Chapter 3 Formation and Geomorphology of the North-Western Negev Sand Dunes

H. Tsoar, D.G. Blumberg, and R. Wenkart

3.1 The Sinai-Negev Dunefield

The coastal plain of the northern Sinai Peninsula is a structural depression that borders several anticline mountains in the south. This coastal plain is covered by a dunefield in a wide area of 20-80 km from north to south, and for 260 krn from the Nile Delta in the west into the northern Negev Desert, where it terminates south of 2 Beer Sheva (Fig. 3.1). The dunefield covers an area of about 12,000km • The Sinai and Negev form one geographical unit subdivided artificially by a political border. The dunefield is located in the northern boundary region of the Eastern Sahara sub­ tropical desert, characterized by a long, hot and dry summer and a cool winter with a mean annual rainfall that is below 200 mm. The political border between the Negev and Sinai has generated two distinctly different landscapes that can be delineated from space-based imagery. The Sinai side of the border tends to be bright and is constituted of bare sand dunes, whereas the Negev side is dark and constituted of vegetated dunes. This political border has thus created a bio-physical border caused -by two distinctly different types of land use - grazing and wood-gathering activities in the Sinai, in contrast to almost no human-induced pressure in the Negev (Tsoar, Chap. 6, this volume). The Negev dunefield is triangular in shape, tapering eastwards because of the northern Negev anticline system that stretches from southwest to northeast and delimits the dunes in the southeast, and because of the storm winds blowing in this direction. The anticline of Har Keren is illustrated above the dunefield in Fig. 3.1. Nahal (wadi) Nizzana, which drains the north-western side of the Ramon anti­ cline, reaches to the south-western side of Har Keren but cannot cross the Agur Sands and, therefore, is diverted westwards where it disappears in the Sinai sands and becomes a defunct wadi. A similar defunct wadi is Nahal Shunra that cannot cross the Haluza Sands from south to north, while Nahal Besor does cross the dunes and drains into the Mediterranean south of Gaza (Fig. 3.1). The Negev dunefield can be subdivided into several sections, based on the geo­ logical structure, the wadis that cross the area, and the morphology of the sand dunes. The main duneftled is located in the North-Western part of the area, and is known as the Haluza-Agur dunefield (Fig. 3.1). This dunefield is delimited in the south by

S.-W. Breckle et al. (eds.) Arid Dune Ecosystems. Ecological Studies 200, 25 © Sprin ger-Verlag Berlin Heidelberg 2008 26 H. Tsoar et al.

~ ; \ ,

~ SandDunes

/ / _ ­ - - , - Nahal (wadi) ' , :t:J.J~a.!'q.l> ; // / ~ Anticline Slope '"(. inclination \ ­ _._._ International , \ -, , Border , \ \ , \ o 5 10 , \ i , kIn

Fig. 3.1 Location map of the sand fields in the north-western Negev

Nahal (wadi) Nizzana, which crosses the area from east to west. In the southeast, the Agur Sands are delimited by the slope of the Har Keren anticline. Eastwards, the Haluza-Agur dunefield sand diminishes towards the floodplain of Nahal Besor where the dunes are smaller. A similar diminishment of the dunes occurs in the north where the dune sand gradually transforms into loess. Surrounded by Nahal Nizzana is the Sde Hallamish dunefield, known also as the Nizzana sand field. Another dunefield is located in the Shunra basin (syncline), separated from the Haluza-Agur dunefield by the Har Qeren anticline. Eastwards of Nahal Besor, the sand becomes thicker where the northern Negev dunefield terminates in a triangular shape in the area of Nahal Sekher (Fig. 3.1).

3.2 Aeolian Sand Incursions into the North-Western Negev During the Upper Quaternary

3.2.1 Period ofAeolian Sand Incursion into the Negev

The dune sand of the Sinai and Negev is composed most!y of quartz with very few other minerals, mostly calcite, magnetite, hematite and other silicates. The source for this aeolian sand is the Nile Delta, since there is no other source for these minerals within the reach of the wadis that flow through the Sinai-Negev dunefield (Almagor 2002). 3 Formation and Geomorphology of the North-Western Negev Sand Dunes 27

