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Geomorphology 257 (2016) 57–74

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Geomorphology

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Morphologic characteristics and migration rate assessment of barchandunes in the Southeastern Western Desert of Egypt

M.A. Hamdan, A.A. Refaat ⁎, M. Abdel WahedGeology Department, Faculty of Science, Cairo University, Giza, Egypt

⁎ Corresponding author.E-mail address: [email protected] (A.A. Ref

http://dx.doi.org/10.1016/j.geomorph.2015.12.0260169-555X/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 3 May 2015Received in revised form 6 December 2015Accepted 30 December 2015Available online 31 December 2015

This work explores themorphologic characteristics of aeolian dune sand in the southeastern part ofWestern De-sert of Egypt. It aims to assess themovement of barchan dunes and evaluate their environmental influence on theToshka Project. Morphometric investigation of barchan dunes in the Toshka area revealed that most barchanshave high length/width (a/c) ratios (fat to pudgy), while one-fifth of the studied barchans have lower a/c ratiosand so appear normal in theirmorphologic forms. Statistical analysis of themain parameters of barchan dunes inToshka and other desert regions in the Kharga (Egypt), Kuwait, Southern Morocco, California and Southern Perudemonstrates that barchans of the Toshka area are distinctive in their appearance. They are characterized by dis-tinct aspect with higher values of length andwidth and greater growth in height. The high-energywind environ-ment in addition to the large amount of drifting sand are principal factors responsible for the unique shape ofToshka barchans.The migration rate of barchan dunes in four chosen test locations, within the central and western Toshka area,ranges from about 3 to 10.82 m/year. The calculated average migration rate of these dunes is about 6 m/yearin a SSW direction. Sand encroachment is more extensive in the central and western parts of the investigatedToshka area. Risk evaluation of sand dune movements in the southeastern part of the Western Desert points tomedium to high sand encroachment risk values. These may represent serious hazards to the newly-establishedToshka Project, threatening roads, as well as cultivated lands in the area.

© 2015 Elsevier B.V. All rights reserved.

Key words:Barchan dunedune migrationdune encroachment hazardWestern Desert of Egypt

1. Introduction

Many areas in theWestern Desert of Egypt are characterized by highaccumulations of wind-blown sand (Fig. 1). Modern aeolian sand of theWestern Desert of Egypt has been the subject of scientific interest sincethe beginning of the 20th century (e.g. Beadnell, 1910; Ball, 1927;Bagnold, 1931, 1941). Numerous geologic, geomorphologic, textural,mineralogical, geochemical and source-area genetic studies of thesand dunes followed (e.g. Embabi, 1970; El-Baz et al., 1979; Besler,1986, 1998, 2000, 2008; Haynes, 1989; Stokes et al., 1998; Hamdanand Refaat, 1999; Hamdan, 2003; El Gammal and Cherif, 2006; AbouEl-Magd et al., 2013; Khedr et al., 2013a, 2013b; Refaat and Hamdan,2015; Hamdan et al., 2015). Several studies have been concerned withestimating the rate of desert dune movement in many areas of Egypt,especially in the Western Desert (Table 1). Sand dune movements areconsidered to be a specific threat to roads, irrigation networks, water re-sources, urban areas, agriculture and infrastructure of the Toshka pro-ject (Wahby, 2004).

The southeastern part of the EgyptianWestern Desert is largely rep-resented by the Toshka area; it covers about 50,000 km2 and is

aat).

characterized by high dune sand accumulation. The area has been cho-sen to serve as a model for the assessment of dune migration because itis affectedmainly by strong tradewindswhich exert a strong control onthe mechanism of dune formation as in the eastern range of the GreatSahara Desert (Besler, 2008). It also represents the focal area of manydune streams and sand accumulations that migrate from other regionsof the Western Desert of Egypt into the northern Toshka area (Fig. 1).Hence, the Toshka area is considered to be an ideal region for dunemea-surements and can be used as a model for other regions in theWesternDesert of Egypt and North Africa.

Sand dunes are a common feature of many desert regions all overthe world and they may exist in many different types. Barchan dunesare the simplest andmost common dune type as it maintains a crescentshape and occurs in areas characterized by steady winds coming from asimilar direction throughout the year, providing there is not enoughsand to cover the entire regional surface (Sauermann et al., 2000). Bar-chan dunes are well developed in many parts of the Egyptian WesternDesert, including the Toshka area, as well as in many deserts areas notonly on the surface of the Earth (e.g. Bagnold, 1941; Finkel, 1959;Long and Sharp, 1964; Hastenrath, 1967; Hesp and Hastings, 1998;Sauermann et al., 2000; Parteli et al., 2007) but also on the surface ofMars (e.g. Goudie and Bourke, 2008; Bourke and Balme, 2008). More-over, several types and forms of barchan exist in these deserts,

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Fig. 1.Map of the Western Desert of Egypt showing the distribution of the major sand dune fields (modified after El-Baz, 1979). The study area is located in the southeastern part of theWestern desert of Egypt. Numbers inside the circle refer to dunemigration rates of the Toshka area and other regions in theWestern Desert of Egyptwhere arrows refer to the direction ofdunemigration at each area. The number at the head of each arrow refers to the corresponding reference: 1= Beadnell (1910); 2=Ashri (1973); 3= Embabi (1979); 4= Sharaky et al.(2002); 5 = El Gammal and Cherif, (2006); 6 = Abou El-Magd et al. (2013); 7 = Present study.

58 M.A. Hamdan et al. / Geomorphology 257 (2016) 57–74

depending upon several factors, such as wind strength and direction,amount of drifting sand, topography and vegetation. The currentpaper is a comparative study that highlights the main morphometriccharacteristic features of Toshka barchans and other barchan shapesfrom different deserts of the world. Evaluation of the main factorsthat control the morphology and existence of these dunes arediscussed.

A further aim of this study is to investigate the distribution of aeoliandune sands in the Toshka area and to identify their types using remotesensing and GIS techniques. The study assesses the rate of barchanmovement in the Toshka area in comparison with other regions in the

Western Desert of Egypt. Finally, the degree of sand dune encroachmentand its effect on the Toshka Project is evaluated.

2. Geomorphologic features

The study area is located in the southeastern part of theWestern De-sert of Egypt between 22° and 24° 00′ N and 30° 15′ and 33° E (Fig. 2).With the exception of Sinn el Kadab Plateau and its pediments, whichlie more than 300 m above sea level and bounding the Toshka Depres-sion from the north, the study area is relatively flat and most of it is

Table 1Rate of dune movement in the east of the Nile Delta and different locations in the Western Desert of Egypt, with their references.

Location area Time period (year) Rate (m/year) Reference

East of the Nile Delta 1898–1990 4.5 Cornish (1900)Kharga Depression 1908–1909 15 Beadnell (1910)Kharga Depression 1971 12 Ashri (1973)Kharga Depression 1978 20–100 Embabi (1979)Dakhla Depression 1990 0.5–14 Sharaky et al. (2002)Abu Moharik dunes–northern of the Bahariya oasis–middle part of the dune field–southern end near the Kharga depression

1974, 1984–2001 17.57.511.5

El Gammal and Cherif, (2006)

Toshka area 2000–2006 1.3–19.3 Abou El-Magd et al. (2013)1987–2007 6.024 Present study

59M.A. Hamdan et al. / Geomorphology 257 (2016) 57–74

occupied by the Toshka Plain (Depression), ranging in elevation from150 to 250 m above sea level.

2.1. Sinn el Kadab Plateau

The extensive Sinn el Kadab plateau, which is underlain mainly bylimestone, consists broadly of horizontal beds of late Cretaceous toearly Eocene age. It is bounded eastwards and southwards by a precip-itous escarpment, overlooking the Nile Valley from its eastern side,while northwards the plateau extends beyond the area under consider-ation to the Kharga Depression (Fig. 1). The escarpment is mainly con-trolled by major faults that are generally trending in N-S and E-Wdirections (Fig. 3).

