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

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.