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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 in the parameter space {θ, N} when the dune develops from a localized sand source.(a) Growth rate ratio σF/σI and (b) RDP/DP-value in the parameter space {θ, N} of bidirectional wind regimes. The growth rates {σI, σF} are computed with equation (2) using the corresponding dune orientations {αI, αF}, γ = 1.6 and the same {H, W}-values (i.e., the same dune shape). The σI/σF-value depends only on the wind regime. For numerical simulations with low sand availability and localized sand source (Fig. 2b), black and white lines show the zones of the parameter space {θ, N} in which fingers and barchans are observed, respectively. Gray lines indicate the zones for transition in dune shape.
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f4: Transitions in dune shape in the parameter space {θ, N} when the dune develops from a localized sand source.(a) Growth rate ratio σF/σI and (b) RDP/DP-value in the parameter space {θ, N} of bidirectional wind regimes. The growth rates {σI, σF} are computed with equation (2) using the corresponding dune orientations {αI, αF}, γ = 1.6 and the same {H, W}-values (i.e., the same dune shape). The σI/σF-value depends only on the wind regime. For numerical simulations with low sand availability and localized sand source (Fig. 2b), black and white lines show the zones of the parameter space {θ, N} in which fingers and barchans are observed, respectively. Gray lines indicate the zones for transition in dune shape.

Mentions: We now address the transition from finger dunes to trains of barchans in the numerical experiments (Fig. 2b). For different transport ratios N, the black lines in Fig. 4 show the ranges of θ-values over which finger dunes are observed. This range decreases with an increasing transport ratio. Close to the transitions, it is common to observe fingers with a finite extension that emit barchans from their tip (e.g., for {60°, 1.5} or {155°, 2} in Fig. 2b). This shows that the two dune growth mechanisms coexist; superimposed dunes in the bed instability mode can develop on the top of dunes in the fingering mode to ultimately break them up into sets of barchans. Note that without a localized sand supply but starting from an initial sand pile, the source of sediment that potentially feeds the extension of a finger dune becomes mobile. In this case, a wider variety of dune shapes would be observed across the entire parameter space {θ, N}. This variety includes barchans, asymmetric barchans with an elongated arm, tear drop and chestnut-like dunes131415.


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 in the parameter space {θ, N} when the dune develops from a localized sand source.(a) Growth rate ratio σF/σI and (b) RDP/DP-value in the parameter space {θ, N} of bidirectional wind regimes. The growth rates {σI, σF} are computed with equation (2) using the corresponding dune orientations {αI, αF}, γ = 1.6 and the same {H, W}-values (i.e., the same dune shape). The σI/σF-value depends only on the wind regime. For numerical simulations with low sand availability and localized sand source (Fig. 2b), black and white lines show the zones of the parameter space {θ, N} in which fingers and barchans are observed, respectively. Gray lines indicate the zones for transition in dune shape.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Transitions in dune shape in the parameter space {θ, N} when the dune develops from a localized sand source.(a) Growth rate ratio σF/σI and (b) RDP/DP-value in the parameter space {θ, N} of bidirectional wind regimes. The growth rates {σI, σF} are computed with equation (2) using the corresponding dune orientations {αI, αF}, γ = 1.6 and the same {H, W}-values (i.e., the same dune shape). The σI/σF-value depends only on the wind regime. For numerical simulations with low sand availability and localized sand source (Fig. 2b), black and white lines show the zones of the parameter space {θ, N} in which fingers and barchans are observed, respectively. Gray lines indicate the zones for transition in dune shape.
Mentions: We now address the transition from finger dunes to trains of barchans in the numerical experiments (Fig. 2b). For different transport ratios N, the black lines in Fig. 4 show the ranges of θ-values over which finger dunes are observed. This range decreases with an increasing transport ratio. Close to the transitions, it is common to observe fingers with a finite extension that emit barchans from their tip (e.g., for {60°, 1.5} or {155°, 2} in Fig. 2b). This shows that the two dune growth mechanisms coexist; superimposed dunes in the bed instability mode can develop on the top of dunes in the fingering mode to ultimately break them up into sets of barchans. Note that without a localized sand supply but starting from an initial sand pile, the source of sediment that potentially feeds the extension of a finger dune becomes mobile. In this case, a wider variety of dune shapes would be observed across the entire parameter space {θ, N}. This variety includes barchans, asymmetric barchans with an elongated arm, tear drop and chestnut-like dunes131415.

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