2.2. The Toshka Plain (Depression)

This plain occupies the central part of the study area and attaining anarea of about 13,142 km2with a lowest point located at 150mabove sealevel (Wadi Toshka). It is limited to the north by the pediments of Sinn

Fig. 2. Satellite image of the Toshka area shows the se

el Kadab Plateau and Lake Nasser from the east, while the Egyptian-Sudanese Border represents its southern boundary. The evolution ofthe Toshka Plain is mainly controlled by the existence of a series offolds and faults (El Shazly et al., 1977). The Toshka Plain is characterizedby outcrops of isolated Nubian sandstone hills and short, wide wadiesthat in the past drained into the Nile Valley. Precambrian igneous andmetamorphic rocks, early Cretaceous Nubian sandstone and QuaternaryNile sediments are all exposed in the Toshka area (Fig. 3).

2.3. Lake Nasser

Lake Nasser, the world's largest man-made lake, is a reservoir in thecourse of the River Nile, formed by construction of the AswanHigh Damin 1960. It is located at the border Egypt and Sudan between latitudes21.8°N to 24.0°N and longitudes 31.3°E to 33.1°E. Its surface area isabout 5200 km2 with a maximum water capacity of 165 km3 and amean depth of 25 m; its surface elevation is 175 m above sea level.The lake is about 550 km long (more than 350 km in Egypt while therest lies in Sudan) and 35 km across at its widest point.

lected locations for dune migration calculations.

Fig. 3. Geological map of the Toshka area showing the exposed rock units and the main structural elements in the study area (modified after Labib and Nashed, 2013).


One of the most interesting features of Lake Nasser is the complexgeometry of its shoreline; which changes enormously according toevenminor changes in the water level. The shoreline possesses numer-ous khors, which are flooded valleys (where the valleys have been cutby streams). Most of these khors are narrow and meander into the de-sert for long distances. A vast number of islands of various sizes,representing the tops of former hills, are scattered throughout thelake. The number, location and size of these islands vary greatly withfluctuations in the lake-level.

3. Materials and methods

Field studies were carried out using topographic maps (scale1:100,000 and 1:50,000), Landsat images and available Google Earthsatellite images, to study the aeolian sand distribution in the southeast-ern region of theWestern Desert of Egypt. The desert dunes in the studyarea constitute four main categories; barchan, linear, lee and shadowdunes. The distribution of different dune types with in theWestern De-sert investigation area is illustrated in Fig. 4. The birth of newly formedbarchan dunes and barchanoids from lee dunes downwind of topo-graphic obstacles is shown in Fig. 5. One hundred and eleven barchandunes in the Toshka area have been subjected to morphological shapeinvestigation according to the classification given by Long and Sharp(1964). In order to assess variation of barchan dune shapes, morpho-metric examinationwas undertaken for 44 barchan dunes in the KhargaDepression belonging to the same dune stream of Toshka area butrepresenting its northern extension. For each selected barchan dune,four parameters were determined; length of the windward slope (a),width (c), height (H) and the a/c (length/width) ratio (Fig. 6). Wemea-sured the barchan dune width as the greatest distance between the tips

of the two horns perpendicular to the wind direction along the dunecrest.

Comparative investigations of morphological shapes of barchansfrom different deserts in the world in the Atlantic Sahara region insouthern Morocco (Sauermann et al., 2000); Kuwaiti Desert (Khalafand Al-Ajmi, 1993); Imperial Valley in California, USA (Long andSharp, 1964) and La Pampa de la Joya in southern Peru (Finkel, 1959)were also evaluated. The four parameters; length, width, height and a/c ratio were also determined for each barchan in different regions; Cal-ifornia (27 dunes); Peru (44 dunes); Kuwaiti Desert (10 barchans) andMorocco (8 dunes). The obtained parameters of the total of 244 studiedsamples are represented as variables that reach 976 data points (fourparameters for each sample). Statistical analysis of such large datasetis not easy without using a convenient statistical software packagesuch as SPSS. This extensively used program for statistical analysis in so-cial science iswidely used bymanygeology researchers. Descriptive sta-tistics for each variable (including the mean, standard deviation,maximum, minimum and range values) were calculated. The interrela-tionships between the different variables of the studied dunes are dem-onstrated in Figs. 7-8. SPSS provides a number of statistical proceduralprograms for doing correlation and a wide variety of analyses such asT-test and One-way ANOVA (Analysis of Variance), which enables thehypothesis that the means of two or more groups are not significantlydifferent to be tested. One-way ANOVA procedure using SPSS wasused for barchan dune parameters of Toshka area and other desert re-gions in order to specify exactly if the means of two or more variablesare significantly different. Once differences between the means of dif-ferent barchan dunes were established, post hoc tests and Dunnett'sT3 pairwise multiple comparisons provided by SPSS were employed todetermine which means differ. These pairwise multiple comparisons

Fig. 4. Satellite image of the Toshka area, showing the distribution of aeolian dune types in the Toshka area.


test the difference between each pair ofmeans and yield amatrixwhereasterisks indicate significantly different group means at an alpha level(probability) of 0.05 (95% confidence level). Table 2 shows the results

Fig. 5. The birth of newly-formed barchan dunes and barchanoids from lee dunes do

of statistical analysis of morphological parameters of barchans in thestudied Kharga and Toshka regions compared with other barchans indifferent deserts across the world. It should be noted that only variable

wnwind as the topographic barrier has no effect on the accumulation of dunes.

Fig. 6. Schematic diagram showing the dimensions of a barchan measured in the field.

Fig. 8. The length/width (a/c) ratio vs. height (H) of Toshka barchan dunes comparedwithdunes from different desert regions in Kuwait, SouthernMorocco, California and SouthernPeru. This relationship clearly demonstrates the separation of Toshka, Kharga and theKuwaiti barchans from other dunes, as these areas enclose more fatty barchan dunes(greater a/c ratio). Variable values that lie within the estimated 95% confidence interval(CI) for means are only used in the diagram.


values that lie within the estimated 95% confidence interval (CI) anddemonstrate a normal distribution are used in the illustrated diagrams(Figs. 7–8), thus the extreme values were excluded. The different pa-rameters and types of barchan dunes and their locations in the Toshkaarea are given in Appendix A.

Determination of the migration trend and rate of dune migration inthe present study is essentially based on remote sensing and GIS tech-niques through accurate comparison of the dune location in satellite im-ages of two different dates separated by a considerable time span.Previous studies (e.g. Abou El-Magd et al., 2013) that dealt with thedunes movement in the southern parts of the Western Desert werebased mainly on ETM images which considerably reduce the accuracyof results. Indeed, the identification of dunes and tracing their bound-aries with significant accuracy is difficult for ETM Landsat images of15 m/pixel resolution, since its maximum mapping scale does not ex-ceed 1:50,000. Considering that the barchan dunes within the studyarea are generally less than 400 m in width, higher resolution images(at least twice that of ETM) should be used.

In the presentwork, Google Earth historical satellite images are usedas a basis for the dune movement calculations. The available GoogleEarth images of the barchan dunes within the area of interest are of a

Fig. 7. Height (H) vs. width (c) of barchan dunes from the Toshka area and other desertsites in the world. The dune width was measured as the longest distance between thetips of the two horns perpendicular to the wind direction along the dune crest. Note thegood linear relationship between these two variables.

maximum resolution ranging between 8 to 4 m/pixel and cover atime-span from 1987 to 2007. The extracted images are geometricallyregistered (their coordinates are identified) using the ArcGIS software.The registration process is carried out by locating 8 control points ofknown coordinates on the Google Earth image, saved as a shape fileand used as a layer of reference points in the ArcGIS database. Barchansoftenmigrate long distanceswith onlyminor changes in their form (e.g.Long and Sharp, 1964; Hastenrath, 1967), and barchan migration rateshave been investigated by many different authors (e.g. Finkel, 1959;Haynes, 1989; Sauermann et al., 2000). Outlines of the barchan dunesin the two images are traced on two different layers in the databaseand are overlain on each other in the final product. To determine the av-erage amount and direction of migration of a group of dunes, points ofwell-defined locations were visually selected on the dune boundariesin both the older and younger images. The distance between two corre-sponding points indicates the point movement during the specifiedtime-span. The average value of distance moved for selected points isconsidered as the average migration of the dune group. Consideringthe time-span between the two satellite images, the yearly rate ofdune migration is then calculated. The distance between two corre-sponding points and the direction of the line connecting them was ob-tained automatically within the ArcGIS environment by creating a linefeature between corresponding points on a separate layer that issaved as a shape file and exporting the data into an Excel spreadsheet.The exported data include the direction and length of each lineconnecting corresponding points.

