Introduction

A barchan dune is a slowly moved crescent-shaped sand body formed by winds that moves the sand on its windward side and forms a slip face on the lee side (Bagnold 1941; Cooke et al. 1993). Barchan has been published under different names, e.g., barcan, barchan, barkan, and barkhan (Glenn 1979). Barchan dunes are developed in areas with low sand availability and unidirectional wind. They occur in coastal and desert areas and can be formed from sand, snow, or a combination of both (e.g.,Beadnell 1910; Lancaster 1989; Pye and Tsoar 1990). They tend to occur in two main areas: (a) on the margins of sand seas (e.g., Smith 1980; Sweet et al. 1988); (b) in areas located in between sand source zones with depositional regions (e.g., Bagnold 1941; Embabi 1982). Generally, barchan dunes do not exist isolated but mainly occur in long dune fields parallel to the prevailing wind direction. However, individual barchans are unstable and continuously growing or shrinking (Hersen et al. 2004).

The aeolian sand deposits cover a large area of Egyptian deserts, amounting to c. 261,941 km2, or c. 26.2% of the total area of Egypt, and they are concentrated in six sand seas and eight dune fields (Bubenzer et al. 2020). Five sand seas and four dune fields exist in the Western Desert, with an area of c. 246,654 km2, or c. 36.3% of the Western Desert area, representing about 94.2% of the total sand forms in Egypt (Hamdan and Hassan 2020). The Great Sand Sea, Salima Sand Sea, and the dunes field south and southeast of the Qattara Depression represent the most significant sand accumulations, with an area of about 83.7% of the total sand deposits in the Western Desert. The aeolian dunes of Egypt, in general, and in the Kharga Depression, in particular, have been studied since the beginning of the twentieth century. Beadnell first studied and mapped the distribution of aeolian dunes in the Western Desert of Egypt (1910). The morphology of the barchans of the Kharga Depression was first studied by Harding King (1918), who measured the slope form of six barchans in Kharga. The first attempt to estimate the rate of movement of Egyptian desert dunes was made by Cornish (1900), who measured a rate of 4.5 m/year in the dune crests east of the Nile Delta. Another estimation of the barchan migration at the Kharga Depression was carried out by Ball (1927, 1939), who estimated a mean southward advance of 10 m/year. El-Baz and his coworkers presented a comprehensive outline for aeolian dunes in the Western Desert using remote sensing techniques (e.g., El-Baz and Maxwell 1982; El-Baz 1992; El-Baz et al. 1998). Intensive studies of the aeolian dunes of the Kharga Depression were carried out by Embabi using field and remote sensing data (1976–1977; 1982; 1986–1987). The textural and mineralogical composition of the aeolian sand of the south Western Desert of Egypt was intensively studied by Hamdan and his coworkers (e.g.,Hamdan 2003; Hamdan et al. 2015, 2016; Refaat and Hamdan 2015; Ramadan et al. 2022).

Barchan dunes in the Egyptian deserts are most vulnerable to wind erosion and deposition; consequently, they constitute a severe threat to agricultural development, rural and urban settlements, road traffic, and public health. The rareness of rainfall, active winds, soil instability, and scarcity of plant cover increased this detrimental phenomenon, especially in the desert areas due to overgrazing and cultivation of marginal land, which led to severe disturbance of the natural equilibrium of the ecosystem (Hegazi et al. 2005).

The current paper aims to evaluate the morphometric parameters and migration rate of Kharga barchans and compares these parameters with those in different deserts worldwide. We also testify to the impact of the different factors (i.e., wind, topography, vegetation, and sand supply) on the barchans’ development and migration rate. The impact of specific parameters, e.g., dune height, width, and grain size, on the dune migration rate is also considered. The current paper also aims to monitor the environmental risk of the Kharga barchans on the roads, urban areas, and archaeological sites.

Geomorphology of the Kharga Depression

Kharga is a natural depression on the surface of the limestone plateau west of the Nile Valley (Figs. 1 and 2) that extends in the N–S direction and is bounded on the east, north, and partly on the west by vast escarpments rising 300–450 m above the depression floor. The geomorphic features exhibited in the Kharga Depression are classified into (1) limestone plateau, (2) escarpments and foothill slopes, and (3) the depression. The southern and western boundaries of the depression are not well defined (El-Shazly 1976). They consist of soft Late Cretaceous clastic rocks. The eastern escarpment runs almost in the N–S direction. It is dissected by numerous gullies directed into the depression (Fig. 3). An upper steep face of white limestone marks the northern escarpment.

Fig. 1
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Location map of the Kharga Depression

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Fig. 2
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Distribution of dune belts in Kharga Depression; (A–C) upper parts of western, central, and eastern belts; (D) middle part of western dune belt; (E) the southern part of the western belt

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Fig. 3
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Shaded relief model showing the location of the studied barchan sites along the Kharga dune field

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The plateau is occupied by limestone rocks dated mainly to the Early Eocene. It attains an elevation of c. 350 m above the depression floor. The surface of this plateau is rough and slopes generally in the eastward direction. The eastern plateau bounds the Nile Basin on its western side, while the western plateau is elevated more than 400 m above the depression floor and is dominated by Late Cretaceous chalky limestone. The depression exhibits an elongated shape, extends in the N–S direction, and slopes gently toward the axial part of the depression at a gradient of c. 7 m/km (El-Shazly 1976). The axial portion of the Kharga Depression is essentially occupied by lacustrine deposits, which are developed into extensive flats or occur in the form of isolated yardangs oriented parallel to the prevailing wind direction in the N–S direction.

Materials and methods of study

The present study is essentially based on remote sensing and GIS techniques. Google Earth historical satellite images are used as a base for the dune morphometric and migration calculations.

Fieldwork includes measurements of morphometric parameters of 30 dunes, along with collecting 103 sand samples from crests of the studied barchans. Several characteristic parameters were measured for selected 160 barchan dunes from 37 sites along the Kharga dune belt (Fig. 3). The dune belt is subdivided geographically from the north to the south into three sectors: the northern sector (latitude 25.730́°–25.517°; sites 1–5; n = 23 barchans), the central sector (latitude 25.478°–24.994°; sites 6–30; n = 102 barchans), and the southern sector (latitude 24.981°–24.527°; sites no. 31–37; n = 35 barchans). The highest urbanization and cultivation activities in the Kharga Depression characterize the central sector.

