Dune formation under bimodal winds

Abstract

The study of dune morphology represents a valuable tool in the investigation of planetary wind systems-the primary factor controlling the dune shape is the wind directionality. However, our understanding of dune formation is still limited to the simplest situation of unidirectional winds: There is no model that solves the equations of sand transport under the most common situation of seasonally varying wind directions. Here we present the calculation of sand transport under bimodal winds using a dune model that is extended to account for more than one wind direction. Our calculations show that dunes align longitudinally to the resultant wind trend if the angle θ w between the wind directions is larger than 90 • . Under high sand availability, linear seif dunes are obtained, the intriguing meandering shape of which is found to be controlled by the dune height and by the time the wind lasts at each one of the two wind directions. Unusual dune shapes including the "wedge dunes" observed on Mars appear within a wide spectrum of bimodal dune morphologies under low sand availability.pattern formation | critical phenomena | aeolian transport | erosion and sedimentation T he majority of the world's sand dunes are formed by winds blowing from more than one wind direction: bimodal winds, which oscillate seasonally between two main directions, are typical of the largest sand deserts of our planet (1-7). Whereas the well-known barchans and transverse dunes appear under unimodal winds for low and high sand availability on the ground, respectively, seif dunes ( Fig. 1) are the prevailing dune type under bimodal wind regimes. These dunes form in the absence of vegetation and align parallel to the resultant wind trend, developing a sharp crest which explains the term "seif" ("sword" in Arabic) and an intriguing meandering shape which has challenged geomorphologists for decades. In recent years, understanding the dynamics of seif dunes has become an issue of major interest also for planetary scientists: fields of seif dunes occur seldom on Mars (8, 9), which led to the hypothesis that Mars wind systems are essentially unidirectional. Is the appearance of seif dunes, indeed, related to the wind regime only? Moreover, are seif dunes the only dune type shaped by bimodal winds? The results of the present work provide insights into these challenging questions.It is still unknown whether other factors rather than wind directionality could also be relevant for dune alignment (10, 11) and which factors determine the meandering shape of seif dunes. Besides, very little is known about the dune morphology resulting from bimodal winds under conditions of low sand availability. In spite of many theoretical advances in the past, a model that solves both the three dimensional equations of aeolian sand transport and computes the evolution of dunes under bimodal wind regimes was lacking (12)(13)(14)(15)(16)(17).In the present work, we extend a well-developed and tested dune model (18,19) to investigate the formation of dunes under bimodal wind r...

Lateral vegetation growth rates exert control on coastal foredune hummockiness and coalescing time

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Process-Based Modeling of Coastal Dune Development

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“…Figure 2. Top view of longitudinal dunes emerged due to a bimodal wind and sand dunes on bedrock with varying angles between directions and characteristic duration (scaled) of these conditions (Parteli et al, 2008).…”

Aeolian processes on the terrestrial planets

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