Seventeen barchan dunes were selected from 111 of Toshka bar-chans that have been examined for their morphologic shape investiga-tion to perform dune movement measurements. Four test locations(Fig. 2) enclosing the selected barchan dunes were chosen within thecentral andwestern parts of Toshka area near the Toshka Lakes throughvisual interpretation of satellite images. The amount and direction ofmigration of all selected points for the test areas in the studied satelliteimages are illustrated in Table 3. Theminimum andmaximum amountsof movement together with the average migration rate of the differenttest locations are summarized in Table 4. The satellite images of dates1987 and 2007 and the drawn interpreted maps of the different loca-tions in the study area are shown in subsequent figures (Figs. 9–12).

4. Results and discussion

4.1. Distribution of the aeolian sand and dune types

Satellite images of the southeasternWesternDesert indicate that ae-olian sand covers about 26.8% of the whole study area (Fig. 4). The esti-mated amount of transported aeolian sand is about 210 × 106 kgannually (Altorkomani, 1999). In fact, these sands show variable

Table 2Mean and range values (measured in meters), ANOVA test (P-values) and Post Hoc Test P-values (Dunnett's T3 pairwise comparisons) for Toshka and Kharga barchans and comparativedunes from different sites around the world.

Location N Length (a) Width (c) Height (H) a/c ratio

Mean values, range in parentheses

Toshka 111 183.24 (31.2–608.2) 175.10 (31.2–476) 16.61 (2.6–45.8) 1.06 (0.47–1.97)Kharga 44 54.4 (18–145.5) 45.4 (7.5–110) 7.8 (1.3–17.4) 1.8 (0.2–8.6)Kuwait(Khalaf and Al-Ajmi, 1993)

10 46.69 (18.2–91) 39.92 (15.7–65) 3.31 (1.8–7) 1.19 (0.74–1.77)

Southern Morocco(Sauermann et al., 2000)

8 37.75 (17.8–79) 57.48 (28.5–96.3) 4.66 (2.3–8.5) 0.65 (0.48–0.88)

California, USA(Long and Sharp, 1964)

27 67.62 (33.5–111.3) 108.32 (41.2–254.5) 5.89 (2.7–12.2) 0.74 (0.23–1.53)

Southern Peru(Finkel, 1959)

44 22.55 (9–40) 36.87 (11.4–66) 3.40 (1.4–5.8) 0.64 (0.43–1.53)

ANOVA test (P-values)

b0.001⁎ b0.001⁎ b0.001⁎ b0.001⁎

Source (I) Source (J) Post hoc Test P-values

T Kh b0.001⁎ b0.001⁎ b0.001⁎ .108K b0.001⁎ b0.001⁎ b0.001⁎ .944M b0.001⁎ b0.001⁎ b0.001⁎ b0.001⁎

C b0.001⁎ b0.001⁎ b0.001⁎ b0.001⁎

P b0.001⁎ b0.001⁎ b0.001⁎ b0.001⁎

Kh K .993 .998 b0.001⁎ .001⁎

M .552 .962 .099 .430C .374 b0.001⁎ .163 .001⁎

P b0.001⁎ .596 b0.001⁎ .003⁎

K M .997 .754 .904 .008⁎

C .253 b0.001⁎ .012⁎ .033⁎

P .075 1.000 1.000 .007⁎

M C .061 .029⁎ .924 .957P .553 .479 .846 1.000

C P b0.001⁎ b0.001⁎ b0.001⁎ .832

N: number of samples; T: Toshka; Kh: Kharga; K: Kuwait; M: Morocco; C: California; P: Peru.⁎ P-values b 0.05 indicate significant differences among sources (bold face type).


distribution patterns across the study area and their accumulation pat-tern is lowest at the top of the Sinn el Kadab Plateau. The high sand ac-cumulation of dunes is recorded in the central part of the Toshka Plain tothe south of Sinn el Kadab escarpment and partly along the westernshore of Lake Nasser (Hamdan et al., 2015).

The free-moving dunes of the southeastern part of the Western De-sert exist mainly in the Toshka Plain. However, a few small barchandunes are also recorded at the top of Sinn el Kadab Plateau. The an-chored dunes (lee dunes and sand shadows) exist over both the Sinnel Kadab Plateau and the Toshka Plain. Sand shadows (falling dunes)were accumulated at the escarpments of the Sinn el Kadab Plateauand in the leeward sides of the Nubian sandstone hills in the ToshkaPlain that may also lead to the development of lee dunes.Where the to-pography no longer has a significant effect, some of the linear lee dunesoften split downwind into individual separate barchan dunes andbarchanoids (Fig. 5). The Toshka dunes primarily originated from anolder (late Pleistocene) dune sand to the north of study area (Hamdanet al., 2015).

The eastern part of the Toshka area is characterized by a relativelylittle sand accumulation in the form of individual barchans and an-chored dunes parallel to the western coast of Lake Nasser (El Shazly etal., 1977). In the western part of the Toshka Plain, linear dunes thatare about 1 km long with sinuous slip faces exist together with solitarysmall barchans (Geofizika, 1966). The central and western parts of thestudy area is characterized by high sand accumulations in the form ofbarchan and linear dune fields, which are 10 to 45 km in length and2–8 km in width, and extended in a north–south direction. The lineardune types occupy the northern part of the dune field, while the bar-chan types exist in its southern part. The observed barchans are of twotypes; small, very rapidly formed dunes without slipfaces, probablymore accurately termed dome dunes (McKee, 1966), and moderate tolarge crescent mega barchans. The former vary in length from 10 to

40 m, in width from 5 to 30 m and from 1 to 4 m in heights; while thelatter vary in length from 140 to 420 m and from 150 to 490 m inwidth at heights of 15–50 m, and with a dune cover density varyingfrom 5.4 to 10.9 per km2. Barchans tend not to remain isolated but canjoin up and form complexes ranging from train-like successions of bar-chans to real dune massifs. This process has been explained in a recenttheoretical work by Melo et al. (2012).

4.2. Morphological classification of barchan dunes

The data of the barchans in the southeastern part of theWestern De-sert provide information that concerns latitudinal variations in bar-chans. The investigated barchan dunes in the Toshka area arecharacterized by a wide range of their morphologic parameters. Theyvary in length from 31 to 608 m, averaging 183.2 m (Table 2), whereasthe mean width of these dunes is 175 m and they may reach 46 m inheight. To the north, the barchan dunes in the Kharga Depression,which represents the northern extension of the same barchan streamthat traverses the Toshka area, also exhibit the same wide range in di-mension, but tend to be much smaller. The dunes located in the Khargarange in length from 18 to 145m, averaging 54m, and between 7.5 and110 m in width, averaging 45 m; they are notably lower in height thanToshka dunes, averaging 7.8 m.

Barchan dunes can be classified as simple, compound or complex, assuggested by McKee (1979). Based on a ratio of the length of the wind-ward slope (a) and thewidth (c), barchan dune shape has been catego-rized by Long and Sharp (1964) as: Fat (≥1), Pudgy (0.75), Normal (0.5)and Slim (0.25). From 111 barchan dunes investigated in Toshka areafor their morphologic classification based on the a/c ratio, only 22 bar-chans were normal in their morphologic forms, representing attainingabout 20% of the recognized barchans. However, most of the investi-gated barchans were fat (54%) or pudgy (24%), characterized by a high

Table 3Dunemigration calculations of the four test locations in the Toshka area, southeastern partof the Western Desert of Egypt.

Testarea

Duneno.

