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Desert, Desertification and Land Degradation
Published in Ajai, Rimjhim Bhatnagar, Desertification and Land Degradation, 2022
After the soil is eroded, it gets deposited at the other place, again through two means, water and wind. After the erosion and transportation, the eroded material gets deposited to form dunes and loess. Dunes are wind-blown sand mounds usually deposited near their source. The size, shape and arrangement of dunes depend on sand supply, direction, the velocity of the prevailing wind and the amount of vegetation. Barchan, longitudinal, transverse, parabolic and star dunes are some of the types of sand dunes. Barchan dunes are crescent-shaped dunes in the downwind direction, opposite to the upwind direction of parabolic dunes. They are formed when sand moves across the pavement. Transverse dunes are perpendicular to wind direction, formed in regions of abundant sand-like beaches. Longitudinal dunes are formed parallel to wind direction especially in the areas of limited sand. Figure 2.9 shows different kinds of sand dunes as captured on satellite images. Loess, too, is deposition by wind but at faraway places, particularly from dry to wet. It consists of layers of windblown silt and clay composed of quartz, feldspar, mica and calcite.
Desertification
Published in A. Monem Balba, in Arid Ecosystems, 2018
For simplicity, the sand dunes are classified into (Imbaby and Ashour24; Schou25): Longitudinal dunes: long dunes formed from several parallel lines. They are oriented parallel to the wind direction and extend excessive distances. This form is familiar in the Great Desert of Africa (the Sahara).Barchan dunes: or Transverse dunes are perpendicular to the wind direction and have the crescent or elliptical form.The Sandy Shades dunes: are formed when winds loaded with sand are faced with an obstacle such as a rock, thus the wind is deflated into two directions at both sides of the rock. A cyclonic wind is formed at each side of the obstacle causing a slower wind speed. Sand particles are deposited because of the slower wind speed forming two heaps of sand which soon join together into one heap with the deposition process of sand.Coastal sand dunes: when the wind blows from the sea side it moves the coastal sand and deposits it into dunes parallel to the coast. Plants growing along the coast prevent the sand movement.
Three-dimensional numerical simulations of barchan dune interactions in unidirectional flow
Published in Particulate Science and Technology, 2019
Xiaosi Zhou, Yuan Wang, Bin Yang
Barchan dunes are highly mobile dunes that possess a remarkable crescent shape with two horns that point downstream. These dunes are observed in vast arid regions (Bagnold 1941; Pye and Tsoar 2008) and at the bottom of seas, rivers and channels (McCulloch and Janda 1964; Hollister and McCave 1984; Kleinhans et al. 2002; Wynn, Masson, and Bett 2002; Pye and Tsoar 2008). Compared with aeolian barchan dunes, subaqueous barchan dunes show similar structures but at smaller temporal and spatial scales (Mantz 1978; Hersen et al. 2002; Hersen 2005; Groh, Rehberg, and Kruelle 2009; Franklin and Charru 2011). Therefore, reproducing crescent-shaped dunes in a laboratory is easier in water than in air, and many fluvial experiments have been conducted to reveal the morphological and dynamical properties of barchan dunes. Hersen, Douady, and Andreotti (2002) focused on reproducing small-scale barchan dunes by using a tray in a water tank and concluded that the inertial length of sediment transport can be used to describe differences in the morphological mechanisms of subaqueous and aeolian barchan dunes. Groh et al. (2008) performed subaqueous experiments to quantitatively validate an existing minimal model (Kroy, Sauermann, and Herrmann 2002), and the researchers emphasized the morphological properties of barchan dunes. Several other studies have focused on binary dune systems. A 2D coherent experiment conducted by Groh, Rehberg, and Kruelle (2009) showed that dunes driven by a unidirectional flow can ultimately reach the shape attractor. Such dune behavior was also observed in experiments completed by Franklin and Charru (2011) and Charru and Laval (2013). Experiments on the interaction of two or more barchan dunes have also been conducted (Endo, Taniguchi, and Katsuki 2004; Hersen 2005; Hersen and Douady 2005). Although interactions among subaqueous barchan dunes have been frequently observed in experiments, the related physical mechanisms remain debatable (Schwammle and Herrmann 2003, 2005; Duran, Schwämmle, and Herrmann 2005; Livingstone, Wiggs, and Baddock 2005; Parteli et al. 2014; Tsoar and Parteli 2016).