Aeolian sand became geologically significant in the sediments that were deposited in the coastal plains of the Sinai and north-western Negev around 25,000-30,000 B.P. Probably most of the aeolian sands in this period were transported and re­ deposited in the flood plains of the area, or they were trapped by the vegetation, creating sand sheets (Zilberman 1991). Sand dunes in considerable amount are found associated with the Upper Epipaleolithic sites that are dated to 18,000­ 10,000 B.P. (Goring-Morris and Goldberg 1990). Thermoluminescence (TL) dat­ ing of the linear sand dunes and the interdune sand of Sde Hallamish reveals that sand was deposited in the interdune area from at least 43,000 until 9,000 B.P. , and that there has been little deposition during the Holocene. The linear dune flanks were stabilized during the last 10,000-6,000 B.P. (Rendell et al. 1993). Radiocarbon dates are available for hearths found in two sand quarries; one, northwest of Revivim, where the age of the base of the sand is 3,030±150 B.P. (Zilberman 1991), the other in the most eastern part of the Negev sand invasion, near Nahal Sekher (Fig. 3.1), where the lower sand wasmobile around 6,100 B.P. (Tsoar and Goodfriend 1994). These sands may represent the mobilization of formerly fixed Upper Epipaleolithic sand found at a Natufian site (ca. 11,000 B.P.) in the Nahal Sekher area (Goring-Morris and Goldberg 1990).

3.2.2 The Sand Red Colour and its Implications

The Negev aeolian sand shows different intensities of redness, with some variation in the hues because of the content of iron-oxide minerals. If there is a common source of all the sand and the climate is homogenous, we can assume that the dif­ ferent hues of red indicate different ages, as has been argued by many (Norris 1969; Folk 1976b; Walker 1979; Gardner and Pye 1981; Wopfner and Twidale 1988; White et al. 1997). Yellowish sand is younger than redder sand. Redness of the Negev sand ensues from iron oxide-bearing clay-sized particles that adhere to the surface of the quartz sand grains. Scanning electron microscope (SEM) analysis shows that the surface of reddened quartz sand is covered by flakes and granular aggregates of iron oxides (Wopfner and Twidale 1988; Pye and Tsoar 1990).

3.2.2.1 The Redness Index of the Sand

Based on the above assumption, we have mapped the red intensity of the sand by measuring the spectral signature of 63 sand samples taken in the field (Fig. 3.2). The redness index (RI) is determined according to the following spectral ratio (Mathieu et al. 1998):

(3.1) 28 H. Tsoar et al.

Fig.3.2 Landsat TM image (bands 4,3,2) of the Negev sand dunes taken in 1987. The blue dots indicate the sand sample locations, and the lines the interpolation of equal values of RI (redness index)

where R is the visible red (640nm), G the visible green (51Onm) and B the visible blue (460 nm) wave band. The spectral signatures were extracted from the spectral reflectance of the sand samples, measured with an ASD Fieldspec spectrometer. The samples were placed in a black plastic dish and illuminated by a 1,000-W high­ intensity halogen lamp at an angle of 45° and a distance of 5em. In order to obtain the bidirectional reflectance of the samples, each measurement was repeated 20 times from each of four directions . Figure 3.2 presents the redness interpolation map produced from the RI results. Analysis of the redness map shows that there are at least three distinct units of sand, based on their colour. Figure 3.3 shows an RI map of three sand incursions into the north-western Negev. A multiple comparison ANOVA test for the various RI values of the sand sam­ ples of the three different sand types (Fig. 3.3) indicates that there are three signifi­ cantly distinct sand units that can be distinguished in terms of their colour (Fig. 3.4). It seems that there were at least three different sand incursions into the Negev Desert during the Upper Quaternary. Sand type 1, which covers all the low and outspread dunes along the north and east side of the dunefield, is the reddest and probably the oldest of the three types. Sand type 2, which is found in the south­ ern part of the dunefield and includes Sde Hallamish and Shunra Sands, is less red. Sand type 3, which includes the Haluza and Agur Sands, is paler than the two other types and is apparently the youngest sand that penetrated into the Negev. 3 Formation and Geomorphology of the North-Western Negev Sand Dunes 29

Fig. 3.3 Map of the three different sand types in the north-western Negev, based on RI values (Fig. 3.2)

200 oMean DMean±1*SD ::I Mean±1.96*SD 180 P=2.54*10·9 160 I F=44.98

140 0

~ IV 120 "Cl I ...... = '" I '"IV 100 0 "Cl= " IV I ~ J 80 I 60 0

40 I 20

0 Sand type! Sand type2 Sand type3

Fig.3.4 Results of the box & whisker analysis for the three sand types of Fig. 3.3. Based on the results of the ANaYA test, there are three significantly different sand units (P=2 .54x lO-9 and F =44.98) 30 H. Tsoar et al.

