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| Titel |
Morphodynamics of star dunes |
| VerfasserIn |
D. Zhang, C. Narteau, O. Rozier, S. Courrech du Pont |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061514
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| Zusammenfassung |
| Star dunes are among the biggest and the most impressive dunes in Earth sand
seas. Nonetheless, they remain poorly studied, probably because of their apparent
complexity. They are massive pyramidal dunes with interlaced arms whose slip faces are
oriented in various directions. Being large, they can integrate wind properties over a
wide range of time scales. Thus, they are observed for wind regimes with multiple
directions, and may result from the amalgamation of dunes or from the development of
arms on a well-established dune pattern. In both cases, the roles of wind directional
variability and secondary flow have been emphasized but not precisely quantified. Here,
we report simulations where the star dune shape results from a a combination of
longitudinal dunes, which form the star dune arms. These arms may radiate and so
interact with the other dunes in the field. This mass exchange, controlled by the
morphodynamics of star dunes arms, must play an important role in the large-scale
arrangement of star dunes networks. We first demonstrate that star dune arms orientation
maximizes the flux in the direction of crests. This is opposed to the usually admit
dunes orientation, which maximizes the sediment transport perpendicular to the
crest. Indeed, depending on sand availability, dunes development results from the
growth of a wave on a sand bed or from a net transport of sediment, which grows and
extends an isolated longitudinal dune over a non-erodible soil. These two different
mechanisms lead to two different modes of crests orientation. Then, we show that the
propagating arms reach a stationary state characterized by constant width, height
and growth rate. These are controlled by the frequency at which the wind changes
direction. Arm width and height increase, whereas the propagation speed decreases with
a decreasing frequency. These morphodynamics properties are helpful to assess
from pattern observation the variability of wind directionality over several time
scales. |
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