Migrationpoints

Azimuth ofthe migration(in D.DD)

Migration inmeters(1987–2007)

Average dunemigration (m)

Area1

1 1–1 189.88 102.32 138.482–2 189.04 167.543–3 201.95 145.59

2 4–4 189.91 142.87 166.965–5 182.09 192.216–6 183.64 165.79

3 7–7 186.25 112.88 145.818–8 185.59 126.129–9 184.57 198.42

4 10–10 182.50 211.30 211.6011–11 184.20 215.9612–12 184.73 207.53

5 13–13 186.58 91.89 118.2014–14 178.02 152.2415–15 210.58 110.46Average 187.97 156.21

Area2

6 1–1 187.18 99.24 76.232–2 199.95 66.663–3 176.22 62.79

7 4–4 186.21 76.53 86.045–5 173.72 94.556–6 192.35 87.05Average 185.94 81.14

Area3

8 1–1 186.03 90.19 113.162–2 193.14 125.003–3 179.45 124.30

9 4–4 180.00 101.23 112.435–5 182.79 97.506–6 182.94 138.57

10 7–7 180.98 123.03 114.708–8 174.38 107.589–9 186.59 113.51Average 182.92 113.43

Area4

11 1–1 195.78 119.39 113.052–2 193.23 72.433–3 186.00 110.224–4 198.93 150.16

12 5–5 191.74 115.27 115.2713 6–6 194.45 138.49 142.96

7–7 179.26 140.118–8 184.87 136.369–9 199.49 156.86

14 10–10 188.68 155.50 155.5015 11–11 186.45 138.45 119.96

12–12 196.59 99.6813–13 192.85 121.74

16 14–14 199.57 123.91 133.6915–15 198.33 143.48

17 16–16 189.79 118.67 130.8117–17 193.89 126.0718–18 191.70 147.70Average 192.31 128.58


a/c ratio (Appendix A). Fat dunes occur in areas where there is a sub-stantial sand influx and lower shear velocity (Parteli et al., 2007). Fewof the recognized barchans are slim. The fat and pudgy barchans arealso common in the investigated dune streams of the Kharga

Table 4Summary of results of dunes migration computations of aeolian dune sands of the four test are

Test area No. ofdunes

Time period(years)

Min.movement(m)

Max.movement(m)

Averagemovement (

Area 1 5 19.97 91.89 215.96 156.21Area 2 2 19.97 62.79 99.24 81.14Area 3 3 19.97 90.19 138.57 113.43Area 4 7 19.71 72.43 156.87 128.58Toshka areaaverage

17 19.90 79.33 152.66 119.84

Depression, representing 65% and 23.7%, respectively, whereas normaland slim dunes (11.3%) are less prominent.

Comparing the studied barchans with other dunes in different sitesindicates that the Toshka dunes seem to bemuch larger than other bar-chans not only in the same dune stream of the same desert (the Khargadunes) but also comparedwith barchans in other deserts: in Kuwait, theAtlantic Sahara in southernMorocco, and La Pampa de la Joya (southernPeru). Larger barchans occur in California but still their dimensions aresmaller than barchans in Toshka area (Table 2). The barchandimensionsin the Toshka area as well as in other regions are linearly related to eachother. An example is the relationship between the dune height and itswidth displayed in Fig. 7, which shows a clear linear (r = 0.76) signifi-cant correlation. Most of the Kharga barchan and some of California'ssamples deviate from the trend line through the majority of points,reflecting that a fair relationship exists between width and height ofbarchan dunes in these areas. Although Tsoar (1985) provides an expla-nation for the steady state response of dunes such as barchans, no expla-nation states why a linear relationship should exist between the heightof barchan dunes and thewidth of the horns (Hesp andHastings, 1998).Performing one-way ANOVA test for the dune dimensions shows signif-icant differences between means of dune parameters (P b 0.001) forboth barchans of the Toshka area and other dunes fromdifferent regions(Table 2). Among the different considered dunes, the Toshka barchansare comparatively characterized by a larger dune shape. Post hoc testsand Dunnett's T3 pairwise multiple comparisons differentiated theToshka dunes as a separate group among the other barchan dunes(Table 2). With the exception of a/c ratio of barchans in the Kuwaiti de-sert (Khalaf and Al-Ajmi, 1993), the Toshka dunes are significantly dif-ferent in their parameters than barchans from other sites. The Khargabarchans, like other barchans from different deserts, are not signifi-cantly different from each other in their parameters. An exception isbarchans from California (Long and Sharp, 1964), which may be shapedifferentiated from barchans in other regions, especially those fromsouthern Peru (Table 2). The previous results obtained from statisticalapproach of the considered dunes can be clarified by plotting the a/cratio vs. height of barchans from the Toshka and the Kharga withother sites (Fig. 8). This diagram clearly shows the complete separationof the investigated barchans of Toshka and Kharga and those fromKuwait from the rest of the dune samples, indicating that they are sig-nificantly different in their length and width, but the Toshka barchansstill increasemore in height. However, theKharga samples are relativelymore dispersed in Fig. 8 than are other dunes, implying that they con-tain barchanswith awide range of a/c ratios. Dunes fromother locations(California, southern Morocco and southern Peru) overlap, which sig-nifies that they are quite similar in their dimensions (Table 2; Fig. 8).

4.3. Hazards of sand encroachment on the Toshka Project

4.3.1. Dune movementThe first attempt to estimate the rate of movement of Egyptian de-

sert dunes was done by Cornish (1900), who measured a rate of4.5 m/year in the dune crests east of the Nile Delta (Table 1). Manyareas in the Western Desert were the subject of evaluation of sanddune movements by different authors (e.g. Beadnell, 1910; Ashri,

as in the southeastern part of the Western Desert of Egypt.

m)Min. migration rate(m/year)

Max. migration rate(m/year)

Average migration rate(m/year)

4.60 10.82 7.823.15 4.97 4.064.52 6.94 5.683.67 7.96 6.533.99 7.67 6.02

Fig. 9. Google Earth satellite images of the first location of dates 1987 (a) and 2007 (b) together with constructed maps illustrating the location of dunes (c) and the points used inmigration calculations (d) during this time period.


1973; Embabi, 1979; Sharaky et al., 2002; El Gammal and Cherif, 2006)who assigned diverse dune rates ranging from 0.5 m/year to be as highas 100m/year. More recently, Abou El-Magd et al. (2013) estimated therate of movement of barchans and transverse dunes in south WesternDesert which ranged from 1.3 to 19.3 m/year.

In the study area, the investigated barchans were concentrated intofour test areas along a narrow N-S dune stream, located immediatelywest of the Toshka lakes in the central and western parts of the Toshkaarea. Seventeen barchans were picked from this dune stream throughvisual interpretation of satellite images of the four chosen test locations.

Fig. 10. Satellite images of the second location of dates 1987 (a) and 2007 (b) in addition with the drawn maps that show the location of dunes (c) and the points used in movementcalculations (d).


The dunes were carefully selected to be of well-defined boundaries andmaintaining their shape in as many successive images as possible. Also,they were considered to be free to move without the presence of anyhindrance, such as surrounding bedrock exposures that may preventor limit their movements.

The first test area is located to the west of the northernmost lake ofthe Toshka region. The satellite images were about 7 m/pixel resolutionwith dates 29/9/1987 and 12/9/2007 and a time-span, between the twoimages, of 19.97 years. Five barchans dunes with sharp boundaries andsimilarmorphology on these two images were recorded in for this loca-tion. Fifteenwell-definedpoints on the dune boundaries are selected for

migration determinations (Fig. 9). Final results of the point to pointcorrelation between the two images are listed in Table 3, in whichthe amount and direction of movement in each point over theassigned time period and the average migration of each dune arepresented. This analysis reveals a minimum and maximum pointmigration of 91.89 and 215.96 m, respectively, with an average mi-gration value of 156.21 m. Consequently, the minimum and maxi-mum migration rates are estimated at 4.60 and 10.82 m/year,respectively, with an annual rate of 7.82 m/year (Table 4). The aver-age direction of movement of these dunes in this area was SSWalong an azimuth of 187.97°.

Fig. 11. Satellite images of both dates 1987 (a) and 2007 (b) together with the constructed maps representing the dune distribution in the third test location (c) and the points used inmigration measurements (d).