Sand type 3 has penetrated from the Sinai into the Negev in a wedge form that tapers towards Har Keren. The RI lines show increasing values in the penetration direction, which indicates that this sand became redder during downwind transport. This fact supports our assumption that the sand becomes redder with time. The distinct sand dune morphology of the Haluza-Agur Sands supports the interpretation that it is a discrete dunefield. According to Fig. 3.3, it is plausible that the Haluza­ Agur Sands overlie sand type I, and that some of sand type 1 has mixed with sand type 3.

3.2.2.2 The Amount of Iron Oxides in the Coated Surface of the Sand

All mafic heavy minerals (with specific gravity greater than 2.8) were separated from quartz grains by submerging the samples in bromoform, which has a spe­ cific gravity of 2.89 at 20°C (Griffiths 1967). The amount of iron in the clay coating of the sand grains was extracted for all the samples by using the sodium dithionite-sodium citrate extraction method (Mehra and Jackson 1960; Smith and Mitchell 1987). The extraction of iron was done for all the coated surfaces of the quartz grains. Iron levels were determined with a Unicam Helios Alpha spectrophotometer. Here, we assume that the cause for the redness of the sand is the amount of iron oxide-bearing, clay-sized particles that adhere to the surface of the quartz sand grains (cf. above). There is a significant regression between the amount of iron (in percent) and the RI (Fig. 3.5).

160...------,

140

120 §: '-' 100 y == 1092.6x- 10.10 ~ "C R2 == 0.67 .5 80 • 7 '" (P == 1.91*10. , 0==27) '"= Q) • • ~ 60 • • • 40 • •

20

O+-----,-----.-----..----..-----.----..------.------i o 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Fe (%)

Fig. 3.5 Regression of dithionite-extractable Fe (in percent) and redness index of the samples 7 collected in the field (P= 1.91x10- ) 3 Formation and Geomorphology of the North-Western Negev Sand Dunes 31

3.3 Wind Climate

The climate in the area of the Negev dunefield is classified as arid in the Haluza­ Agur and Nahal Sekher Sands, and becomes hyper-arid in the Sde Hallamish and Shunra Sands. A comprehensive description and analysis of the northern Negev climate is given by Littmann and Berkowicz (Chap. 4, this volume). Wind is a vector characterized by magnitude and direction. The energy of the wind can be calculated from the kinetic energy (KE) equation:

I KE= _pV3 At (3.2) 2

where p is the density of air, V the wind speed, A the cross-sectional area of the airflow, and t the time the wind blew at wind speed U. In a similar way, we can calculate the drift potential (DP) of the wind (Fryberger 1979), which is based on Eq. (3.2) and the equation of sand transport (Lettau and Lettau 1978):

(3.3)

where 'Lq is related to the total potential sand flux from all wind directions, V is the wind velocity (in knots), measured at a height of 10m, VI the threshold wind veloc­ ity for sand transport (=12 knots), and t the amount of time the wind blows above the threshold velocity (in annual %). The drift potential (DP) is given in vector units (v.u.). An index of wind direction variability is represented by the ratio between the resultant drift potential and the drift potential (RDPIDP), where values close to 1 indicate a narrow unidirectional drift potential, and values close to zero indicate a multidirectional drift potential. The drift potential of the Nizzana area was calculated from two different wind recorders at the Sde Hallamish research site (one from 1991-1995, the other from 1995-2002), and from another recorder at Qetziot (for 1981-1982), which is 11km southeast of the Sde Hallamish research site. The data of these three wind recorders were converted into values at 10m height, based on the von Karman-Prandtl loga­ rithmic velocity profile law (Pye and Tsoar 1990). Figure 3.6 shows sand transport roses for these three stations. The drift potential (DP) of the Nizzana area in the southern part of the North-Western Negev dunefield is between 21 and 108 V.u. (Fig. 3.6). The differences in the DP values ensue from different wind recorders and different periods of records. However, all these values indicate a low-energy wind environment (Fryberger 1979). The drift potential for some active dunes in various humid areas reaches values of 2,000 v.u. and higher (Tsoar 2001, 2005; Yizhaq et al. 2007), which indicate a very high wind energy. The sand-transporting winds in the Nizzana area have a seasonal shift. The strongest winds occur during winter and spring when a depression exists over 32 H. Tsoar et al.

