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

Gao X, Narteau C, Rozier O, Courrech du Pont S - Sci Rep (2015)

Bottom Line: New evidence indicates that sand availability does not only control dune type but also the underlying dune growth mechanism and the subsequent dune orientation.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.There are systematic transitions in dune shape from barchans to linear dunes extending away from the localized sand source, and vice-versa.

View Article: PubMed Central - PubMed

Affiliation: Equipe de Dynamique des Fluides Géologiques, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France.

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.

No MeSH data available.


Related in: MedlinePlus

Transitions in dune shape around the Tibesti Massif (east central Sahara), a zone of low sand availability.(a) Map of σF/σI-value derived from the wind data. The continuous map is generated by linear interpolation based on the horizontal spatial resolution of 0.25° × 0.25° of the wind data. Local dune fields exhibit (b) barchans (image credit: Google Earth), (c) fingers breaking up into barchans (image credit: Google Earth) and (d) fingers elongating in the direction of the resultant sand flux (image credit: Google Earth). All these different dune types are reported in (a) using black symbols. Insets in (b–d) show the local flux roses. The map used in figure (a) is generated by GMT (The general mapping tools).
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f6: Transitions in dune shape around the Tibesti Massif (east central Sahara), a zone of low sand availability.(a) Map of σF/σI-value derived from the wind data. The continuous map is generated by linear interpolation based on the horizontal spatial resolution of 0.25° × 0.25° of the wind data. Local dune fields exhibit (b) barchans (image credit: Google Earth), (c) fingers breaking up into barchans (image credit: Google Earth) and (d) fingers elongating in the direction of the resultant sand flux (image credit: Google Earth). All these different dune types are reported in (a) using black symbols. Insets in (b–d) show the local flux roses. The map used in figure (a) is generated by GMT (The general mapping tools).

Mentions: Here, we concentrate on the ergs around the Tibesti Massif in east central Sahara, where Wilson30 and Mainguet and Callot31 have identified major changes in flow orientations and a variety of dune shapes. In this area of low sand availability where the inter-dune area is free of sand or composed of coarse-grain sediment, we identify and locate the different dune types from recent satellite images. Then, we compare the spatial distribution of barchans and linear finger dunes to the σF/σI-value map derived from the wind data (Fig. 6a). Barchans occur in zones where σF/σI < 0.2 (Fig. 6b), whereas linear finger dunes are always observed where σF/σI > 0.4 (Fig. 6d). In most cases, zones of intermediate values of σF/σI exhibit both barchans and finger dunes (Fig. 6c). Although the transition is not observed for the same σF/σI-value in the simulations and in the field, these observations are consistent with our numerical results and predictions. The quantitative discrepancy between the transition values of σF/σI could be ascribed to non fully accurate ERA-Interim wind data in this mountainous area, which is distant of any meteorological station.


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

Gao X, Narteau C, Rozier O, Courrech du Pont S - Sci Rep (2015)

Transitions in dune shape around the Tibesti Massif (east central Sahara), a zone of low sand availability.(a) Map of σF/σI-value derived from the wind data. The continuous map is generated by linear interpolation based on the horizontal spatial resolution of 0.25° × 0.25° of the wind data. Local dune fields exhibit (b) barchans (image credit: Google Earth), (c) fingers breaking up into barchans (image credit: Google Earth) and (d) fingers elongating in the direction of the resultant sand flux (image credit: Google Earth). All these different dune types are reported in (a) using black symbols. Insets in (b–d) show the local flux roses. The map used in figure (a) is generated by GMT (The general mapping tools).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4588573&req=5

f6: Transitions in dune shape around the Tibesti Massif (east central Sahara), a zone of low sand availability.(a) Map of σF/σI-value derived from the wind data. The continuous map is generated by linear interpolation based on the horizontal spatial resolution of 0.25° × 0.25° of the wind data. Local dune fields exhibit (b) barchans (image credit: Google Earth), (c) fingers breaking up into barchans (image credit: Google Earth) and (d) fingers elongating in the direction of the resultant sand flux (image credit: Google Earth). All these different dune types are reported in (a) using black symbols. Insets in (b–d) show the local flux roses. The map used in figure (a) is generated by GMT (The general mapping tools).
Mentions: Here, we concentrate on the ergs around the Tibesti Massif in east central Sahara, where Wilson30 and Mainguet and Callot31 have identified major changes in flow orientations and a variety of dune shapes. In this area of low sand availability where the inter-dune area is free of sand or composed of coarse-grain sediment, we identify and locate the different dune types from recent satellite images. Then, we compare the spatial distribution of barchans and linear finger dunes to the σF/σI-value map derived from the wind data (Fig. 6a). Barchans occur in zones where σF/σI < 0.2 (Fig. 6b), whereas linear finger dunes are always observed where σF/σI > 0.4 (Fig. 6d). In most cases, zones of intermediate values of σF/σI exhibit both barchans and finger dunes (Fig. 6c). Although the transition is not observed for the same σF/σI-value in the simulations and in the field, these observations are consistent with our numerical results and predictions. The quantitative discrepancy between the transition values of σF/σI could be ascribed to non fully accurate ERA-Interim wind data in this mountainous area, which is distant of any meteorological station.

Bottom Line: New evidence indicates that sand availability does not only control dune type but also the underlying dune growth mechanism and the subsequent dune orientation.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.There are systematic transitions in dune shape from barchans to linear dunes extending away from the localized sand source, and vice-versa.

View Article: PubMed Central - PubMed

Affiliation: Equipe de Dynamique des Fluides Géologiques, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France.

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.

No MeSH data available.


Related in: MedlinePlus