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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

Steady-state dune field morphologies in the parameter space {θ, N} of bidirectional wind regimes for two conditions of sand availability.(a) Dunes grow in height and wavelength from a flat bed with no restriction in sand availability; (b) Dunes extend or propagate 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. In all cases, the dominant wind blows from left to right. 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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f2: Steady-state dune field morphologies in the parameter space {θ, N} of bidirectional wind regimes for two conditions of sand availability.(a) Dunes grow in height and wavelength from a flat bed with no restriction in sand availability; (b) Dunes extend or propagate 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. In all cases, the dominant wind blows from left to right. 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: Figure 2a,b show the dune fields once they have reached a steady-state in morphology and orientation in the parameter space {θ, N} of bidirectional wind regimes under the same two conditions of sand availability as in Fig. 1. When bedforms grow from a flat sand bed (Fig. 2a), a periodic dune alignment is systematically observed. For θ < 90°, the two winds blow always from the same side of the dune and both participate to the formation of linear transverse dunes with gentle upstream slopes (~10°) and slip faces in the lee12. For θ > 90°, the two winds blow alternatively from both sides of the dune. As a consequence, the slip face switches alternatively from one side to the other. Dunes are more symmetric in shape with slopes of approximately 20°. Their orientations are always more perpendicular than parallel to the dominant wind direction. Hence, the dune aspect ratio seen by the secondary wind is smaller than the dune aspect ratio seen by the primary wind. It explains why superimposed bedforms perpendicular to the secondary wind are observed more easily than those perpendicular to the dominant wind (e.g., Fig. 2a for θ = 110°). However, they are transient dune features, which are rapidly blown away by the next wind (see Supplementary Note 2 and Movie S1). All these observed dune orientations are in good agreement with the GBNR.


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

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

Steady-state dune field morphologies in the parameter space {θ, N} of bidirectional wind regimes for two conditions of sand availability.(a) Dunes grow in height and wavelength from a flat bed with no restriction in sand availability; (b) Dunes extend or propagate 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. In all cases, the dominant wind blows from left to right. 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

f2: Steady-state dune field morphologies in the parameter space {θ, N} of bidirectional wind regimes for two conditions of sand availability.(a) Dunes grow in height and wavelength from a flat bed with no restriction in sand availability; (b) Dunes extend or propagate 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. In all cases, the dominant wind blows from left to right. 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: Figure 2a,b show the dune fields once they have reached a steady-state in morphology and orientation in the parameter space {θ, N} of bidirectional wind regimes under the same two conditions of sand availability as in Fig. 1. When bedforms grow from a flat sand bed (Fig. 2a), a periodic dune alignment is systematically observed. For θ < 90°, the two winds blow always from the same side of the dune and both participate to the formation of linear transverse dunes with gentle upstream slopes (~10°) and slip faces in the lee12. For θ > 90°, the two winds blow alternatively from both sides of the dune. As a consequence, the slip face switches alternatively from one side to the other. Dunes are more symmetric in shape with slopes of approximately 20°. Their orientations are always more perpendicular than parallel to the dominant wind direction. Hence, the dune aspect ratio seen by the secondary wind is smaller than the dune aspect ratio seen by the primary wind. It explains why superimposed bedforms perpendicular to the secondary wind are observed more easily than those perpendicular to the dominant wind (e.g., Fig. 2a for θ = 110°). However, they are transient dune features, which are rapidly blown away by the next wind (see Supplementary Note 2 and Movie S1). All these observed dune orientations are in good agreement with the GBNR.

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