Qtziot 1981-1982 DP= 108 RDP = 75 RDP/DP = 0.7 RDD = 280" 1= 16.8%

Sde Hallamish Sde Hallamish 1991-1995 1995-2002 DP=21 DP=56 RDP= 15 RDP=27 RDPIDP = 0.73 RDPIDP = 0.48 RDD=241 " RDD = 289" t= 6.2% 1= 10.6%

o 5 10 I DP(v.u)

Fig.3.6 Three sand roses for Sde Hallami sh and the nearby Qtziot. DP (drift potential) total vector units (v.u.) for all wind directions, RDPIDP index of wind direction variability where values close to 1 indicate narrow unidirectional drift potential, and values close to zero indicate multidirectional drift potential , RDD direction of the RDP shown by the red arrow in the downwind direction , t percentage of time the wind was above the threshold velocity for sand transport the north-eastern Mediterranean. Strong southwest to west winds are commonly blowing under this synoptic condition. The winter storm winds are neither constant nor frequent. In contrast to winter and spring, the summer winds ensue from a con­ stant synoptic condition corresponding to a difference in pressure between the Mediterranean and the Negev during the daytime. As a result, a sea breeze is devel­ oped that blows regularly everyday from the north-northwest, from noon until the early evening. The summer storm winds are of low magnitude (usually not above 8 mJs) and show a high degree of constancy, while the winter and spring storm winds are of high magnitude (up to 20mJs) and low constancy.

3.3.1 Factors Affecting Mobility and Stability ofthe Negev Sand Dunes

For sand dunes worldwide, there is no direct relationship between the amount of rainfall and the vegetation cover. Active dunes with no vegetation cover are found in humid areas - e.g. the Oregon coastal dunes (Hunter et a1. 1983), the coastal sand dunes in NE Brazil (Jimenez et a1. 1999), or the Alexandria coastal dunes in South Africa (Illenberger and Rust 1988) - while in the Negev Desert, the dunes are fully stabilized by microphytes and macrophytes. Because of the high rate of infiltration 3 Formation and Geomorphology of the North-Westem Negev Sand Dunes 33 in dune sand, most of the rain in humid climates is lost to the groundwater and is not available to the plants. Hence, the amount of rainfall is not a decisive factor in sand dune stabilization, whether in humid or in arid climates, except for temporary increases in the cohesiveness of wet sand. The deep infiltration of rainwater into dune sand reduces the effect of evapora­ tion from the ground (Chap. 6, this volume), mostly in arid areas, contrasting with other soils composed of fine particles of silt and clay. The wind energy is thus the most important factor that determines sand dune mobility, because of the non­ cohesiveness of the sand. High-energy wind has the power to erode sand to such an extent that it prevents seeds from germinating in the sand and stabilizing it (Chap. 6, this volume). A drift potential (DP) above 400 V.u. would be needed in the Negev sand dunes in order to obtain active dunes with little or no vegetation (Tsoar and Illenberger 1998).

3.4 The Negev Dune Forms and Their Evolution

3.4.1 Linear Dunes

The various dunefields in the western Negev are dominated by linear dunes. The linear dunes fall into two type categories, characterized by a simple, longitudinal pattern corresponding to vegetated and unvegetated surfaces. While the former are known as vegetated linear dunes (VLD), the latter are better named self (sword in Arabic). All linear dunes possess one common characteristic of elongation that dif­ ferentiates these from transverse and barchan dunes where the whole body of the dune advances.

3.4.1.1 Vegetated Linear Dunes (VLDs)

VLDs are known from many semi-arid and arid regions in Australia, the Kalahari in South Africa, and the Southwest US (Twidale 1981; Wiggs et al. 1996; Wopfner and Twidale 2001). Unlike the seif dunes, the VLDs are straighter and do not mean­ der, are partly or fully vegetated, and have a blunt crest line and round profile (Figs. 3.7,3.8). An exclusive attribute of VLDs is the tendency for two adjacent dunes to converge and continue as a single ridge. Convergence is in the form of a Yjunction (the tuning fork shape; Fig. 3.8), commonly open to the effect of wind (Folk 1971; Twidale 1972a; Mabbutt and Wooding 1983; Thomas 1986). Yjunctions are a symmetrical or an asymmetrical coalescence of juxtaposed VLDs. This coales­ cence has been attributed to a deflection by cross wind of the extreme of the ridge during the elongating process (Madigan 1946; Mabbutt and Sullivan 1968; Thomas 1986, 1997). The uniform spacing between the VLDs is a phenomenon common to linear dunes (Twidale 1972a). It is attributed to statistical occurrences (Madigan 1936; Goudie 1969) or to dynamic processes (Folk 1976a). Mabbutt and Wooding (1983) interpret VLD junctions as a response to changes in the dynamic control of 34 H. Tsoar et al.