Among the 160 barchan dunes, 104 dunes are measured for their morphometric parameters. The morphometric parameters measured include dune width, dune length, windward length, slip-face length, and length of each horn (as used in Hamdan et al. 2016). In the field, the morphometric measurements of a small sample (10 barchans) were manually measured using a measuring tape (50 m). The heights of these barchans were calculated from the measurements of the length of the slope faces using a measuring tape, and the corresponding angles of the slope face were measured by Brunton compass. We found that the measured heights are correctable with those calculated heights using Finkel’s (1959) equation. Therefore, we measured the heights of all studied barchans using Finkel’s (1959) equation. The empirical equation of Finkel (1959): W = 10.3 H + 4.0, where W is the dune width. Dune volume V was calculated as a half pyramid using the width and height of the dune (Hersen et al. 2004) as follows:

$$V=1/6W2\times H$$

where V is the dune volume, W is the dune width, and H is the dune height.

The perimeter and area of the studied dunes, as well as the dune densities, were also measured using ArcGIS software. All the previously mentioned parameters were measured on registered high-resolution Google Earth images. A total of 103 sand samples were collected from the dune crests to determine sedimentary characteristics and the engineering parameters of 30 barchan dunes. Statistical grain size parameters were calculated using Folk and Ward (1957). They include graphic mean size (MZ), inclusive graphic standard deviation (sorting) (σ1), inclusive graphic skewness (SKI), graphic kurtosis (KG), and median size (Md).

The available Google Earth images of the barchan dunes of the studied area vary in resolution from 8 to 4 m/pixel and are dated from 2002 to 2018. The extracted images are geometrically registered (their coordinates are identified) using the ArcGIS software. The registration process is carried out by locating eight control points of known coordinates on the Google Earth image and used as a layer of reference points in the ArcGIS database. Barchans often migrate long distances with only minor changes in their form (e.g.,Finkel 1959; Long and Sharp 1964; Hastenrath 1967; Sauermann et al. 2000). The dune migrated distance and direction were measured in well-defined locations visually selected on the dune boundaries in both the older and younger images. The distance between two corresponding points indicates the point movement during the specified timespan. The distance between two corresponding points and the direction of the line connecting them was obtained automatically by ArcGIS. The exported data include the direction and length of each line connecting corresponding points.

Thirty-six test locations (Fig. 3) enclosing the selected barchan dunes were chosen within the northern, central, and southern sectors of the Kharga Depression based on visual interpretation of satellite images. The mean amount and direction of migration of all selected points for the test areas in the study area are calculated. The drawn interpreted maps of dates ranging between 2002 and 2018 of chosen example locations in the study area.

Results and discussion

Distribution of barchan dunes in the Kharga Depression

In the Kharga Depression, aeolian deposits attain an area of c. 1600 km2, where the barchan dunes are one of the most critical sand shapes that spread on its floor and sides. In turn, they represent one of the geomorphological phenomena in the study area, and their movement represents one of the natural geographical problems facing the manifestations of urbanization in the depression. Dunes in the Kharga Depression mainly exist in the eastern and central parts of the depression and on the plateau to the north (Fig. 2). Dunes on the depression are arranged in discrete patterns extended in a north–south field and consist mainly of barchan dunes (Bagnold 1941; Gifford et al. 1979; Embabi 1982). Because the wind within the depression comes consistently from the north, the Kharga dunes proceed southward, maintaining the form of the individual belts. Barchans are the most common dune forms and are arranged into three parallel belts (western, central, and eastern belts (Fig. 2A–C)), extending from north to south in a manner consistent with the direction of the prevailing winds.

The western belt is the longest and largest within Kharga Depression (Fig. 2A), with a total length and width attaining c. 195 km and 19 km, respectively. This belt consists of three parts from north to south; the northern part is represented by narrow sand streaks extended from the northwestern escarpment to the south of Gebel Tarif for c. 20 km (at latitude 25.730́°–25.517°). Figure 2A shows c. 7 of these sand streaks; each attains a mean width of about 1 km and is separated by corridors with an average width of c. 1.5 km. The northern part of the western belt is dominated by simple barchan dunes in its western part and complex barchanoids in its eastern part. The sand streaks of the northern part are joined together in the central part in one massive dune belt (latitude 25.478°–24.994°) and extend in a north–south direction for c. 70 km in length and 3 km in width (Fig. 2D).

The central belt (Fig. 2B) is divided into three sub-belts: (a) east of the western belt, extends from the eastern end of the northwestern escarpment to the Kharga Oasis and begins in the form of an immobile linear dune for 5 km, then turns into mobile barchans. (b) Belt begins in the far north of the depression in the form of a group of separate barchans and continues for c. 2.5 km. (c) A belt consisting of several complex barchans and barchanoids extending at the east of the belt.

The eastern belt extends adjacent to the foot slope of the eastern escarpment of the depression (Fig. 2C) for 25 km in the form of separate sand streaks. It collides with the delicate terrain of the eastern edge, especially east of the Baris Plain, and takes different forms. It seems that the three belts of the Kharga Depression are joined together in one thick sand sheet (c. 40 km) near the southern edge of the depression (Fig. 2E).

Morphometric parameters of Kharga barchans

Morphometry can classify, discriminate, and describe barchan dunes and study relationships between development, migration, and environmental aspects (Long and Sharp 1964). Sauermann et al. (2000) used morphometric relationships (i.e., height, width, and length) and suggested that a fixed relationship can occur within a particular dune field. In the current study, we measured main morphometric parameters, such as width, length, height, length of windward and leeward sides, the length of eastern and western horns, and the direction of barchan movement. These measurements are performed to test their interrelations, impact on the migration rate, and geographical distribution variations.

Width, length, and height of Kharga dunes

The measured barchan dunes in the Kharga Depression are characterized by a variable range of morphologic parameters (Figs. 4, 5, 6, and 7; Table 1). The horn-to-horn dune width of all studied dunes varies between 453.6 and 32.9 m, with a mean value of 146.9 m. The northern sector exhibits lower mean width values of 130 m of the studied dunes. The mean width values increase drastically in the central sector, attaining 165.1 m, then slightly decrease in the southern sector, measured as 112.7 m.