The second location was at a distance of about 13 km southwards ofthe first test area. The satellite images are of resolution of about 5 m/pixel, and have dates and a time-span between images similar tothose of the first location. The location encloses only two barchandunes typically suitable for migration measurements, i.e. well definedoutlines and maintenance of shape in the two images (Fig. 10). Sixpoints were located on the dune boundaries. Processing estimates alowest and highest point movement of 62.79 and 99.24m, respectively,led to an average movement of 81.14 m. Computation indicates 3.15,4.97 and 4.06 m/year as minimum, maximum and average rates ofdune migration, respectively. The trend of dune migration is due SSWalong an average azimuth of 185.94°.

The third test area is located at a distance of about 11.5 km south-west of the second location. Satellite images of this location are of6 m/pixel resolution and have also the same dates as the first test

area. Four barchan dunes are recorded; three of them were suitablefor analysis, while the fourth had a poorly defined boundary (Fig. 11).Nine selected points were projected along the dunes borders. Point topoint correlation of the two images indicates the greatest and leastvalue of point migration as 138.57 and 90.19 m, respectively, with anaverage migration of 113.43 m. The calculated minimum, maximumand averagemigration rates are 4.517, 6.9 and 5.68m/year, respectively(Table 4). The average direction of dune movement is SSW (azimuth of182.92°).

The fourth test area is located to the south of Toshka Lakes at a dis-tance of about 55 km to the southwest of the third test area. The satelliteimages are of about 8m/pixel resolution andhave dates 31/12/1987 and12/9/2007. The time-span between the two images is 19.71 years. Thislocation enclosed 7 barchan dunes with well-defined boundaries. Eigh-teen points were chosen for migration determination (Fig. 12) and the

Fig. 12. Satellite images of a successive row of barchans in the fourth location of dates 1987 (a) and 2007 (b) and the drawn interpreted maps showing the distribution of dunes in thefourth location (c) and the points used in migration calculations (d).


results of dune migration analyses are shown in Table 3. The minimumandmaximum pointmovement were 72.43 and 156.87m, respectively,with an average value of 128.58 m. The minimum and maximum ratesof dune migration were 3.67 and 7.96 m/year. The average migrationof these dunes is 6.53 m/year, along a SSW direction (azimuth of192.31°).

The minimum and maximum values of dune movement togetherwith their rates and the average rate of migration representing all thetest areas are presented in Table 4. This table shows that the first testarea exhibits the highest migration of barchan dunes in the Toshkaarea of a maximum point movement of about 216 m and a maximummigration rate (10.82 m/year), averaging 7.82 m/year. This contrastswith dunes in the second test area which show the lowest dune move-ment, with only 4 m/year as an average rate of migration. This secondlocation contains the largest studied barchans as well as the ‘fatty’

nature of theses dunes (Appendix A), whereas the first location areacontains the smaller normal dunes that movemore speedily. By averag-ing dune migration measurements of the 17 representative barchandunes in the central and western parts of Toshka area during the19.9 year time-span, the minimum point movement is 79.33 m andthe maximum is 152.66 m, with an average point movement of119.84 m (Table 4). The annual rate of migration of the studied sanddunes varies fromabout 4 to 7.67m/year. The calculated averagemigra-tion of these dunes is 6.02 m/year along a SSW direction (azimuth of187.29°). This calculated average migration rate is very similar to thatgiven by Besler (1986), who measured the rate of migration of 5 m/year to the dunes of the Toshka area.

In order to investigate the relationship between dune morphologyand dune movement, a correlation of the length/width (a/c) ratio withthe dune migration of the selected barchans is illustrated in Fig. 13.

Fig. 13. The length/width (a/c) ratio vs. dunemigration of the investigated barchans of theToshka area in the southeastern Western Desert shows clearly that small dunes migratemore rapidly than larger ones.


This diagram shows clearly the inverse relationship between these twovariables, where normal barchans (the smallest a/c ratio) migrate morerapidly than fat barchan dunes. It is obvious that small dunes (e.g.Table 2; Dunes No. 2, 4 and 14) move faster than large fat ones(Table 2; Dunes No. 5, 6, 7, 10, 15 and 16). Similar observations arealso reported by previous studies (e.g. Pye and Tsoar, 2009). Some ob-served barchans are also characterized by strongly elongated hornsand asymmetric progress of their plan shapes (e.g. Table 2; Dunes No.8, 10, 14 and 16). These could be as a result of superimposing of two suc-cessive barchans on one another. Wind direction variations and asym-metric sand influx due to a complex distribution of upwind sandsources (complex dune patterns upwind of the dune) may also causedune asymmetry (Bourke, 2010; Parteli et al., 2014). In fact in the inves-tigated images (e.g. Fig. 12) some asymmetric barchans not directly par-ticipating in collisionmay have the origin of their asymmetry associatedwith the presence of upwind dunes. Therefore the influx asymmetrymay be a possible origin of the asymmetry of some dunes in the Toshkaregion.

Many factors affect dune displacement, such as differences in localwind regime, sand supply, topography, and vegetation but size of thedunes, especially height of the slip face, is generally regarded as themost important factor (Long and Sharp, 1964). In evaluating the influ-ence of the a/c ratio on movement, it appears important to knowwhether the barchan is growing or is in a steady-state condition. As-suming a uniform sand flux, a growing fat barchan should move moreslowly than a growing barchan that is slim because a greater volumeof sand is required to produce corresponding increase in height of thefat dune. On the other hand, in a steady-state condition, a fat barchanmightmove more rapidly since most of the sand causing advance is de-rived from thewindward slope of the dune itself and a fat barchan has alongwindward slope as a source of supply. The sand coming to a steady-state dune from upwind serves principally to set the dune's sand intomotion and to replace the loss from the tip of the horns (Long andSharp, 1964). Plottingdunemigration of each of the studied 17 barchans

Fig. 14. Height of slip face vs. dune migration of the investigated barchans of the Toshkaarea in the southeastern Western Desert shows clearly that higher elevated barchandunes move more slowly than smaller ones.

against their height (Fig. 14) shows an inverse relationship. Somepointsdepart from the linear relationship which may be due to the shape ofthese dunes as most of them are of the ‘fatty’ type. This implies thatdunes which are greater in their heightmovemuch slowly than smallerones.

4.3.2. Sand drift potentialIn recent years, amajor project developed in the southeastern part of

theWestern Desert (the Toshka project) and has provided many bene-fits to Egypt (Wahby, 2004). It helped in doubling the cultivated andreclaimed land for Upper Egypt, attracting workforce, constructing anefficient network of main and side roads, facilitating power generationand utilizing the massive amounts of water sorted in the Lake Nasser.However, aeolian sand encroachment may cause serious hazards tothe sustainable development of the Toshka Project (Abdelmoaty et al.,2011). The southeastern division of the Western Desert is locatedwithin the hyper-arid zone where precipitation occurs once every twoor three decades. Data obtained from the Tropical Rainfall MeasuringMission (TRMM) shows that the Toshka area received b20 cm of totalrainfall from September 1999 to September 2006 (Bastawesy et al.,2008). Meteorologically, the mean air pressure in this area varies be-tween 940 and 1001 mbar, whereas the mean air temperature rangesfrom 16 to 40.5 °C (Table 5). The mean humidity is low and ranges be-tween 15.6 and 48.8%, whereas the mean wind speed ranges between3 and 7.1 m/s (Khedr et al., 2013a). El-Baz and Wolfe (1982) studiedthe wind regime of the Western Desert of Egypt and revealed that twomajor wind regimes exist; a narrow band of predominantly westerlyalong the Mediterranean coast and generally a north northwesterlythough the Western Desert. They also noticed some local variations inthe general wind trend and attributed these to topographic effectssuch as that at the Farafra, Kharga and Aswan areas. At Aswan (east ofthe study area), the general wind trend is imposed by the north–southtopographic highs of the Eastern Desert and Sinn el Kadab Plateau.