Fig. 3.7 VLDs on the westem Shunra dunefield

Fig. 3.8 Aerial photograph of north-eastern Sde Hallarnish taken in 1989. Nahal Nizzana is on the right and upper side. The coalescence of two linear dunes is typical for VLDs. Small dunelets, superimposed on the linear dunes, were formed where vegetation had been removed as a result of human activities (grazing and shrub gathering) dune pattern seeking its adjustment through equilibrium spacing. When one VLD converges with a dune adjacent to it, a new linear dune is formed downwind in the space that was formed, or where the linear ridges are closest together the constant space is maintained by coalescence of two dunes (Fig. 3.8). Most VLDs worldwide 3 Formation and Geomorphology of the North-Western Negev Sand Dunes 35 comply with the above descriptions, but still there are patterns of linear dunes that are not parallel to each other but nevertheless have Y-junctions (Thomas 1986; Bullard et al. 1995). In many areas, vegetated linear dunes have been reported as aligning approxi­ mately parallel to the dominant strong wind direction (Madigan 1936, 1946; Clarke and Priestley 1970; Folk 1971; Higgins et al. 1974; Lancaster 1981, 1982; Hyde and Wasson 1983). This is corroborated by the way they swerve around topo­ graphic obstacles (Mainguet 1984). Secondary side winds usually exert a modify­ ing influence on the crest and account for either the symmetry or the asymmetry of the whole dune. Although the elongation process of VLDs is evident (Harrison and Yair 1998), the exact mechanism of elongation is not completely clear because most VLDs worldwide are covered by vegetation and stabilized; there are, however, some hypotheses dealing with this aspect (Tsoar 1989; Tseo 1993). The area shown in Fig. 3.8, located at Sde Hallamish, was under heavy grazing by Bedouins from the Sinai region until 1982 when the current international border between Israel and Egypt was established. Since then, vegetation has recovered on the Israeli side while the Egyptian side has continued to be impacted by animal grazing and wood­ gathering (Chap. 6, this volume). Since vegetation accompanies all VLDs worldwide, it can be assumed that their formation is related to vegetation. When vegetation is removed from the linear dunes, the pattern changes to the braided form (Figs. 3.8, 3.9). Vegetation is not a solid obstacle to sand-moving winds, and sand tends to penetrate and to be trapped

Fig. 3.9 Aerial photograph s of VLDs in the eastern Sinai (immediately west of Sde Hallamish) where all vegetation has been removed as a result of human activities (grazing, agriculture, shrub gathering). The formation of small dunelets superimposed on the VLDs (braided pattern) is due to the removal of vegetation 36 H. Tsoar et al. by the vegetation. This results in the formation of coppice dunes or nebkhas (Fig. 3.10), which are sand mounds capped and protected by vegetation. The nebkhas are formed when the sand is free of phytogenic crust. Such a condition occurs on the crest of the dune and the upper south-facing slopes (Fig. 3.10). In arid areas with low wind energy, such as the Negev, vegetation thrives more easily on sand than on other, finer soils (Chap. 6, this volume). Therefore, additional vegetation will even­ tually clutch to the shadow lee dune, formed either from a solid obstacle or a shrub, in a process of self-propagation, thereby forming a vegetated linear dune along the direction of the strongest dominant wind. This phenomenon is known from other areas where vegetation forms nebkhas and lee (shadow) dunes that coalesce into linear ridges (see, for example, Hesp 2004). Crosswinds may and will add sand to the linear dune and impart it, in some cases, with an asymmetric profile. Vegetation, as an element of surface roughness, tends to decrease the impact of wind on sand. Hence, only strong winds are effective for vegetative dune development, and that is the reason why VLDs are aligned along the dominant strongest wind.

