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Although contemporary dust storms are frequent in arid and semi‐arid areas, desert loess deposits are poorly developed. Much of the World's loess occurs in mid‐latitude areas which experienced glaciation during the Pleistocene. Ocean core evidence indicates that dust transport from sub‐tropical deserts increased during cold stages of the Pleistocene, but loess formed only on certain desert margins, for reasons which have not been fully explained. This paper re‐examines the mechanisms of dust transport and deposition, and the circumstances leading to the accumulation of thick loess. Typical loess is composed mainly of medium silt grains which are transported in short‐term suspension a few metres above the ground. Significant thicknesses of loess form only when dust is trapped within a limited area, often relatively close to the source. Dust particles finer than 20 μm are transported mainly in long‐term suspension over a greater height range and may be widely dispersed. The availability of silt and the frequency, magnitude and direction of dust‐transporting winds are important factors governing the potential for loess formation, but the existence of a suitable dust trap is particularly important. Traps may be formed by topographic obstacles, areas of moist ground, or vegetated surfaces. Vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid‐latitudes during the Pleistocene. Dust blown during glacial periods from certain deserts, notably in Sinai, Soviet Central Asia and China, accumulated as loess in neighbouring semi‐arid regions. On the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps. During most of the Holocene net dust deposition rates in all desert‐marginal areas have been too low for significant loess accumulation. This is mainly due to a reduction in silt availability and a tendency towards landscape stability. Reported dust storm frequencies during the past 50 years over‐estimate the longer‐term Holocene dust flux due to the effects of human activities. Much modern dust owes its origin to erosion of cultivated soils in semi‐arid areas and is finer than typical loess.

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Field measurements were made on a longitudinal dune in the Sinai Desert in order to understand its morphology and dynamics. The field measurements contradicted the wind structure indicated by the helicoidal flow theory. Rather, it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank. The essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line. The occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line. When this angle is < 40° the velocity of the deflected wind is higher than on the crest line or the windward flank and longitudinal sand transport occurs. When the angle is less acute (> 40°) the velocity of the deflected wind drops and deposition takes place on the lee flank. The angle of incidence in each wind storm is changed intermittently between 30° and 100° along the dune because the dune meanders and because of the sinuous outline of the crest line. In this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line. As a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month. Peaks and saddles along the crest line advance at an average rate of 0.7 m per month. The lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition. This can explain the formation of peaks along the crest line of the dune.

The Mediterranean coastal dunes of Israel underwent a land-use change during the second half of the 20th century. Due to intense agricultural and pastoral activity, the coastal dunes were stripped of natural vegetation until the end of the first half of the 20th century. The barchan and transverse dunes were shaped by strong southwesterly winter winds. A decrease in human activity during the second half of the 20th century brought about a renewal of natural vegetation on the dune crest -the only area with neither erosion nor deposition. The establishment of vegetation on the crest changes the dynamics of these barchan and transverse dunes, so that not all of the sand eroded from the windward side is carried to the lee slip-face; some is trapped by plants. Consequently, there is a change in the shape of the windward slope from convex to concave, and the dune gradually becomes parabolic. In this study we trace the morphodynamics of the dunes by analysing 12 sets of aerial photographs, which were taken from 1944 to 1995. The average rate of advance of 15 dunes has decreased from 3Ð4 m a 1 to 1Ð9 m a 1 , while the vegetation cover has increased from 4Ð3 to 17 per cent during this period.

No abstract

Sand dunes can be active (mobile) or stable, mainly as a function of vegetation cover and wind power. However, there exists as yet unexplained evidence for the coexistence of bare mobile dunes and vegetated stabilized dunes under the same climatic conditions. We propose a model for dune vegetation cover driven by wind power that exhibits bistabilty and hysteresis with respect to the wind power. For intermediate wind power, mobile and stabilized dunes can coexist, whereas for low (or high) wind power they can be fixed (or mobile). Climatic change or human intervention can turn active dunes into stable ones and vice versa; our model predicts that prolonged droughts with stronger winds can result in dune reactivation.

[1] We provide several examples for the coexistence of active and fixed sand dunes under similar climatic conditions, namely, with respect to wind power and precipitation rate. A model is developed for dune vegetation cover that includes wind power, precipitation rate, and anthropogenic effects, such as grazing and wood gathering. The model reproduces the observed dune's bistability and shows that under intense human pressure and prolonged droughts the fixed dunes may turn active. Moreover, the model shows that the dune reactivation process is almost irreversible, as a fixed dune will become active only under the action of very strong winds and can then return to the fixed state only when wind power decreases far below the levels under which the initial dune maintained its stability. Similar hysteretic behavior of dune mobility is predicted by the model with respect to changing precipitation and human pressure parameters.