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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

Formation and evolution of dune fields for θ = 130°, N = 2 and two conditions of sand availability.(a) Dunes grow in height and wavelength from a flat bed with no restriction in sand availability; (b) A finger dune extends on a non-erodible ground from a localized sand source. The red and blue arrows show the sand flux vectors of the dominant and secondary winds, respectively. The non-erodible ground is shown in gray. The cellular space has a square basis of side L = 600 l0. As shown by the orientation of the superimposed bedforms in (a) and the orientation of the finger tip in (b), images are taken after the secondary wind at the end of the cycle of wind reorientation.
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f1: Formation and evolution of dune fields for θ = 130°, N = 2 and two conditions of sand availability.(a) Dunes grow in height and wavelength from a flat bed with no restriction in sand availability; (b) A finger dune extends on a non-erodible ground from a localized sand source. The red and blue arrows show the sand flux vectors of the dominant and secondary winds, respectively. The non-erodible ground is shown in gray. The cellular space has a square basis of side L = 600 l0. As shown by the orientation of the superimposed bedforms in (a) and the orientation of the finger tip in (b), images are taken after the secondary wind at the end of the cycle of wind reorientation.

Mentions: In a cellular automaton dune model (see Methods), we set two types of numerical experiments to investigate the effect of sand availability. In a first set of experiments dedicated to conditions of high sand availability, a flat sand bed with a thickness larger than the flow depth ensures that bedforms never reach the non-erodible ground at the bottom of the cellular space. In addition, boundary conditions are periodic. The vertical edges of the cubic lattice are pasted together (see Fig. 1a at t = 0). In a second set of experiments dedicated to conditions of low sand availability, a circular source of sediment of diameter 20l0 is located upstream of a non-erodible ground (see Fig. 1b at t = 0). The input flux is not fixed, but there is always at least one layer of sedimentary cells on the circular source. Thus, the input flux adapts to the experimental conditions. In these simulations, boundary conditions are open to remove all sedimentary cells that reach the downstream border of the cellular space. For these two conditions of sand availability, we systematically investigate the dune morphology and orientation for periodic bidirectional wind regimes. The wind transport capacity is kept constant but the direction and the duration of the secondary wind are changed from one simulation to another (see Supplementary Note 1 and Table S1). Then, the periodic bidirectional wind regime is fully characterized by the divergence angle θ and the transport ratio N between the two winds. This transport ratio is simply the ratio between the time spent in the primary and secondary winds over a period of wind reorientation (i.e., N ≥ 1).


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

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

Formation and evolution of dune fields for θ = 130°, N = 2 and two conditions of sand availability.(a) Dunes grow in height and wavelength from a flat bed with no restriction in sand availability; (b) A finger dune extends on a non-erodible ground from a localized sand source. The red and blue arrows show the sand flux vectors of the dominant and secondary winds, respectively. The non-erodible ground is shown in gray. The cellular space has a square basis of side L = 600 l0. As shown by the orientation of the superimposed bedforms in (a) and the orientation of the finger tip in (b), images are taken after the secondary wind at the end of the cycle of wind reorientation.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Formation and evolution of dune fields for θ = 130°, N = 2 and two conditions of sand availability.(a) Dunes grow in height and wavelength from a flat bed with no restriction in sand availability; (b) A finger dune extends on a non-erodible ground from a localized sand source. The red and blue arrows show the sand flux vectors of the dominant and secondary winds, respectively. The non-erodible ground is shown in gray. The cellular space has a square basis of side L = 600 l0. As shown by the orientation of the superimposed bedforms in (a) and the orientation of the finger tip in (b), images are taken after the secondary wind at the end of the cycle of wind reorientation.
Mentions: In a cellular automaton dune model (see Methods), we set two types of numerical experiments to investigate the effect of sand availability. In a first set of experiments dedicated to conditions of high sand availability, a flat sand bed with a thickness larger than the flow depth ensures that bedforms never reach the non-erodible ground at the bottom of the cellular space. In addition, boundary conditions are periodic. The vertical edges of the cubic lattice are pasted together (see Fig. 1a at t = 0). In a second set of experiments dedicated to conditions of low sand availability, a circular source of sediment of diameter 20l0 is located upstream of a non-erodible ground (see Fig. 1b at t = 0). The input flux is not fixed, but there is always at least one layer of sedimentary cells on the circular source. Thus, the input flux adapts to the experimental conditions. In these simulations, boundary conditions are open to remove all sedimentary cells that reach the downstream border of the cellular space. For these two conditions of sand availability, we systematically investigate the dune morphology and orientation for periodic bidirectional wind regimes. The wind transport capacity is kept constant but the direction and the duration of the secondary wind are changed from one simulation to another (see Supplementary Note 1 and Table S1). Then, the periodic bidirectional wind regime is fully characterized by the divergence angle θ and the transport ratio N between the two winds. This transport ratio is simply the ratio between the time spent in the primary and secondary winds over a period of wind reorientation (i.e., N ≥ 1).

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