Fig. 4
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A–D Rose diagrams of migration direction of the Kharga barchans. A All dunes in the study area. B Northern sector. C Central sector. D Southern sector. E Percentages of asymmetry of Kharga barchans

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Fig. 5
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The shape of the Kharga barchans. A Type of barchan. B Slimness of Kharga barchans

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Fig. 6
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Relationship of the morphometric parameters of Kharga barchans. A Relationship between the length and width of Kharga barchans. B Relationship between length and height. C Relationship between the height and dune area. D Relationship between dune height and dune spacing

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Fig. 7
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A Histogram showing the variation of barchans dunes heights in different sites worldwide localities. 1. Present study; 2. The Western Desert, Egypt (Beadnell 1910); 3. Kharga Oasis, Egypt (Stokes et al. 1999); 4. Pampa de la Joya, Peru (Finkel 1959); 5. Salton Sand Sea, California (Long and Sharp 1964); 6. Pampa de la Joya, Peru (Hastenrath 1967); 7. Jafurah Sand Sea, Saudi Arabia (Fryberger et al 1984); 8. Pampa de la Joya, Peru (Hastenrath 1987); 9. Kuiseb River delta, Namibia (Slattery 1990); 10. Eastern Sahara (Haynes 1989); 11. Jafurah Sand Sea, Saudi Arabia (Shehata et al 1992); 12. Qatar Peninsula (Embabi and Ashour 1993); 13. Salton Sand Sea, California (Haff and Prestim 1995). B–D Relation between length, width, and wavelength of the Kharga dune field and those from different dearests around the world. B Length–width relationships. C Dune wavelength and dune width. D Dune wavelength and length

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Table 1 Morphometric parameters of Kharga barchans
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The barchan dunes of the Kharga Depression range in length from 55.3 to 688 m, averaging 199 m. The pattern of length values of the studied dunes is similar to that of the width values, where the mean length in the northern, central, and southern sectors are measured as 169.4, 220, and 165.8 m, respectively. The mean value of the height of all studied dunes is measured as 13.88 m (maximum 43.7; minimum 2.8) and measured in the northern sector as 12.2 m. In the central sector, the mean height values are measured as 15.6 m and 10.5 m in the southern sector. The barchan dunes in the Kharga Depression, which represent the northern extension of the same barchan fields in the Toshka area (Hamdan et al. 2016), also exhibit the same wide range in dimension but tend to be much smaller (Table 1).

Although Bagnold (1941) suggested that the maximum height of barchans is c. 30 m, most published barchan heights are much smaller (cf.Finkel 1959; Long and Sharp 1964; Hastenrath 1967, 1987). Generally, the mean heights of the Kharga barchans are smaller than those of the Abu Moharik and Toshka dune belts to the north and south of the depression and lie in the middle of the range recorded from studies around the World (Fig. 7A).

The current measurements of the morphometric parameters are compatible with the previous publications that concluded that barchan dunes have a maximum height of 1/10th of both the length and width (Finkel 1959; Hesp and Hastings 1998).

Dune perimeter, area, volume, and density

The mean perimeter and the area measured for 375 dunes, including barchan dunes and other dune forms recorded in the studied sites of the Kharga Depression, are 0.66 km (maximum 3.03 km; minimum 0.062 km) and 0.0284 km2 (maximum 0.246 km2; minimum 0.002 km2), respectively (Table 2). The northern and southern sectors show a mean perimeter of 0.62 and 0.51 km, and areas of 0.0159 and 0.021 km2, respectively. The dunes of the central sector are more significant than the dunes of the other two sectors, with a mean perimeter and dune area of 0.77 km and 0.0347 km2, respectively.

Table 2 Dune perimeter, area, density, volume, percent, and spacing in the study sectors
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The mean dune density (number of dunes/km2) and percentage of area occupied by sand in all sites in the study area are 12.62/km2 and 20.4%, respectively. The volume of all studied dunes varies between 1497.37 × 10−6 and 0.51 × 10−6 km3, averaging 123.71 × 10−6 km3. The volume of the studied dunes is more extensive in the central sector, where it is 152.78 × 10−6 km3, than the mean volume of dunes in the northern and southern sectors, which attains 76.15 × 10−6 km3 and 87.3 × 10−6 km3, respectively (Table 2).

Dune spacing

Crest to crest spacing of the Kharga barchan dunes shows a mean value of 264.8 m (maximum 828.9 m and minimum 52.3 m). Generally, the largest dunes, often of compound form, are found on the western side of the Kharga dune field with dune spacing greater than 350 m. In the east of the dune, field spacing decreases to less than 200 m and averages 120 m. The mean dune spacing shows an increased pattern at the central part of the depression; it attains about 194.5 m in the northern sector, about 291.6 m in the central sector, and 246.8 m in the southern sector.

Migration direction, asymmetry, and dune collision

Figure 4A shows the direction of migration of all studied barchans (n = 160), which is mostly in the S–SE direction (S5°E), with a subsidiary S–SW direction. The dune migration direction in the northern and central sectors is similar to that of all studied dunes with little modification (Fig. 4B, C). In the southern sector, the direction of migration is measured as S15°E with subordinate directions of S5°E and S5°W (Fig. 4D).

Barchan dune asymmetry denotes the extension of one barchan horn downwind. This phenomenon often displays an asymmetric shape of the barchan dunes, a common form of dunes on Earth, and also occurs on Mars (Bourke 2010). Bourke (2010) classified dominant horns of asymmetric barchans into beaded, linear, and kinked. Linear horns are usually straight, while kinked horns usually display a curved or angled morphology, and beaded limbs represent a line of beads extended in the downwind direction. The asymmetric barchans of Kharga belong to the former type (i.e., linear).

About 87% of the studied barchans of the Kharga Depression are asymmetric, and only 13% are symmetric to nearly symmetric. The length (from the center of the crest) of the eastern and western horns for 104 barchans was measured (Table 1). The mean length of all measured dunes’ eastern and western horns are 89.7 m and 92.1 m, respectively.

The studied barchans are mostly east-horn dominated (c. 53%), whereas 34% of the studied barchans are west-horn dominated (Fig. 4E). In the northern sector, 25% of the studied barchans are symmetric; 44% are of eastern dominant horn, and 31% are western dominant horn (Fig. 4E). In the central and southern sectors, the same pattern is also observed, where the eastern horn is dominant. However, it was found that the mean of the western horns’ lengths is slightly shorter than the eastern horn (Table 1). The ratio of the lengths of the eastern horns to the western horns of the studied barchans is 0.99, probably due to the eastward slope of the floor of the depression. This ratio of elongation of the horns to the east is also probably the result of the side effect of a small component of the wind blowing transversely to the predominant direction. The ratio of the length of the eastern horn to the length of the western horn was plotted against the height of the dune for all the measured cases, and no correlation was found.

There are four main factors causing barchan asymmetry: local wind regime, topography, dune collision, and variable sand supply (Bourke 2010). It seems that the dune collision and sand supply are the most important factors affecting the dune asymmetry of the barchans of the Kharga Depression. The existence of the east-limbed and west-limbed dunes in the studied field supports the fact that the barchan collisions and heterogeneous sand supply cause an alternate elongation of the left and right limbs throughout the field.