3.4.1.2 Seifs

Unlike VLDs, seifs are completely devoid of vegetation on both slopes. This accounts for the formation of a sharp crest, which explains the term seif. Another typical characteristic of the seif is the tortuosity of its crest line. The vegetative

Fig.3.10 Nebkhas (coppice dunes) formed on the crest of VLDs by shrubs that trap sand. Note that the crest is devoid of phytogenic crust, which promotes sand movement 3 Formation and Geomorphology ofthe North-Western Negev Sand Dunes 37 cover that flourishes in the Negev, the Israeli side of the border, stabilized all seifs that were active there when the area was under human pressure (Chap. 6, this vol­ ume). Large, active seif dunes can be observed on the Sinai side of the border, which is still subjected to anthropogenic pressure (Fig. 3.11). Seif dunes are the only type of linear dune shaped under bidirectional wind regimes impacting the dunes obliquely, the dunes extending parallel to the resultant direction of the wind (Tsoar 1983). Seif dunes are known to run parallel for scores of kilometres but they do not show any tendency for two adjacent dunes to converge into a single dune, as is common with VLDs. Seif dunes, the only elongating dunes devoid of vegetation, have a complicated mechanism of sand transport and deposition. When winds encounter the seif dune crest line at acute angles, the flow at the leeside is deflected to parallel the crest line (Tsoar 1983). Accordingly, the leeward slope is not merely a zone of deposition but also a zone of erosion by the diverted wind flow (Tsoar et al. 1985). The occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line. When this angle is <40°, the deflected wind has the power to erode sand along the lee slope. When the angle is close to 90°, the velocity of the diverted wind decreases and deposition mainly occurs. As a result, the linear seif dunes develop a waveform (Fig. 3.12) that then changes the angle of incidence of the wind. This sand, eroded from the leeside by the deflected wind flow, is deposited on the same side as where the dune meanders, and the angle of incidence changes to around 90°. The consequence is erosion of one side of the winding dune by one wind direc­ tion, and deposition on the other side by the other wind direction. The elongation of seif dunes is performed by migration of the waveforms along the dune.

Fig. 3.11 Seif dunes on the Sinai side of the border at Agur Sands, north of Nahal Nizzana. This linear dune meanders with peaks and saddles lengthwise 38 H. Tsoar et al.

Fig. 3.12 Meandering seif dunes formed in north-eastern Sde Hallamish from a VLD that aligned obliquely to the storm wind direction, and of which the vegetation was removed between 1970 and 1982

The two main wind directions that affect and form seif dunes, and cause erosion and deposition along the leeside always differ in their incidence angle and in their total yearly wind power. In winter, most storm winds (from west and southwest) encounter the southern slope of the seif obliquely, while in summer the storm wind is mostly from the northwest (Fig. 3.6). This lack of uniformity in total wind power from both sides consequently brings about a lack of uniformity in the rates of ero­ sion and deposition. The response of the dune to this lack of uniformity is to form peaks and saddles along its length (Figs. 3.11,3.12). The peaks experience deposi­ tion from the more effective wind direction, and erosion from the less effective one. Conversely, the saddles receive deposition from the less effective wind direction, and are eroded by the more effective one (Tsoar 1983). The distinction between VLDs and seif dunes is not widely accepted. Some see similarity between both types, which brought them to conclude that VLDs were originally formed as seifs during the late Pleistocene and have since become stabi­ lized as the climate became more humid and less windy (Lancaster 1994). It is well demonstrated that when vegetation is removed from VLDs, these dunes do not evolve into seif dunes but rather into braided forms (Fig. 3.9). As was stressed above, VLDs are aligned parallel to the direction of the strongest prevailing winds, while seif dunes are formed under bidirectional wind regimes impacting the dunes obliquely from the two sides. However, seif dunes are known to develop from those parts of VLDs, such as the Y-junctions, which deviate from the usual alignment parallel to the strongest wind, and have also experienced removal of vegetation . When such deviation reaches 15-20° from the dominant wind direction, the dune is under the influence of oblique winds that divert on the leeside, and meandering forms develop (Tsoar 1989; Fig. 3.8). 3 Formation and Geomorphology ofthe North-Western Negev Sand Dunes 39

3.4.1.3 Linear Dunes with Barchans in the Interdune Area

The sand dunes in the Haluza-Agur dunefield are thicker, and composed of VLDs with a braided pattern that extends from west to east. In the interdune areas, there are many stabilized, crescentic slip faces arranged along transverse lines (Figs. 3.13,3.14). There are two hypotheses for these unique dune forms. Transverse or barchan dunes were formed first by westerly winds when the dunes were com­ pletely bare of vegetation. VLDs commenced to form later when the dunes were sparsely covered by vegetation. The arrangement of the transverse dunes along transverse lines (Fig. 3.14) supports this hypothesis. The second hypothesis states that the VLDs were formed first when the dunes were sparsely covered by vegetation. The transverse dunes with crescentic slip faces formed after the dunes were grazed and all the vegetation removed. The wind was funnelled along the interdune trough and there formed barchans. The crescen­ tic slip faces are stabilized today but their form is very obvious, which indicates that activity was very recent (Figs. 3.13, 3.14). This dune morphology is different from the morphology of the linear dunes found at Sde Hallamish and Shunra, where the interdunes are not covered with any bedform. As we postulated above, the Haluza-Agur dunefield probably marks the youngest and thickest invasion of sand into the Negev.

