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Earth System Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/esd-2019-73
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esd-2019-73
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 04 Dec 2019

Submitted as: research article | 04 Dec 2019

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This preprint was under review for the journal ESD. A revision for further review has not been submitted.

Yardangs and Dunes: Minimum- and Maximum-Dissipation Aeolian Landforms

Ralph D. Lorenz Ralph D. Lorenz
  • Johns Hopkins Applied Physics Laboratory, Laurel, MD 20723, USA

Abstract. Yardangs are ridges formed in soft rock by aeolian erosion in a unidirectional wind environment, and often have a 4 : 1 length : width ratio that is associated with a minimum-drag shape for a given width. Dunes are emergent aeolian landforms formed by accumulation and removal of sand particles. Dunes have a range of morphologies (barchans, stars, linear, transverse etc.) which can be mapped to the sand supply and the diversity of wind directions. The dune pattern that generally emerges is one that maximizes gross bedform normal transport (GBNT). For fixed imposed wind speed, a minimum drag force corresponds to a minimum dissipation, whereas maximum sand transport corresponds to maximum dissipation. These examples illuminate a more general paradox in non-equilibrium thermodynamics, wherein entropy production rates may be maximized or minimized depending, vaguely, on the degrees of freedom in the system. In these geomorphological examples, however, the difference is informatively clear: whereas yardangs emerge simply by removal of material alone and dissipation is minimized, dunes form by the much less constrained removal and accumulation to maximize dissipation.

Ralph D. Lorenz

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Ralph D. Lorenz

Ralph D. Lorenz

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Short summary
There is a paradox between systems that maximize the production of work or entropy (e.g. those with many degrees of freedom like the climate) and those that minimize it (constrained ones, like a heated rod). How free is free enough? Desert landforms provide an instructive analogy – sand dunes are simultaneously built and destroyed, and thus are free and maximize sand transport or work. Yardangs form only by erosion, are more constrained, and emerge with streamlined shape, minimizing work.
There is a paradox between systems that maximize the production of work or entropy (e.g. those...
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