the ratio between secondary and primary transport rates is
larger than 25%. Calculations of dune formation on an inclined surface
under constant wind direction also lead to barchan asymmetry, however no
seif dune is obtained from surface tilting alone. Asymmetric barchans
migrating along a tilted surface move laterally, with transverse
migration velocity proportional to the slope of the terrain. Limb
elongation induced by topography can occur when a barchan crosses a
topographic rise. Furthermore, transient asymmetric barchan shapes with
extended limb also emerge during collisions between dunes or due to an
asymmetric influx. Our findings can be useful for making quantitative
inference on local wind regimes or spatial heterogeneities in transport
conditions of planetary dune fields hosting asymmetric barchans. (C)
2013 Elsevier B.V. All rights reserved.
Users
Please
log in to take part in the discussion (add own reviews or comments).