Figure 1

Deactivation of migrating dunes under the influence of vegetation. On top, a dune field in White Sand, New Mexico, that shows barchanoid ridges on the left, where vegetation is absent, developing towards a mixture of active and inactive parabolic dunes on the right (wind blows from left to right). Dark regions indicate abundance of vegetation. This suggests a transition between both types of dunes when the vegetation cover increases. This transition is illustrated with various dune types found in the White Sand dune field (pictures in the middle), reinforcing the idea of their common evolution from a crescent dune. Satellite images courtesy Google Earth/ DigitalGlobe. Below, same transition obtained by the numerical solution of a model that accounts for the coupling between sand transport and vegetation, with fixation index $\theta =0.22$. The vegetation cover is represented in gray. (Parts a–e are mentioned in the text.)Reuse & Permissions

Figure 2

Evolution of the normalized dune velocity ($v/v_0$) for simulations with initial condition given by the parameters $V$ ($\times {10}^3{\mathrm{m}}^3$), $Q$ (${\mathrm{m}}^2/\mathrm{yr}$), and $V_v$ ($\mathrm{m}/\mathrm{yr}$): (1) 200, 286 and 78; (2) 200, 286 and 26; (3) 44, 529 and 52; and (4) 200, 529 and 26. They represent fixation indexes $\theta$: 0.06 (1), 0.19 (2), 0.30 (3), and 0.35 (4), respectively. Inset (a): the normalized dune inactivation time $t_s/t_m$ diverges when $\theta \to {\theta }_c$ (solid line). Inset (b): the normalized final parabolic dune length $L/V^{1/3}$ is proportional to $t_s/t_m$. Dots indicate simulation results.Reuse & Permissions

Figure 3

Examples of parabolic dune shapes at fixation indexes $\theta$: (a) 0.16, (b) 0.22, and (c) 0.27. (d) is an example of an intermediate state at $\theta =0.38$ where the dune still is barchanlike, in sharp contrast to the rest. Each figure is compared with real examples from White Sand, New Mexico. Note the similarities in the contour lines market in red. Satellite images courtesy Google Earth/ DigitalGlobe.Reuse & Permissions

Figure 4

The fraction of available sand surface $1-⟨{\rho }_v⟩$, where $⟨{\rho }_v⟩$ is the mean vegetation cover density, shows an exponential decay during the evolution of a vegetated barchan dune for small $\theta$ when time is normalized by $t_m$. Inset: proportionality between the mean sand flux $⟨q⟩$, normalized by the saturated upwind value $Q$, and the fraction of active sand surface.Reuse & Permissions