The shape of Kharga barchans

McKee (1979) classified barchan dunes into simple (solitary), compound, complex, or barchanoid ridges. Simple (solitary) dunes are the basic form of the barchan’s dune type. Compound dunes are formed by the superimposition of dunes of the same morphological type (Breed and Grow 1979; Lancaster 1989). On the other hand, complex dunes are formed when dunes of two different types are superimposed or merged (e.g., barchan dunes on the flanks of larger linear or star dunes). Kocurek et al. (1992) mentioned that as sand supply increases, several barchans coalesce to form barchanoid ridges that consist of a series of connected crescents in plan view. Among 375 dunes in the studied Kharga dunes, 76% are simple (solitary) barchan type, 20% are compound barchans, 1% are complex dunes, and only 3% are barchanoids (Fig. 5A).

Slimness

The slimness value of barchan dunes is a ratio of the length of the windward slope and the horn-to-horn width, classified by Long and Sharp (1964) as slim = 0.15–0.35, normal = 0.43–0.58, pudgy = 0.64–0.76, and fat = 0.89–2.02). Bourke and Goudie (2009) reclassified dune shape ratios to incorporate a wider range of values (e.g., slim = 0.125–0.375; normal = 0.376–0.625; pudgy = 0.626–0.875; fat = 0.876 to > 1.0). Figure 5B shows that the studied barchan dunes (c. 104 dunes) in the Kharga Depression are mostly pudgy (44.7%) or fat (35.9%), little normal (15.5%), and rare slim (3.9%). The barchan dunes in the northern sector are dominated by normal (37.5%) and fat (31.3%). In the central sector, pudgy barchans are the most abundant (57.6%), followed by fat barchans (32.2%) and very few normal types. In contrast, slim dune types are not recorded (Fig. 5B). In the southern sector, fat (44.4%) and pudgy (37.0%) are the most abundant dune types, and a few normal dunes (14.8%) are also recorded.

Comparing the slimness of the studied barchans with other dunes in different sites in Egypt and worldwide indicates that the Kharga dunes seem to be much smaller than other barchans not only in Egypt (Toshka dunes, Hamdan et al. 2016) but also in other deserts, e.g., in Kuwait, the Atlantic Sahara, southern Morocco, and Peru.

The relationships between morphometric parameters

The correlation matrix of the studied morphometric parameters of Kharga barchans is listed in Table 3, and their relationships are cited in Fig. 6. This figure shows a positive correlation between dune heights, width, lengths, and dune-occupied areas. Most of the relationships are strongly positive and moderate as linear (R2 ranges between 0.69 and 0.89) with correlation coefficients between 0.94 and 0.99. The best model representing the relationships between them is the linear model depending on the R2 values (see Fig. 6A and B). It is obvious that as the height of the barchan dunes increases, other parameters increase too.

Table 3 Correlation matrix of the morphometric parameters and rate of migration of Kharga barchans
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Moreover, the positive correlation between dune height and dune width and area (Table 3; Fig. 6C) is attributed to the fact that the sand transporting wind tends to diverge on the windward side of a barchan and increase the width of the dune (Hastenrath 1967). With a stable sand supply and constant direction of wind regime, the barchan will grow progressively, including all morphometric parameters. Moreover, the sand supply affects the size of the barchans, which supports the growth of the barchan dunes. Barchans need to be supplied with sand at a rate that exceeds the erosion rate from their windward slope (Warren 1988). Generally, the strong winds led to a larger amount of sand that could be moved, so the larger the dunes formed (Fryberger 1979). The size and form of the studied barchan dunes are also influenced by other factors such as wind velocity, variability of wind direction, grain size saturation of the interdune sand flow, and aerodynamic roughness (Howard et al. 1978). Trap efficiency, where the very closely spaced dune belts are more efficient at trapping sand than large, spaced ones (Lettau and Lettau 1969). The roughness of the windward slope is correlated with the dune height, where the greater roughness of the slope always occurs in the higher dunes (Howard et al. 1978). With all other things being equal, the height a dune has reached will depend on the time it has to grow (Wilson 1973).

There is a good positive correlation (R2 = 0.69) between dune height and spacing (Fig. 6D). Similar relationships have been established by Wilson (1972) for Saharan dunes and by Lancaster (1982) for linear dunes in the main Namib sand sea (R2 = 0.59), implying some overall control, probable aerodynamics of dune morphology.

There is a positive correlation between the dune size and crest-to-crest distance (spacing) (Table 3). Most of the barchan dunes of the Kharga dune field are much smaller than those in Toshka in the south and Abu Moharik dune belt in the north, and the coastal parts of the Namib sand sea (Breed and Grow 1979). The Kharga dunes are comparable in terms of overall morphology, size, and spacing to the dunes at Guerrero Negro, Baja, California (Inman et al. 1966). The spacing of the studied barchan dunes compares well with those in Qatar, the United Arab Emirates, Saudi Arabia, and the Thar desert (Breed and Grow 1979).

Comparison of the morphometric parameters of Kharga barchans with other barchans of worldwide deserts

Figure 7 compares the morphometric parameters of Kharga barchans and other barchans worldwide. The heights of studied barchans are similar to those in the previously published works in Kharga (e.g.,Beadnell 1910; Stokes et al. 1999) and Qatar Peninsula (Embabi and Ashour 1993) but relatively less than what so-called Bagnold dunes in the southwestern desert of Egypt (Haynes 1989) (Fig. 7A). The heights of Kharga barchans are higher than those of worldwide deserts, e.g., Pampa de la Joya, Peru (Finkel 1959); Salton Sand Sea, California (Long & Sharp 1964); Pampa de la Joya, Peru (Hastenrath 1967); Jafurah Sand Sea, Saudi Arabia (Fryberger et al. 1984); Pampa de la Joya, Peru (Hastenrath 1987); and Kuiseb River delta, Namibia (Slattery 1990). Kharga barchans exhibit relatively smaller length, width, and dune spacing measurements than other barchans worldwide (Fig. 7B–D).

