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Geomorphology

Flow and form

Dune fields often exhibit complex patterns of vegetation and morphology over relatively short distances. An analysis of the White Sands dune field in New Mexico attributes the shift in dune form to the development of an internal boundary layer over the rough dune-field surface.

Aeolian dunes form in areas where sand is abundant, and wind flow is sufficient to mobilize and transport it. On Earth, a confluence of these two factors typically occurs in some coastal regions and in desert basins. Dune fields, particularly those in deserts, can extend from tens to hundreds of kilometres and form over long time-spans that often cover substantial changes in regional climate. It is therefore unclear whether the shape of dune fields that we observe today is the result of current environmental conditions1. Writing in Nature Geoscience, Jerolmack et al.2 show that the complexity of the dune field at White Sands, New Mexico, can be explained by modern processes, specifically the development of an internal boundary and its effects on sand redistribution.

The moderately sized dune field in White Sands National Monument covers about 350 km2 (Fig. 1). Winds come from the southwest, and the field is covered by two types of dunes: barchanoid ridges and parabolic dunes. Both forms are typical of environments with airflow from a single, prevailing direction. Upwind of the dunes is the White Sands Alkali Flat, a vegetation-free plain stripped of all loose sand above the shallow water-table. The transition from flat to dune field is marked by the presence of a dune ridge. The ridge marks the start of a series of barchanoid ridges, which continues for about 7 km. The subsequent transition to parabolic dunes occurs over a short distance. The dune field abruptly terminates in a plain marked by herbs and shrubs. The field essentially has a closed sediment budget, with the downwind progression of dunes leaving behind a sand-barren addition to the Alkali Flat.

Figure 1: Patterns in White Sands.
figure1

a, The White Sands dune field in New Mexico contains two dune types separated by an abrupt transition. b,c, Jerolmack et al.2 demonstrate that the transition from barchanoid ridges (b) to parabolic dunes (c) can be attributed to a downwind decline in sand flux caused by the development of an internal boundary layer. Decreased sand movement allows more vegetation to colonize the area between dunes, promoting the transition to parabolic dunes. In these images, airflow is from left to right. The distance from the lee slope of one barchanoid dune ridge to the next is 120 m (b), and the distance across the parabolic dune in the forefront of c is 150 m. (Images from Google Earth.)

Airflow is driven by regional pressure gradients, and it is the pattern of irregularities on the underlying surface — known as roughness — that controls the friction exerted on the surface by that flow. In dune fields, the transition from the relatively flat plain to the roughness of the sand hills causes air speed near the surface to drop. The decline in speed will slowly move upwards through the air column as the flow progresses across the field, triggering the development of an internal boundary layer3.

Jerolmack et al.2 hypothesize that it is the presence of this internal boundary layer that drives the morphology of the White Sands field. In their scenario, the internal boundary layer generated by the transition from the Alkali Flat to the ridges causes the energy available for sand transport to decline gradually as the air flows downstream. Based on the well-established fluid dynamics of internal boundary layers4, Jerolmack et al. created an analytical expression that describes the atmospheric dynamics over the dune field. Their calculations suggest that bed stress declines with distance from the upwind border of the field. They then used data from meteorological stations near the White Sands to calibrate atmospheric parameters for this setting. Once they determined bed shear stress, it was straightforward5 to predict the variability of sand transport across the dune field.

The study relies on a number of simplifications, but nevertheless the predicted sand transport compares well to measurements of transport rate derived from detailed LIDAR-mapping of surface topography. Moreover, the primary results can be tested against measurements of annual sand flux or deposition rates derived from optically stimulated thermo-luminescence6.

The calculations presented by Jerolmack et al. can also explain — but not predict — the transition from barchanoid to parabolic dunes at White Sands. This sudden transition in dune type is an example of a geomorphic threshold: vigorous sand movement prevents the establishment of plants. However, as the movement drops below a certain threshold, plants successfully root and colonize the area between dunes. This alters sand availability, and favours the formation of parabolic dunes (Fig. 1).

The successful colonization of vegetation also affects the water table in White Sands. In the sparsely vegetated areas between the barchanoid ridges, the water table lies about 50 cm below the surface. Yet between the parabolic dunes, it extends to a depth of 1 m, thanks to the roots of the denser vegetation. The water beneath unvegetated dunes is also more saline than that below vegetated ones, pointing to a tantalizing interplay between biogeochemistry, vegetation and dune form that remains to be identified.

Nevertheless, the model proposed by Jerolmack et al.2 provides an explanation for the complexity of dune form and vegetation found in the White Sands, and holds promise for extrapolation to other dune fields.

References

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Correspondence to Keld R. Rasmussen.

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Rasmussen, K. Flow and form. Nature Geosci 5, 164–165 (2012). https://doi.org/10.1038/ngeo1408

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