Fig. 3.13 Stabilized slip face in the interdune of two linear dunes at Haluza Sands 40 H. Tsoar et al.

Fig. 3.14 Aerial photograph of Haluza Sands, taken in 2003. Note the VLDs with braided super­ imposed dunes (braided pattern) and the crescentic slip faces in the interdune areas

3.5 The Effect of Destruction of Vegetation on the Morphology and Dynamics of the Sand Dunes

The sand dunes of the Sinai and Negev have experienced several cycles of vegetation coveting and removal (Tsoar 1995; Meir and Tsoar 1996; Chap. 6, this volume). VLDs were formed in the Sinai and Negev when vegetation grew on the dunes in the absence of human disturbance. When vegetation was removed, the shape and the profile of the dunes became subject to change by the creation of secondary, superim­ posed transverse dunelets with slip faces facing downwind (Figs. 3.8,3.9). This type, known as linear-braided (Tsoar and M011er 1986), is also known from Australia (Madigan 1936; Twidale 1972b; Mabbutt and Wooding 1983). Destruction of vegetation on VLDs that change their azimuth of alignment in the order of 16 to 25°, which occurs when they converge to form a Y-junction, pro­ motes the formation of seifs (Figs. 3.8, 3.12). Therefore, the transformation takes place after the destruction of vegetation in those areas (such as Y-junctions) where VLDs became aligned obliquely to the strongest dominant winter winds as well as to the frequent summer winds (Tsoar 1989). The occurrence of seif dunes in the Sinai and Negev (Figs. 3.8, 3.11) was initiated after the vegetation was destroyed. According to the rate of elongation of seifs in the eastern part of the Sinai, and their maximum length, it is assumed that seif dunes 3 Formation and Geomorphology of the North- Western Negev Sand Dunes 41 started their formation in the 18th century, after the inhabitation of the Bedouin in the Sinai and the long dry period during most of the 17th and 18th century (Tsoar 1995). Sinai dunes have been bare of vegetation since then, while the vegetation of the Negev dunes recovered in the second half of the 20th century when human pressure ceased (Chap. 6, this volume). The seif dunes of Sde Hallamish had a low shape with a round profile in 1968 when the dunes were covered by vegetation. The removal of vegetation by the Sinai Bedouin between the late 1970s and 1982 resulted in the formation of sharp-edged crest lines in those areas where the linear dunes stretch in an average azimuth of 285 and 290°, which is between the summer's northerly and north-westerly winds and the winter's westerly to south-westerly winds. Figure 3.15 shows four corre­

s N :[ 220 ,r " '' ~ 210 , ' , , ' , " ~ ~ , io 200 ~ 'v ::r: 190 +----r-.-----r---,-----,--,------,--+ o 100 200 300 400 Length (m) s N '8 220 "'--" ::::j' 210 '" ~ .:ti 200 ,, I 190 +-----r--,-----,--,--.------,--,.---__+_ o 100 200 300 400 Length (m) s N :[ 215

::::;vi 205 ~ r r io 195

~ 185 +-----r--,-----,----.---.------,--,.--I­ o 100 200 300 400 Length (m) s N '8 220 <;»: ,. ::::; ,, , vi 210 , , , , ~ . , "'"' 200 ~ .-­ 'v ' ...... ::r: 190 -f-----r­ -,-----,­ ---.-­- r----r­ ,---,­ ,----,-­ -,----,-­ -,-­ rt­ a 100 200 300 400 500 600 700 Length (m)

Fig. 3.15 Four cross sections made across VLDs in Sde Hallamish from topographical maps of 1968 and 1982. The VLDs were vegetated in 1968, and devoid of vegetation in 1982 (after Tsoar and Meller 1986) 42 H. Tsoar et al.

630 640 650

343 343

34 2 342

630 640 650 Scale

M a( CH S 5000

Fig.3.16 Landsat image from 1987 showing the north-western Negev, the main geographic units (azure, blue and green) and the major wadis (red and yellow)

sponding cross sections of nine linear dunes in Sde Hallamish in 1968 and 1982. All the VLDs that were in a position to become seifs with a conspicuous, sharp­ edged crest have increased in height by up to 6 m. VLDs that were in a position to change to the linear-braided pattern did not increase in height or change their profile (Fig. 3.15).