Grain size characteristics of the Kharga barchans

Grain size distribution in the barchans of the Kharga dune field is characterized by a unimodal pattern with the modal class of 250–125 µm (i.e., medium to fine-grained sand size) (Fig. 8A). The mean size (MZ) of the sand varies between 0.6 and 3.3 ɸ with an average value of 2.1 ɸ (Table 4). There is little variation in the mean size in different parts of the Kharga dune field. The sand of the central sector is a little coarse with a mean size of 1.9 ɸ (maximum 2.8 ɸ and minimum 0.2 ɸ), while the sand of the northern and southern sectors is similar to the mean size of all samples (Ramadan et al. 2022). The mean sorting of all studied sand samples varies from well sorted to moderately well sorted (i.e., 0.1–0.9 ɸ), with an average value of 0.62 ɸ (moderately well sorted). The sands of the southern sector are of lower sorting than those of the other two sectors (Table 4, Fig. 8B). Figure 8B and C show the relation between the mean size, sorting, and skewness, respectively; the former shows a weak positive correlation between mean size and sorting (R2 = 0.1), while the mean size–skewness relationship shows a very weak positive correlation (R2 = 0.001). Furthermore, the sand of Kharga barchans is nearly symmetrically skewed (− 0.001 and 0.1) and mesokurtic to leptokurtic (− 1.01 to 1.1).

Fig. 8
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A Grain size distribution of sand of Kharga barchan. B Relationship between mean size (ɸ) and sorting. C Relationship between mean size and skewness. D Relationship between mean size and sorting of Kharga barchans and barchan sands from other deserts around the world

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Table 4 Mean, maximum, and minimum of grain size parameters of 103 sand samples of Kharga barchans
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Migration of Kharga barchans; rate and direction

Previous studies

The first published work about the migration rate of the dunes of Egyptian desert dunes was done by Cornish (1900), who estimated a rate of dune migration of 4.5 m/year in the east of the Nile Delta. Several papers concerning the rate of dune migration in the Western Desert of Egypt were published by different authors (e.g.,Ashri 1973; El-Gammal and Cherif 2006; Hamdan et al. 2016) who assigned dune migration rates c. 0.5 to 100 m/year. Figure 9 shows previous results of the rate of barchans dune at and around Kharga Depression. Five of these studies were carried out on the Kharga barchans; two on the plateau north of Kharga (Hassan and El-Leithy 2004; Embabi et al. 2012) and one on the Toshka area to the south of the Kharga Depression. Investigations by Embabi (1986–1987), Stokes et al. (1999), and Hassan and El-Leithy (2004) in the Kharga Depression calculated similar rates of movement (9.0–9.9 m/year). Embabi (1979) measured the migration rate of 25 barchan dunes over only 1 year with high movement rates for individuals. Hereher (2010) estimated the annual rate of migration of Kharga barchans to be 7.5 m/year. Hamdan et al. (2016) mentioned that the migration rate of Toshka dunes varies from c. 4 to 7.67 m/year, averaging 6 m/year. Hassan and El-Leithy (2004) estimated a rate of movement on the plateau north of the Kharga Depression of c. 5.8 m/year and conclude that the wind regime of the plateau is weaker than that in depression. Barchan dune migration has been studied quantitatively and qualitatively in several locations around the world. The calculated values of the migration rate of barchan dunes worldwide range between 3 and 43 m/year (Table 5).

Fig. 9
figure 9

Histogram of the rate of migration in barchans of Kharga and other localities in Egypt

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Table 5 Rate of migration of barchan dunes at different deserts along the world
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Wind characteristics

Thirty years (1980–2010) records of El-Kharga metrological station reveal that the prevailing winds in the study area are the north and northwest winds (Fig. 10A). At the Kharga Depression, 50.3% of wind blew in from the North direction, most likely because the depression extends in a north–south direction (Fig. 2). Other wind directions, e.g., W and NE winds, are less prevailing probably due to the presence of the high Abu Tartur plateau of c. 500 m (asl; above sea level). It is noted that the rate of stillness reaches the lowest value at only 1%, which is probably due to the difference in elevation between the high plateau in the north and the low depression, c. 2 m ASL, which sustains continuous blowing of northern wind throughout the year. During the winter season, the northern winds prevailed at (c. 42.2%) (Fig. 10A). This is followed by the predominance of the northwestern direction at (22.5%). The prevailing wind directions do not change much in the summer, as the northern winds continue to prevail with a percentage reaches to c. 65% and a slight increase in the rate of northwesterly winds of 28.9%. The Kharga Depression continues to be affected by the northern winds in the spring and autumn, although the rate of their blowing during the fall is higher than the rate of blowing in the spring, reaching about c. 72% in the fall and 65% in the spring. The spring and autumn seasons are among the months of the year in which the wind speed that blows on the Kharga depression increases, reaching 15 km/h. The study area’s wind speed during the summer season is 14.8 km/h, which is approximately the same as the wind speed in the two equinoxes, and this is because summer is characterized by a high wind temperature, which helps to increase wind speed. In contrast, it is noted that the wind speed during the winter is relatively low, about 11.6 km/h, where it records the highest rates of wind rest, in addition to the fact that it is the slowest season of the year in terms of wind gusts.

Fig. 10
figure 10

A Mean seasonal variation of wind direction and speed of the Kharga meteoric station (1980–2010). B Mean annual variation of sand drift potential (DP) and resultant drift potential (RDP) of the Kharga meteoric station

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Rate of migration of Kharga barchans

In the present study, the movement of 160 barchans for a total of 16 years (2002–2018) varies from 3.3 to 26.6 m/year, averaging 13 m/year (Table 6), which indicates that the barchan dunes migrate at different speeds in the southward direction parallel to the prevailed wind. There was little variation in the barchans’ migration rate during that period. The barchans rate of migration during the period 2002–2003 to 2016 is 10.5 and 9.85 m/year, respectively. From the years 2007 to years 2011, 2014, and 2017, the rate of migration is measured as 16.4, 7.8, and 9 m/year, respectively. The rate of barchans migration is also varying geographically from north to south as follows (Figs. 11, 12, 13; Table 6); in the northern sector, the mean rate of barchans migration is 16.4 m/year, while the mean rate of migration in the central and southern sectors is measured as 10.9 m and 12.7 m/year, respectively. Based on the dune migration, the barchan dune could be classified into three types: migrating, elongating, and accumulating dunes (Tsoar et al. 2004). Kharga barchans belong to the first type (i.e., migrating dunes), where the whole dune body advances with little or no change in shape and dimensions (see Figs. 11, 12, 13).