Based on the surfacewinddata of 9 years (2000–2008) of theKhargaand East Uwinat meteorological stations; the wind in the Toshka areablows mainly from the north (about 42%) and the northeast (nearly35%). The Kharga Depression has dominant winds mostly from thenorthwest (34.2% of time) and from the north (32.6% of time) direc-tions. Wind data also reveal a considerable variation of strength. Effec-tive wind strength (12 knots/h) is maximal in this area and blows for46.7% of time (Hereher, 2010). Sand drift potential (DP) refers to theamount of sand moved by the surface wind that includes values abovethe threshold velocity. It is calculated using Fryberger (1979) equation,as follows:

DP ¼ V2 V−Vtð Þt:

where, DP refers to the sand drift potential in vector units (VU); V is theaverage wind speed in knots at 10 m height; Vt is the threshold velocity(the velocity required to entrain sediment) in knots; and t is the percentof time during which wind velocity is greater than the threshold veloc-ity required to entrain the sediment. Based on the average grain size ofToshka sandswhich is 0.35mm(Hamdan et al., 2015), the threshold ve-locity is estimated to be 12 knots (6.2 m/s) under dry conditions(Fryberger, 1979).

Table 5Summary of meteorological data of the area west of Lake Nasser (adopted from Khedr etal., 2013a).

Meteorological parameter Min. Max. Average

Pressure (mbar) 940 1001 989.55Temperature (°C) 16 40.5 27.05Humidity (%) 15.6 48.5 29.59Wind speed (m/s) 3 7.1 4.79


Resultant drift potential (RDP) represents the magnitude of DP,whereas the resultant drift direction (RDD) refers to the direction inwhich sandwill be transported. The ratio of the resultant drift potential(RDP) to the drift potential (DP) is known as the directional variabilityof surface wind (RDP/DP). Higher RDP/DP values indicate that thewind comes from the same direction (a narrowly unidirectional windregime) whereas lower values indicate a multidirectional wind regimewith multiple significant drift directions.

Sand drift potential is always coincident with both the strength anddirection of the effective winds. Drift potentials are measures of the en-ergy of surface winds in terms of sand movement. Fryberger (1979)classified energy of surface winds in arid regions, based upon the aver-age annual drift potential, into three groups: (1) low-energy environ-ment with drift potential (DP) less than 200 vector units (VU);(2) intermediate-energy wind environment with DP ranges from 200to 399 VU; and (3) high-energy wind environment with DP greaterthan 400 VU (Table 6a). The calculated potential sand transport param-eters showed that the potential sand drift of the Kharga area attainsabout 173 VU (low energy environment), which is much lower thanthat of the Toshka area which reaches 571 VU (Table 6b). The greatestDP value of the Toshka area is classified as high-energy wind environ-ment. Fryberger et al. (1984) estimated the amount of sand creep aseach VU represents 0.07 m3 of sand moving across 1 m width of land.The corresponding amount of sand drifting per 1 m width of land inthe Toshka area, based on Fryberger's rate, is about 40 m3/year. In theKharga area, the amount of sand drift is estimated at about 12 m3/year, which is less than that in the Toshka region. High directional var-iability index is recorded in both the Kharga and Toshka regions (RDP/DP = 0.82 and 0.78, respectively); this indicates that wind blows froma narrow wind direction (from the north and northwest) and wherebarchans form chains extending parallel to the RDD. The resultantdrift direction (RDD) is due S and SSW (azimuth of 181.2° and 192.3°)for the studied Kharga and Toshka areas, respectively (Table 6b).These results reveal that the Toshka area is characterized by the highestsand activity and drifting in the Western Desert of Egypt. Such a highamount of transported sand every year will result in hazards to the sus-tainable development with in the Toshka Project.

The adverse impacts of sand encroachment and mobile dunes are agreat threat, especially to the agricultural activity areas (Fig. 15a). Ef-fects extend to the roads (Fig. 15b), irrigation canals and to the ToshkaLakes (Fig. 15c) causing significant economic losses and considerableexpense for mitigation measures. What happens to a moving dunewhen it reaches a lake shore is that it stops shortly after entering thelake. Lake Nasser itself may also be influenced by the sand encroach-ment that can be expected to diminish its water storage capacity(Abdelmoaty et al., 2011). The Google Earth images and field investiga-tions of the present study show that the central and western parts ofToshka area are highly affected by sand encroachment compared tothe eastern part. The Toshka lake area is threatened by about 10 dunestreams, each stream is about 5–10 km in length and 0.3–1 km in

Table 6a) The classification of wind energy environments using drift potential (DP) and directional vaRDP, and RDD) as well as the directional variability index (RDP/DP) of the studied Kharga and

a)

DP (VU) Wind energy environment RDP/DP

200 Low 0.3200–399 Intermediate 0.3–0.8400 or more High 0.8

b)

AreaDP(VU)

Sand drift (m3/m-w/year)

Kharga 173 12.11Toshka 571 39.97

width. These dune streams begin at the foot of Sinn el Kadab Plateauas lee dunes then change to be linear dunes and finally become barchanand barchanoid types. Because of the variance in the rate of dunemigra-tion, two ormore barchan dunesmaymerge to form a small linear duneconnecting the main dune stream.

4.3.3. Aeolian sand risk assessment of the Toshka areaStopping dune movement is extremely costly and it is only a practi-

cal and economic solution in areas likely to be affected with dune en-croachment (Tag El Din, 1986). Sand dune movement risk evaluationis important for countries that have migrating dunes that threatentheir towns, transportation routes, or agricultural fields, like the West-ern Desert of Egypt (Effat et al., 2011). Remote sensing data has facili-tated desert exploration and modeling risk and vulnerability of sandencroachment. The rate of desertification, especially formation and ex-pansion of dune fields, depends mainly on six parameters; these are:terrain surface elevation, dune slope angle, dune slope direction (as-pect), relative moisture index, prevailing wind direction, and prevailingwind speed. These six parameterswere obtained from Shuttle Radar To-pographic Mission data (SRTM) and the land cover map produced bythe Food and Agricultural Organization (FAO), which were used byEffat et al. (2011) tomodel potential dunemigration in theWestern De-sert of Egypt. Based on risk scale given by Effat et al. (2011), the risk as-sessment of aeolian sand dune encroachment in the southeasternWestern Desert can be evaluated (Table 7). The elevation of the desertarea is represented as elevation risk index, where higher elevationareas are more at risk to sand dune activity. According to the SRTMdata and topographic maps of the Toshka area, altitude ranges from114 to 478mabove sea level.When it is represented as a dune elevationrisk index, medium risk is indicated. On the other hand, dune slope inthe desert area is represented as a slope risk index, where sand accumu-lated on steeper slope dunes is more risky. The dune slope value of theToshka area varies between 40 and 60°. Consequently, the dune sloperisk index here is medium to high risk. The third measured index isthe dune slope direction (aspect), where dunes facing the prevalentwind direction are most likely to experience dune movement and thushave the highest risk aspect. The greater aspect angle the greater isthe risk. The azimuth of the dune migration in the study area rangesfrom 173.72° to 210.58° (Table 3). Hence, the aspect risk index ofToshka area is measured to be low to medium risk. The average windspeed of Toshka area varies between 9.5 and 11 m/s (Effat et al.,2011), so its wind speed risk value is thus high. The spatial distributionof wind directions, especially the dominant directions, are more effec-tive in generating a sand movement risk. Meteorological data(Hereher, 2010) indicate that the annual average prevailingwind direc-tion of the Toshka area varies between 0° and 45° (NNE) and its winddirection risk is thus, medium. Finally, moisture is measured as a rela-tive moisture index, where sand dunes are more active when the rela-tive moisture is low (dry). As the study area is hyper-arid, the sand isvery dry and the relative moisture risk index will be the highest.

riability (adopted from Fryberger, 1979). b) The potential sand transport parameters (DP,Toshka regions.

Directional variability Wind regime

High Complex or obtuse bimodalIntermediate Obtuse or acute bimodalLow Acute Unimodal

RDP RDP/DP RDD

142 0.82 181.2447 0.78 192.3

Fig. 15. a) Sand dunes encroaching on the cultivated land of the Toshka area. b) Dune encroachment on Aswan-Abu asphaltic road in the Toshka area. c) A series of barchan dunes crossingthe Toshka Lakes from the north in a dune stream trending in N–S direction.

Table 7Sand dune risk evaluation of the aeolian sand dunes of Toshka area in the southeastern Western Desert of Egypt.

