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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 {theta}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.

Sand is blown across beaches and deserts by turbulent winds. This seemingly chaotic process creates two dominant bedforms: decametre-scale dunes and centimetre-scale ripples, but hardly anything in between. By the very same process, grains are constantly sorted. Smaller grains advance faster, while heavier grains trail behind. Here, we argue that, under erosive conditions, sand sorting and structure formation can conspire to create distinct bedforms in the ‘forbidden wavelength gap’ between aeolian ripples and dunes. These so-called megaripples are shown to co-evolve with an unusual, predominantly bimodal grain-size distribution. Combining theory and field measurements, we develop a mechanistic understanding of their formation, shape and migration, as well as their cyclic ageing, renewal and sedimentary memory, in terms of the intermittent wind statistics. Our results demonstrate that megaripples exhibit close similarities to dunes and can indeed be mechanistically characterized as a special type of (‘reptation’) dune.

Vegetation cover on sand dunes mainly depends on wind power (drift potential—DP) and precipitation. When this cover decreases below a minimal percentage, dunes will start moving. It is therefore necessary to study the effects of DP and precipitation on contemporary dune activity in order to predict likely future dune mobility in the coming decades. We concentrate on the future activity of the currently fixed dune fields of the Kalahari and the Australian deserts. These sand seas include the largest areas of stabilized dunes in the world, and changes in their mobility have significant economic implications. Global maps of DP are introduced, based on real and reanalysis data. Analyses of two global circulation models (GFDL and CGCM3.1) provide future predictions under the SRES-A1B IPCC scenario, which is a moderate global warming scenario. According to the GFDL model, both the Australian and Kalahari basin dunes will apparently remain stable towards the end of the 21st century because the DP will stay small, while the rate of precipitation is expected to remain much above the minimal threshold necessary for the vegetative growth that leads to dune stabilization. The CGCM model predicts insignificant changes in DPs and shows that the precipitation rate is above 500 mm/year for almost the entire Kalahari basin. The central-northern part of Australia is predicted to have larger DPs and greater precipitation than the southern part. Since the predicted changes in DP and precipitation are generally not drastic, both the Australian desert and Kalahari basin dunes are not likely to become active. Still, the Australian dunes are more likely to remobilize than the Kalahari ones due to some decrease in precipitation and an increase in wind power.

Barchan dunes—which have a crescent shape with two horns pointing downwind—may undergo a transition to a longitudinal (seif) dune under a bimodal wind regime. Understanding the barchan–seif dune transition is important for the research of dune field evolution and for the investigation of planetary climate and wind regimes. Two models have been proposed to explain the barchan–seif dune transition: Bagnold (The physics of blown sand and desert dunes. Methuen, London, 1941 ) and Tsoar (Z Geomorphol 28:99–103, 1984 ). The significance of both models has been investigated through much field and modeling works over the last few decades. However, the conditions for the barchan–seif dune transition as well as the models proposed to explain it are still poorly understood. To correct this situation, here we present and discuss some examples of asymmetric barchans and barchan–seif transitional dune morphologies occurring in nature and show how to characterize wind regimes and identify the relevance of different factors leading to the observed patterns (in addition to wind directionality). Bagnold’s and Tsoar’s models were conceived to explain the barchan–seif dune transition under asymmetric bimodal winds. They were not conceived to explain all types of barchan asymmetry. However, these models must be evaluated in the light of an insight that has been gained more recently, from field investigations, experiments and numerical simulations: The seif dune forms only if the divergence angle between the two main wind directions is ≥90°.

Recent research suggests that, under unconstrained human circumstances, pastoral nomads within arid environments have at their disposal means of evading ecological stress that could impel them to cause damage to their grazing and land resources. The Israeli-Egyptian border has imposed a severe constraint upon the range management strategy of the Bedouin whose traditional territory it bisects. The border forced them to exert an increased pressure upon local resources. Considerable damage was thus caused to the perennial vegetation cover (both macrophytes and microphytes) and to the structure of sand dunes on the Egyptian side of the border, with opposite effects on the Israeli side to which the Bedouin had no access. This case study adds a further dimension to the discussion of range management by pastoral nomads in arid and semi-arid areas.

Sand dunes that are subjected to different wind directions attain different steady state profiles. Two fundamental types of profile are known: the convex asymmetric one, characterizing barchans and transverse dunes, and the symmetric triangular one of seif dune. The distribution of wind velocity along these two fundamental profiles indicates that the convex profile of barchan and transverse dunes are in a steady state to uni-directional winds. A steady state profile having a separate crest and brink ensues a higher advance of slip-face relative to the windward slope. A triangular sharp-edged profile of seif dunes is not in a steady state under long-term perpendicular uni-directional wind. Steady state can be attained, however, by bi-directional winds of oblique approach to the dune. In this case seif dunes are in a steady state with respect to profile oblique to dune axis and parallel to wind direction. A sharp-edged profile causes a substantial separation of perpendicular flow over the lee slope. The separation vortex reduces the wind-velocity's rate of increase near the crest line which keep its sharp profile during a short period of perpendicular wind. In the case of the wind shifting to a perpendicular direction for a longer period, the triangular shape will turn into a convex asymmetric one.

Barchan dunes --- crescent-shaped dunes that form in areas of unidirectional winds and low sand availability --- commonly display an asymmetric shape, with one limb extended downwind. Several factors have been identified as potential causes for barchan dune asymmetry on Earth and Mars: asymmetric bimodal wind regime, topography, influx asymmetry and dune collision. However, the dynamics and potential range of barchan morphologies emerging under each specific scenario that leads to dune asymmetry are far from being understood. In the present work, we use dune modeling in order to investigate the formation and evolution of asymmetric barchans. We find that a bimodal wind regime causes limb extension when the divergence angle between primary and secondary winds is larger than \\(90^\\), whereas the extended limb evolves into a seif dune if the ratio between secondary and primary transport rates is larger than 25%. Calculations of dune formation on an inclined surface under constant wind direction also lead to barchan asymmetry, however no seif dune is obtained from surface tilting alone. Asymmetric barchans migrating along a tilted surface move laterally, with transverse migration velocity proportional to the slope of the terrain. Limb elongation induced by topography can occur when a barchan crosses a topographic rise. Furthermore, transient asymmetric barchan shapes with extended limb also emerge during collisions between dunes or due to an asymmetric influx. Our findings can be useful for making quantitative inference on local wind regimes or spatial heterogeneities in transport conditions of planetary dune fields hosting asymmetric barchans.