Table 6 Average rate and trends of migration of barchans of 36 sites in Kharga Depression (sites 21 and 23 are not measured)
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Fig. 11
figure 11

Barchan displacement of 4 sites in the northern sector. a Site 1. Used images are dated 7/2/2007 and 5/4/2010. b Site 2. Used images are dated 7/2/2007 and 5/4/2010. c Site 3. Used images are dated 7/2/2007 and 5/4/2010. d Site 5–1. Used images are dated 1/9/2002 and 29/1/2016

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Fig. 12
figure 12

Barchan displacement of 4 sites at the central sector. a Site 9. Used images are dated 7/2/2007 and 11/2/2014. b Site 12. Used images are dated 1/9/2002 and 30/11/2016. c Site 14. Used images are dated 22/9/2003 and 29/1/2016. d Site 29. Used images are dated 11/2/2014 and 30/11/2016

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Fig. 13
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Barchan displacement of 4 sites at the southern sectors. a Site 31. Used images are dated 7/2/2007 and 11/2/2014. b Site 34. Used images are dated 22/9/2003 and 24/12/2013. c Site 35. Used images are dated 12/11/2002 and 25/10/2016. d Site 36. Used images are dated 20/3/2004 and 24/12/2013

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Seasonal variations in barchan movement

Sand drift potential (DP) is usually used to study the capacity of aeolian sand transport. The monthly variation of the amount of sand expected to move the drift potential (DP) and the resultant drift potential (RDP) of the Kharga Depression is given in Fig. 10B. The maximum amount expected to move during June and September reached c. 16% and 14% of the year value of DP, respectively, reaching its lowest level during January and December. The sand drift potential (DP) was determined in vector units (VU) using wind direction. Mohamed (2003) mentioned that the total amount of sand expected to be moved could be estimated as c. 2388.01 VU. Thus, the Kharga Depression environment can be classified among the environments dominated by high-energy winds according to Fryberger’s classification. Thus, it is similar to the environment of the north of the Arabian Peninsula and Libya (Fryberger 1979). Figure 10B also shows that spring is the most season of the year in terms of the amount of sand expected to move (DP), and then the movement of dunes based on wind energy, as it occupies about 30.4% of the total amount of sand expected to move during the whole year, followed by the summer season with 29%. These results were almost supported by field techniques by Mohamed (2003), who concluded that autumn is the most season of barchan migration in Kharga barchans, with an average movement distance between 4.10 and 31.40 m, averaging 11.68 m. The summer season follows autumn in the distance of movement of the dunes, averaging 9.62 m (3.4–25.3 m), then the spring season, with a rate of movement ranging between 2.30 and 23.9 m, averaging 6.85 m. The least movement distance occurs during the winter season, ranging between 1.1 and 15.5 m, with an average of 4.25 m. These results are compatible with wind energy and direction during the seasons of the year in the Kharga Depression, where there is a strong correlation between the wind direction in the seasons of the year and the rates of dune movement in these seasons. Mohamed (2003) also concluded that the effect of wind direction on the rate of movement of the Kharga barchans is stronger than the effect of wind energy, and the direction responsible for most of the movement of the dunes in the region is the northern wind, followed by the northwestern wind. Accordingly, the autumn season records the largest range of dune movement during the year in the study area, where the percentage of the north direction during this season is about 57.9%.

Factors that influence rates of barchan dune movement

Several factors, including local wind, dune size (e.g., height), dune shape, local topography, and vegetation, are the most driving factors of barchan dune migration (Bagnold 1941; Long and Sharp 1964; Norris 1966; Lancaster 1983; Fryberger et al. 1984).

Impact of dune size on the migration of Kharga barchans

Dune height

The correlation coefficient between the migration rate and the barchan's heights is − 0.64 (Table 3 and Fig. 14A), indicating a strong inverse relationship between the moving distance and dune height. The strength of this relationship between the height of the dune and the distance of its movement is since the increase in the height of the dune is followed by an increase in its other dimensions (see Fig. 6), especially the width of the dune and the length of the windward (Embabi 1982). The rate of barchan migration is dependent upon the size of the dunes, where the correlation coefficient between the rate of migration and dune width, length, and volume are − 0.64, − 0.56, and − 0.43, respectively (Table 3 and Fig. 14B).

Fig. 14
figure 14

Relationship between the rate of migration of Kharga barchans and height (A), width (B), windward length (C), and mean grain size (D). (E) represents dune height-mean grain size relation. F Migration rate-dune spacing relation

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Moreover, the dune area negatively correlates with the average annual migration rate (R2 =  − 0.594, not shown). The larger the dune, the greater the amount of material to be moved; hence, the degree of movement in each unit of time will be smaller. The current result is well matched with several published studies that show that as dune size increases, the rate of movement decreases (e.g., Bagnold 1941; Hastenrath 1967; Lancaster 1982; Hamdan et al. 2016).

Length of windward side

The length of the windward side varies between 26.1 and 538.2 m, averaging 122.1 m (Table 1), and the correlation coefficient between the rate of migration and the windward length is equal to − 0.66 (Table 3). Figure 14C indicates a strong inverse relationship between them. Therefore, it is noted that the dunes with short windward moved at a faster rate than dunes of long windward length.

The field measurements indicate that the average slope of the windward side ranges between 4 and 17°, and it is noted that dunes with the lowest slope of the windward side moved a greater distance than those dunes with the steepest windward side. Furthermore, there is an inverse relationship between the slope and the length of the windward side, i.e., the low slope angle of the windward side exhibits a longer windward side. Therefore, there is an average reverse relationship between the dune’s moving distance and the windward surface’s average slope.

Impact of the grain size on the migration of Kharga barchans

A positive relationship exists between the heights and the mean grain size of the studied barchans (Fig. 14E), where the mean of the dune sands is always fine sand with an increase in the dune heights. Figure 14D indicates a positive relationship between mean sand grain size ɸ and migration rate, where finer grain size dunes are transported at higher rates. The mean size of Kharga dune sands is similar to those of other deserts worldwide (Hastenrath 1967; Ahlbrandt 1979; Watson 1985; Sagga 1998). Bagnold (1941) also mentioned that large dunes contain much finer sands than smaller ones, and this is because the increase in barchan height leads to an increase in the windward slope, which affects the transportation of sand over the dune. The coarse grains are too heavy to blow up the steeper windward slope; therefore, coarse sand tends to be deposited on the base of the slope as a lag deposit. On the other hand, finer grains are easily transported over a steeper slope and deposited at the crest.

Impact of dune shape on the migration of Kharga barchans

Generally, the impact of barchan shapes on the migration rate has been mentioned by previous researchers (e.g., Long and Sharp 1964; Hamdan et al. 2016). In the current study, no significant correlation is evident between the current a/c ratio (i.e., length/width) and the average rate of movement (R2 =  − 0.004). Moreover, the relationship between the dune's slimness and rate of migration shows that, generally, fatter barchans dunes (i.e., a/c ≥ 1) migrate slower than slimmer ones. Therefore, in the central and southern sectors, which are dominated by fat and pudgy barchan types (Fig. 5B), the rate of migration (10.9 and 12.7 m/year, respectively) are much less than those of the northern sector (16.1 m/year) which includes much slimmer barchans. It seems that the effect of the slimness on the rate of barchans migration could be outweighed by other factors, e.g., wind speed, topography, and vegetation.

