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Primary reference(s)

FAO, 2010. . Accessed 26 November 2019.

Additional scientific description

Different factors and processes foster the formation and movement of sand masses, such as violent wind blowing over large areas, sparse or stunted vegetation, and degraded soil that is mobile, dry or bare (Khalaf and Al-ajmi, 1993; FAO, 2010).

Sand particles in movement are the site of various interactions, the main ones being:

• Avalanche effect - the avalanche effect is the result of saltation. As the grains of sand fall back, they cause the displacement of a larger quantity of particles, so that the more intense the saltation process caused by the wind, the greater the number of particles set in motion, until a maximum or saturation point is reached, where the quantity lost is equal to the quantity gained at any given moment. The distance needed to reach the saturation point depends on the sensitivity of a soil to erosion: on a very fragile soil, it can occur over a distance of about 50 m, but requires more than 1000 m on a very cohesive soil (FAO, 2010).
• Sorting - the sorting mechanism concerns the wind’s displacement of the finest and lightest particles, leaving behind the larger particles. This process gradually impoverishes the soil, since the organic matter made up of small light elements is the first to be removed (FAO, 2010).
• Corrosion - corrosion is the mechanical attack on the surface as the sand-laden wind blows over it. In arid regions, it is the aggravating cause of soil erosion and is seen in parallel streaks or the polishing of rocks (FAO, 2010).

When the wind grows lighter, it loses its capacity to carry sand particles, which are then dropped (FAO, 2010).

Forms of sandy accumulation vary widely, depending on landform, the nature of the soil on which they encroach, the presence or lack of vegetation, and the size of the grains of sand (Hamdan et al., 2016).

Metrics and numeric limits

The scale of the event varies depending on the wind speed, which is an essential factor, for it determines the force of sand removal; the greater the speed, the greater the carrying capacity. The second factor is the size and density of sand particles. Sandy encroachments can vary from 50 cm in height, 150 cm in length and 40 cm in breadth, to 20 to 40 km long and 50 to 200 m wide (Al-Helal and Al-Awadhi, 2006). Sand encroachment reporting should indicate the accumulation location, scale, sand sources, and transport zones.

Examples of drivers, outcomes and risk management

All types of sand encroachment, no matter the size or duration, can create hazardous conditions affecting especially soil, vegetation, villages, roads (Boulghobra et al., 2015), railways and irrigation channels:

• The wind first carries off the finer parts of the soil, thus weakening the soil structure. As the soil becomes sandier, it is more vulnerable to the wind and has a reduced water retention capacity. Its colour turns from grey to white and then to red as it is scoured. The terrain is gradually broken up by the creation of small mounds surrounding the woody and grassy vegetation as this degrades. The land gradually becomes unsuitable for cultivation (FAO, 2010).
• The wind has both mechanical and physiological effects on vegetation: 
  • Mechanical effects. The soil particles that are carried off collide with stalks and leaves with a force that abrades their tissue. In the zones from which the particles are carried off, roots are uncovered, and the vegetation risks being uprooted, while in zones where the particles are deposited the vegetation is steadily buried (FAO, 2010).
  • Physiological effects. The wind increases evaporation and dries out plants, mainly in the Kuwait dry season. The air’s evaporating power is proportional to the square root of the wind speed. Moreover, the soil’s water retention capacity is reduced, leading to water stress. The surrounding or moving mass of dry air tends to absorb humidity and exacerbate water deficit and this deficit is the main factor determining local vegetation, inasmuch as the latter has to adapt to the severe shortage of water (Khalaf and Al-ajmi, 1993).