Parameter Risk scale rangea Toshka area Risk evaluation

Elevation(m)

4–1033 114–478 Medium

Dune slope angle(degrees)

0–90 40–60 Medium–High

Slope aspect(azimuth degrees)

0–360 173–210 Low–Medium

Relative Moisture Index −4.5–12.6 −4.5 to −3.2a HighPrevailing wind speed(knots/h)

0–23 18.5–21.6aa High

Prevailing wind direction(azimuth degrees)

0–360 0–45 Medium

a Data from Effat et al. (2011).


123


Accordingly, the sand dune risk for the Toshka area is about a me-dium to high risk and the road vulnerability for sand encroachmentrisk is high. These may cause serious hazards to the Toshka projectand agricultural lands in this area.

4.4. Factors controlling dune occurrence, morphology and movement

Factors controlling the occurrence and morphology of sand depositsare complex (Gifford et al., 1979). They include the wind direction,strength and duration; the nature, extent and rate of erosion at the sed-iment source; the distance from the source; the grain and fragment size;the underlying and surrounding topography; the nature of the surface(rough or smooth); the amount and type of vegetation and the amountof rainfall. It is important to stress thatwe do not take into considerationin this discussion all these factors, but only those that we have consid-ered in our field studies and laboratory investigations for barchandunes in the southeastern Western Desert of Egypt and from compari-sonswith barchans in other deserts of theworld. Themorphological dif-ference between barchan in the Toshka area and other regions in theworld could be due to increasedwind activity, to increased sand supply,or both.Meteorological data frommany sources proved that the averagewind speed of Toshka area varies between 9.5 and 11 m/s, which isgreater than that of Kharga in the Western Desert of Egypt (2.9 m/s)and California, USA that reach 4 m/s, and both of the Pampa de la Joyain Peru and in the Kuwaiti Desert (5.4 m/s), while the average windspeed in southernMorocco varies from7 to 8.5m/s. Increased sand sup-ply seems likely to be the most important factor. In the Kharga Depres-sion, the sand supply is limited due to the presence of the northernescarpment of the depression, which prevents much sand fromreaching the depression floor (Hereher, 2010). Limited supply of sandis funneled, forming the barchan dunes. On the other hand, large bar-chans in the Toshka area were formed due to the great availability ofsand, i.e. the Great Sand Sea, located to the north. Considering these fac-tors that affect the barchan dunes in the Toshka area and the resultsdiscussed in the preceding sections, we can conclude that the highwind speed (9.5–11 m/s) and the considerable quantity of driftingsand (40 m3/year across 1 m width of land) in these dunes comesfrom dune streams crossing the north of the Bahariya Oases throughthe Farafra, Dakhla and Kharga Oases to reach the Toshka area in thesoutheastern Western Desert (Fig. 1) and corresponds with a high-energy wind environment (571 VU). All these factors together haveassisted barchan dunes in the Toshka area to increase in size signifi-cantly compared with barchan dunes in other desert areas.Sauermann et al. (2000) showed that barchan dunes move proportion-ally to wind velocity and inversely proportionally to their height. How-ever, the lowestmovement of Toshka dunes comparedwith other areasin the northern and central Western Desert of Egypt (Fig. 1) may be at-tributed to several topographic and morphologic elements. From thesefeatures (Figs. 3-4) are: 1) the existence of Sinn el Kadab plateau, onthe northern margin of the Toshka Plain; 2) the rugged ground surfaceof the western part of the studied Toshka area; 3) the occurrence ofToshka Lakes in the central part of the Toshka area, which intercept bar-chan streams crossing from the north to south; 4) frequent small hills ofNubian sandstone that are scattered in different parts of the Toshkaarea; and 5) large dune shape (‘fatty’ nature) and elevated height ofthe Toshka barchans. This could support the conclusion that the Toshkadunes are unique in their morphometric characteristics.

5. Conclusions

The southeastern part of theWestern Desert represented mainly bythe Toshka area is characterized by considerable aeolian sand accumu-lations. These sands can be categorized into free-moving barchan andlinear dunes and anchored (lee and shadow) dunes. A relatively lowsand accumulation in the form of solitary barchan, lee dunes andshadow dunes are scattered in the eastern part of the Toshka area,

whereas the central and western parts of the study area are distin-guished by high sand accumulations in the form of barchan and lineardune streams in a north–south direction. Linear dune types occupythe northern part of the dune field, while the barchan types exist in itssouthern part.

The barchan dunes of the Toshka area have been studied for theirmorphometric analysis in the form of length, width and height andtheir relationships in order to identify their morphologic characteristics.Most of the recognized barchans in the Toshka area are fat and pudgy;while normal barchan dunes are less common. The main parametersof Toshka barchans are compared statistically with barchans from theKhargaDepression (WesternDesert of Egypt) and dunes in different de-sert regions in Kuwait, SouthernMorocco, California and Southern Peru.The Toshka barchans are unique in their dimensions; characterized bygreater aspect (higher values of length and width and increase morein height).

The potential for sand drift in the Toshka area is considerable as it is ahigh-energy wind environment. The area is characterized by the maxi-mum sand activity and drifting in theWestern Desert of Egypt. Thismayexplain the unique large form of barchan dunes in the Toshka area. Fieldinvestigations show that the central and western parts of this area arehighly affected by sand encroachment. Risk assessment of the south-eastern Western Desert indicates a medium to high sand dune riskand high sand encroachment risk to vulnerable areas. This may repre-sent serious hazards to the newly-established Toshka Project, roads, aswell as the reclaimed agricultural lands in this area.

The trend and rate ofmigration of barchan dunes is essentially basedon remote sensing andGIS techniques through the accurate comparisonof dune location in satellite images of two different dates (nearly20 years interval). The mean movement of 17 representative barchandunes in the central and western parts of Toshka area varies fromabout 4 to 7.67 m/year. The calculated average mean of migration ofthese dunes is nearly 6 m/year along SSW direction (azimuth of187.29°).

The high-energy wind environment combined with the highamount of drifting sand are principal factors thatmay facilitate theprev-alence of large fat and pudgy barchan dunes in the Toshka area. Thegreat shape and fatty nature of the Toshka barchans together with therugged bedrock topography may explain the reduction of the barchanmigration rate compared with dunes from other desert regions in theWestern Desert of Egypt.

Acknowledgements

Field work of this paper was supported by the Ministry of State ForEnvironmental Affairs. We would like to express our thanks to Prof.Roger J. Flower, Department of Geography, UCL — University CollegeLondon for carefully reading the manuscript and his valuable notes,which have greatly improved the text. The authors are also indebtedto anonymous reviewers and the editor for their helpful commentsthat improving the final version of the manuscript.

Appendix A. The morphologic parameters; length (a), width (c) andheight (H) measured in meters and the a/c ratios and the locationsof the investigated barchans in the southeastern part of theWesternDesert of Egypt. The first 17 barchans have been investigated for thedune movement study

Dune no.
Latitude Longitude Length(a)
Width(c)

Height(H)

a/cratio

Dunetypea

23.36933549
30.46212531 180.55 214.05 20.39 0.84 P 23.35829092 30.4590697 150.89 161.09 15.25 0.94 P 23.35921976 30.46244395 215.25 297.43 28.49 0.72 P

( (


continued)

Dune no.