Impact of dune density on the migration of Kharga barchans

Dunes themselves act as barriers to the wind and consequently cause a reduced wind velocity and, in turn, a decreased migration rate. Therefore, theoretically, as dune density increases, the advance rate will decrease (Embabi and Ashour 1993). In the case of the Kharga dune field, there is clear negative regression (R2 = 0.4) between the rate of migration and dune spacing (Fig. 14F). The effect of dune spacing on the migration rate may explain the difference in the migration rate of some dunes. The density of the dunes also affects the trap efficiency; the closer together they are, the more sand they will trap, the larger they will grow, and the slower they will move, with other conditions being equal. A very low negative correlation occurs between the mean migration rate and the dune density in each study site with R2 = 0.014 (not shown).

Impact of local topography on the migration of Kharga barchans

Local topography, including variation in the ground levels and the existence of specific geomorphic forms, may affect, positively or negatively, the rate of migration of Kharga barchans. Fieldwork aided with topographical maps 1:100,000 and 1:250,000 illustrates that the existence of some isolated hills (e.g., Gebels Tarif and Tair) with a height of more than 100 m can stop the movement of any dune, as well as lead to the scattering of the barchan dunes on their sides. On the other hand, the rate of barchan migration increases when the ground surface slopes in the direction of the movement.

Lettau and Lettau (1969) mentioned that dunes on the higher ground migrate more consistently faster than dunes on the lower ground. In the study area, the ground surface slopes in a southward direction, and the northern sector is located at a higher elevation (100–125 m ASL) than the central and southern sectors (Fig. 2). The gradient is markedly high in the northern sector which shows a higher rate of migration (16.1 m/year). The central sector is located in the low-level area (less than 25 m ASL), and the southern sector, which lies on a slightly rising ground surface, show relatively lower rates of barchans migration (i.e., 10.9 and 12.7 m/year, respectively).

Impact of average humidity and vegetation on the migration of Kharga barchans

The average humidity in the Kharga Depression is generally low, as it does not exceed 51%. The highest relative humidity rate is found in the winter season, while its lowest rate reaches only 29% during summer (Kharga Meteoric station). The evaporation rate is generally high, which confirms the scientific fact that there is a direct relationship between temperature and evaporation rate; it is 9 mm during June, while it decreases in the winter to reach c. 6.2 mm during January. It is also noted that evaporation rises during the autumn season, reaching about 17.1 mm in September, while it decreases in the spring season, and this is due to the wind speed affected by the spring depressions. The relative humidity has little or no impact on the Kharga barchans’ migration rate.

Generally, the vegetation cover in the Kharga dune belt is very low; however, it has a slight effect on the dune migration rate, whereas the central and southern sectors show slightly higher vegetation cover and exhibit a lower rate of barchans migration (Table 6). Field examination shows the existence of thin salt crusts at the windward side of some barchans, especially in low-level areas, i.e., the central sector. Salt crusts were formed due to rising groundwater levels adjacent to newly reclaimed cultivation land. These salt crusts are most likely retard rates of migration of the dunes by inhibiting the entrainment of material.

Environmental hazards of Kharga barchan dunes

Kharga Depression, as part of the hyper-arid North African belt, is characterized by rainless and scarcity of vegetation cover, therefore, leading to the predominance of wind erosion activity and the movement of wind-blown sand sediments. Dune movement risk evaluation is important for countries with migrating dunes threatening their towns, transportation routes, or agricultural fields, like the Western Desert of Egypt (Effat et al. 2011; Hamdan et al. 2016). Moreover, the barchan dunes are extended in specific paths that follow the direction of the prevailing winds (see above), which causes many environmental problems represented in the threat and danger on roads, villages, agricultural lands, and archaeological sites. In addition to the field study, these threats were monitored by studying topographic maps at a scale of 1:100,000 and 1:250,000 and from Google Earth historical satellite images.

Risk and vulnerability to the roads

The barchan dunes constantly threaten the main and secondary roads in the Kharga Depression, which comprises four main roads that connect the Kharga City with Assiut, Baris, Luxor, and Dakhla oases (Figs. 15 and 16).

Fig. 15
figure 15

Map showing the risk and vulnerability of the barchan dunes to the urban, cultivated areas and archaeological sites of the Kharga Depression. 1. Kharga-Assiut road; 2. Kharga-Baris road; 3. Kharga-Dakhla road; 4. Baris-Luxor Road

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Fig. 16
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Satellite images showing the risk of barchan dune encroachments in A Kharga-Assiut asphaltic road; B Kharga-Dakhla road near Abu Tartur; C barchan dunes encroaching agricultural lands; E barchan dunes encroaching urban area

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Assiut-Kharga road

This main road that connects Kharga City with Assiut extends in almost N–S direction from Kharga City to the edge of the Northern Plateau (Naqb Assiut), then turns eastward above the plateau (Fig. 15). The Assiut-Kharga road is threatened by sand risk when approaching the surface of the plateau because it passes across the north–south migrating barchan direction (Fig. 16A). The most threaten parts of the Assiut-Kharga road as noticed in this study, at the distances 173, 167, and 117–118 km from Al-Kharga, and in the Naqb region. The danger of the tyranny of sand on the road is exacerbated by the steepness of the feet of the cliffs so that the passing cars are not allowed to turn around the part covered by the sand (Embabi 1970).

Kharga-Baris road

This road also suffers from the attack of migrating dunes (Fig. 17A). It extends in the south-ward direction, passing through highly populated villages with intensive agricultural activity (Fig. 15). It is exposed to the danger of the barchan sand encroaching in the central and southern sectors of the Kharga barchan belt. In these sectors, the three barchan belts previously described are coalescing in one large barchan belt (see Fig. 2E).

Fig. 17
figure 17

Field photos of the sand dune encroachment; A secondary road from Kharga-Baris main road; B at the top of the Eocene plateau; C barchan dune encroachment to a desert track at Umm Dabadib area; D sand covers the pre-existing Abu Tartur phosphate railway

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Kharga-Dakhla road

This road extends in an E–W direction and crosses the western barchan belt (Figs. 2A and 15). The western belt of the barchan dunes threatens the straight road that connects the villages of the Kharga Depression to the villages of the Dakhla Depression, which is perpendicular to the directions of these craters; therefore, it constantly overwhelms the road and obstructs the traffic of cars, and cars must turn around it, which exposes it to many risks.

Baris-Luxor road

This road is a newly constructed road connecting Kharga City and Luxor city via Baris city (Fig. 15). There are several land reclamation projects along the western part of the road. Both the road and the reclaimed land are threatened by the encroachment of barchan sand of the eastern belt (Fig. 2). More recently, Abdel Motamed and Abdel Ghafa (2022) studied in detail the risk of the barchan dunes to the Luxor-Kharga Road and mentioned that the western part of this road varies between moderate to high sand dune risk.

Risk and vulnerability to the railway

There is sand encroachment on the Kharga-Qena railway, which serves the Abu Tartur Phosphate Project (Fig. 16D). However, this railway currently does not exist. However, another plan is to establish a new railway passing through the eastern escarpment. Sands are away from the railways by about 8 m in the closest cases. It is worth noting that the factors which are responsible for the encroachment of sand into the railway in the study area are the direction of the northern winds prevailing in the area most days of the year, in addition to the flatness of the relatively low surface, which helped the movement of sand dunes, and the absence of obstacles preventing their movement in those areas.

Risk and vulnerability to the wells and farms

The problem of the burial of wells and their agricultural reins represent a serious environmental problem for the movement of the barchan dunes. The permanent move of the barchan dunes to the south led to the burial of some of these wells and farmlands and the threat to others (Fig. 16C, D).

Jinnah Village is located about 12 km south of Kharga City. It was directly exposed to the encroachment of sand dunes to its residential mass and agricultural wells in 1975, forcing its residents to move and rebuild their homes 2 km southeast of the old place (Mohamed 2003). However, due to its location in the central sector of the Kharga barchan belt, in addition to its intertwining with the western barchan belt range (Fig. 2), it was exposed to the encroachment of the dunes more than once, which resulted in the rebuilding of Jinnah Village four times over a hundred years, meaning that the dunes bury the whole village once every 20 years since its establishment.

The wells of the old village of Jinnah were buried in 1975, and the well of Jinnah 3 in 1979 and its agricultural control of 141 acres are affected. It is worth noting that these lands were cultivated during the period from 1960 to 1980. They were subjected to wind erosion and dunes encroachment in 1999; consequently, their lands became desertified.

Risk and vulnerability to the archaeological sites

The Kharga Depression includes highly important archaeological sites dating from Prehistory (middle Paleolithic–Neolithic) to Pharaonic Roman time (Fig. 18). Currently, most prehistoric sites are affected by the agriculture reclamation projects in the Kharga Depression and sand encroachment along the northern part and at the escarpments (Ismael et al. 2020). The prehistoric archaeological sites are disturbed due to wind deflation; the archaeological remains (i.e., lithics, potsherds, and charcoal) suffered from wind erosion. Sometimes the whole site is completely covered by the aeolian sand.

Fig. 18
figure 18

Archaeological sites of Northern Kharga Depression; A Umm Dabadib site surrounded with phytogenic dunes; B collapsed house walls of Umm Dabadib site; C Ain Labkha site surrounded by aeolian sands; D sandblasted mud bricks at Ain Labkha

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The Roman monuments, which are formed from mudbricks, also suffer from various patterns of processes such as differential weathering, scratching, and undercutting. The Pharaonic temples, formed mainly of limestone and sandstone (e.g., Hibis, El Ghawita, and El Zayyan temples), are subject to wind erosion and salt weathering (Salman et al. 2010). It seems that the archaeological sites in the northern part of the Kharga Depression (i.e., Umm Dabadib, Fig. 18A, B; Ain Amur and Ain Labkha, Fig. 18C, D) are highly in danger concerning sand encroachment (Fig. 15). Archaeological sites in the middle of the depression are less affected by dune migration, while those in the southern part (Dush temple and cemeteries) are moderately affected by the sand encroachment (Fig. 15).

Conclusions

Barchans are the most dominant dune types in the Kharga Depression and exist in three parallel dune belts controlled by the N–S prevailing wind regime. The sand transported by incessant effective winds of the Kharga Depression caused serious hazards to urban areas and roads. However, due to the uniform nature of these winds, they do so in a somewhat predictable manner. The Kharga barchans are small and medium sized; however, they are smaller than those in other parts of Egypt, e.g., Toshka, Abu Muharak, and Northern Sinai. Mean length, width, and height are measured at 199, 146.9, and 13.9 m, respectively, which seems smaller than the barchan of most worldwide deserts.

The measured barchan dunes in the Kharga Depression are characterized by a variable range of morphometric parameters similar to those to the south in the Toshka area. The morphometric parameters (width, length, height, perimeter, volume, and crest-to-crest spacing) show increased mean values in the central sector of the depression. Also, a strong inverse relationship is indicated between the migration rate of the barchan dunes and their height, windward length, and dune spacing. It seems that the relative humidity during the year has little impact on the Kharga barchans’ migration rate. Among 375 dunes in the studied Kharga dunes, 76% are simple (solitary) barchan type, 20% are compound barchans, 1% are complex dunes, and only 3% are barchanoids. They are mostly pudgy (44.7%) or fat (35.9%), little normal (15.5%), and rare slim (3.9%). Among 104 barchans, c. 87% are asymmetric dunes with dominated eastern and western horns measuring 53% and 34%, respectively. Kharga barchans’ asymmetry is mainly controlled by local topography and dune collision.

Kharga barchans’ migration rate for a total period of 16 years (2002–2018) varies between 3.3 and 26.6 m/year, averaging 13 m/year. This migration rate varies geographically and chronologically; from 2002–2003 to 2016, it is 10.5 and 9.85 m/year, respectively. From the years 2007 to the years 2011, 2014, and 2017, the migration rate was measured as 16.4, 7.8, and 9 m/year, respectively. The mean rate of migration in the northern, central, and southern sectors are 16.4, 10.9, and 12.7 m/year, respectively. There is a strong preferred orientation for the dune migration in the direction of the prevailing wind. The main trend of migration of dunes in the northern and central sectors is S5°E and deviates to S15°E in the southern sector of the Kharga Depression. The central sector of the depression attains the highest urbanization and cultivation activity. It shows the higher values of morphometric parameters of the barchan dunes and a lower value of the migration rate than the northern and southern sectors. The migration rate of dunes is remarkably higher in the northern sector and represents serious road problems. The previously mentioned conclusions provide important data for the mitigation of the hazards of these dunes on the urbanization and cultivation in the Kharga Depression.