456789111111111122222222223333333333444444444455555555556666666666777777777

Latitude
Longitude Length(a)
Width(c)

Height(H)

a/cratio

Dunetypea

7
23.34696085 30.46887841 139.02 207.45 19.75 0.67 N 8 23.33753278 30.46370905 407.56 405.04 38.94 1.01 F 8 23.23588047 30.45889261 413.26 380.14 36.52 1.09 F 8 23.22761538 30.45503355 402.86 290.22 27.79 1.39 F 8 23.20138828 30.55587043 252.59 245.16 23.41 1.03 F 8 23.19046799 30.5637644 255.1 244.53 23.35 1.04 F 8 0 23.1840701 30.56082285 269.58 285.41 27.32 0.94 P 8 1 22.78776306 30.84440371 241.57 145.05 13.69 1.67 F 8 2 22.77895616 30.84367206 175.3 156.7 14.83 1.12 F 8 3 22.77575491 30.84404791 151.5 237.82 22.70 0.64 N 8 4 22.76828231 30.84396202 128.96 189.6 18.02 0.68 N 9 5 22.7659679 30.84541657 219.54 113.95 10.67 1.93 F 9 6 22.76011405 30.84404791 274.68 322.77 30.95 0.85 P 9 7 22.7542602 30.84321989 143.53 199.1 18.94 0.72 N 9 8 23.44493897 30.46785953 203.05 192.54 18.30 1.05 F 9 9 23.44012016 30.47124002 151.14 145.23 13.71 1.04 F 9 0 23.43767169 30.46868864 100.05 124.27 11.68 0.81 P 9 1 23.4428341 30.47682367 217.35 146.58 13.84 1.48 F 9 2 23.44053177 30.48173063 105.14 146.22 13.81 0.72 N 9 3 23.41673741 30.46969831 202.26 173.42 16.45 1.17 F 9 4 23.391083 30.46513303 278.59 298.55 28.60 0.93 P 1 5 23.37662353 30.47225545 242.17 233.48 22.28 1.04 F 1 6 23.36032157 30.4691588 152.84 173.28 16.43 0.88 P 1 7 23.33067584 30.42639765 404.26 383.75 36.87 1.05 F 1 8 23.28885417 30.4239268 151.47 200.35 19.06 0.76 P 1 9 23.27937621 30.47587728 405.29 324.15 31.08 1.25 F 1 0 23.27482882 30.47133054 360.11 217.33 20.71 1.66 F 1 1 23.26903 30.47997856 358.33 208.38 19.84 1.72 F 1 2 23.28869836 30.46565101 608.15 475.98 45.82 1.28 F 1 3 23.30002953 30.47940546 234.13 196.55 18.69 1.19 F 1 4 23.23034323 30.38731255 363.19 284.27 27.21 1.28 F 1 5 23.24960017 30.39157826 100.05 211.04 20.10 0.47 N
6
23.24996503 30.38661708 153.84 156.33 14.79 0.98 P 7 23.21923155 30.48913429 271.36 213.75 20.36 1.27 F 8 23.21806763 30.45270948 231.84 125.24 11.77 1.85 F 9 23.2060776 30.4560242 245.28 185.28 17.60 1.32 F 0 23.20057696 30.4668068 74.61 153.67 14.53 0.49 S 1 23.2014249 30.46801553 31.15 57.14 5.16 0.55 N 2 23.19892236 30.46824438 82.88 143.99 13.59 0.58 N 3 23.1945989 30.46729698 179.35 206.95 19.70 0.87 P 4 23.19058559 30.46659568 54.93 83.24 7.69 0.66 N 5 23.18815604 30.46827496 44.75 62.77 5.71 0.71 N 6 23.17831012 30.46506728 120.35 141.86 13.38 0.85 P 7 23.16904693 30.46547547 87.52 150.82 14.25 0.58 N 8 23.16455303 30.47193746 61.21 87.63 8.12 0.70 N 9 23.15850649 30.46474155 48.26 89.09 8.26 0.54 N 0 23.15310341 30.47606925 172.18 186.21 17.69 0.92 P 1 23.14301148 30.51678317 71.97 123.98 11.65 0.58 N 2 23.14038973 30.51570579 109.06 67.23 6.14 1.62 F 3 23.1376941 30.51633254 61.52 60.02 5.44 1.02 F 4 23.13522808 30.51597731 62.9 81.23 7.50 0.77 P 5 23.04762236 30.69678934 312.53 238.99 22.81 1.31 F 6 23.06075424 30.72914359 132.89 156.58 14.81 0.85 P 7 22.69149307 30.74341194 145.06 124.91 11.74 1.16 F 8 22.65410361 30.74218445 123.59 129.11 12.15 0.96 P 9 22.64846431 30.74323677 222.97 159.18 15.07 1.40 F 0 22.62234727 30.73930558 166.44 172.85 16.39 0.96 P 1 22.60134139 30.73885185 146.19 228.57 21.80 0.64 N 2 22.59001816 30.73902934 212.54 331.25 31.77 0.64 N 3 22.32164098 30.73150679 118.59 128.5 12.09 0.92 P 4 22.31747065 30.73184094 200.09 157.37 14.89 1.27 F 5 22.27448457 30.71713632 181.91 171.17 16.23 1.06 F 6 22.3021565 30.4502637 238.35 160.24 15.17 1.49 F 7 22.30055202 30.45501669 256.63 156.78 14.83 1.64 F 8 22.3028744 30.4600076 355.73 258.27 24.69 1.38 F 9 22.31941233 30.27394258 189.99 106.9 9.99 1.78 F 0 22.31269298 30.27870335 241.38 166.31 15.76 1.45 F 1 22.30738183 30.27368256 209.91 111.75 10.46 1.88 F 2 22.29964921 30.27650778 202.35 231.11 22.05 0.88 P 3 22.30365692 30.28116411 51.62 49.5 4.42 1.04 F 4 22.27499235 30.27352602 180.73 154.7 14.63 1.17 F 5 22.27852508 30.29780967 103.5 164.51 15.58 0.63 N 6 22.27356442 30.30240768 190.42 127.15 11.96 1.50 F 7 22.2699087 30.301921 92.1 169.05 16.02 0.54 N 8 22.26495231 30.30112851 77.3 103.24 9.63 0.75 P
continued)

Dune no.
Latitude Longitude Length(a)
Width(c)

Height(H)

a/cratio

Dunetypea

9
22.25961191 30.30182966 231.55 175.5 16.65 1.32 F 0 22.3551144 30.2959085 208.17 176.12 16.71 1.18 F 1 22.57901379 30.37153469 128.13 100.69 9.39 1.27 F 2 22.58024132 30.37251815 58.29 57.3 5.17 1.02 F 3 22.57369245 30.37036733 46.55 59.21 5.36 0.79 P 4 22.57151289 30.37182659 113.86 75.4 6.93 1.51 F 5 22.5711963 30.36772501 127.01 92.76 8.62 1.37 F 6 22.57681215 30.36736598 172.1 87.34 8.09 1.97 F 7 22.57891424 30.36920683 44.81 31.21 2.64 1.44 F 8 22.61329469 30.36759474 138.88 121.38 11.40 1.14 F 9 22.61128786 30.36310305 77.23 60.71 5.51 1.27 F 0 22.60882584 30.36510272 132.7 94.75 8.81 1.40 F 1 23.23241428 30.68558973 111.79 126.65 11.91 0.88 P 2 23.19400162 30.61439429 187.23 165.2 15.65 1.13 F 3 23.24501744 30.59414593 166.65 165.77 15.71 1.01 F 4 23.23552556 31.55812087 382.22 293.3 28.09 1.30 F 5 23.20599913 31.55815218 141.72 95.28 8.86 1.49 F 6 22.6277727 30.44665095 286.08 222.34 21.20 1.29 F 7 22.62887239 30.39196837 130.89 165.05 15.64 0.79 P 8 23.05951844 30.46294336 68.35 70.67 6.47 0.97 P 9 23.06196491 30.46279297 51.33 105 9.81 0.49 S 00 23.06634194 30.46344588 56.25 45.57 4.04 1.23 F 01 23.21205156 30.48616649 198.7 264 25.24 0.75 P 02 23.20643583 30.48986848 290.99 346.35 33.24 0.84 P 03 23.20002991 30.4784155 120.12 96.58 8.99 1.24 F 04 23.19599443 30.49193214 202.35 122.89 11.54 1.65 F 05 23.17765626 30.49023068 365.62 291.3 27.89 1.26 F 06 22.7383956 30.84338745 175.7 176.6 16.76 0.99 F 07 22.73311866 30.843468 156.25 218.95 20.87 0.71 N 08 22.7240155 30.84188888 189.87 211.6 20.16 0.90 P 09 22.68641578 30.84281142 161.25 218.2 20.80 0.74 N 10 22.70459028 30.87324951 124.7 165.1 15.64 0.76 N 11 22.71723548 30.87405127 115.77 172.27 16.34 0.67 N 1
a F = fat; P = pudgy; N = normal; S = slim.

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