3.6 Buried Channels

The cycles of dune activity, be they caused by climate or human activity, have resulted in a complex interaction between the drainage patterns and the sand mantle. Many dry channels (wadis) that start from rock outcrops (mostly chalk and limestone) and continue to the sand dunes are nowadays covered by sand and have essentially become defunct wadis. Some of these wadis have thus been covered by the sand dunes and are now buried channels. Mapping this complex drainage system cannot be achieved merely by means of aerial photography or by using existing topographic maps. The drainage systems, including the buried and unburied drainage systems, were mapped by using visible and near-infrared (Landsat) as well as synthetic aperture radar (SLR) imagery (Blumberg et al. 2004), 3 Formation and Geomorphology of the North-Western Negev Sand Dunes 43 with the objective of better understanding this interaction between the aeolian geomorphology and the fluvial one.

3.6.1 Methods ofMapping Buried Drainage Systems

The regional drainage system was mapped using Landsat data and synthetic aper­ ture radar data from the Spaceborne Radar Laboratory (SRL) missions. The Landsat TM data have six visible and near-infrared channels (there is also a thermal channel that was not used here), and the SRL mission acquired data in L-, C- and X-band in several polarizations. In this study, the C- and L-band data were used in HH and HV polarizations. All the data were used to generate a series of drainage maps each showing a different drainage pattern. Each spectral combination was used to generate its own vector map of the drainage pattern, and the four most distinct combinations were used. These amount to a map from visible data, and near-infrared, C-band HV, and L-band HV data. All four maps were then combined (Fig. 3.17) to create a map of the combined drainage patterns.

Scale = Meters 5000 5000

Fig. 3.17 Map showing the combined set of drainage patterns based on visible, near-infrared, C-band HV, and L-band HV data. The areas in the centre are most strongly suspected of having buried drainage patterns 44 H. Tsoar et al.

Differences between the maps produced from the radar and Landsat imagery are related to the different processes governing the backscatter of radar energy versus reflectance in the visible and near-infrared region. Some instances where such differences occur were attributed to surface penetration and backscattering from subsurface inhomogeneities. Several continuous features were identified where the L-HV backscatter is enhanced, compared to the C-band A=5.6cm, and to some extent compared to the L-HH data (note: the L-band had a wavelength of 24cm, and HV denotes the polarization of the transmitted and received electromagnetic waveforms, i.e. horizontally polarized transmitted wave and vertically polarized received wave). These continuous features do not appear as vegetated in Landsat data and, therefore, the enhancement of the cross-polarized backscattering is attributed to scattering from inhomogeneities within the sand layer. The differ­ ence between the two wavelengths suggests that these inhomogeneities in the subsurface are deeper than the C-band wavelength.

3.6.2 Nahal Nizzana and Buried Drainage Systems in the Shunra and Haluza Sand Fields

Figure 3.18 shows the drainage route of Nahal Nizzana, as can be seen in a Landsat image. Nahal Nizzana meanders from the southeast to the northwest across the

o 10

Fig. 3.18 Landsat image showing the northern Sinai, specifically the Wadi El-Arish and Nahal Nizzana. Nahal Nizzana disappears in the sand after crossing from the Negev side into the Sinai, and is mantled by sand dunes 3 Formation and Geomorphology of the North-Western Negev Sand Dunes 45 I N

33'.5',31'.14'

SCALE 9 W l UG 2,°km

Fig. 3.19 SRL radar image generated from L-band HV data, showing a different and more complete meandering of Nahal Nizzana all the way to the Mediterranean

Israel-Egypt border. This wadi is one of the largest hydrological systems in the northern Negev, and drains the Negev Highlands before it disappears in the sand mantle of the Haredin dunefield in the Sinai, This is clearly observed in the Landsat 5 TM image of the region (Fig. 3.18). The regional slope in this area is 0.14° towards the northwest. Despite this slope, there is no visible central channel but rather a gradual split into many minor channels, eventually disappearing in the sands. Medial reaches of Nahal Nizzana exhibit a wide channel and suggest that an ancient channel, buried by aeolian sediments, may still be present beneath the sandy mantle of the Haluza-Agur Sands, The area was subsequently studied by means of a radar image from the Spaceborne Radar Laboratory Mission of 1994, using the L- and C-bands. The overlapping radar image of the area using the Spaceborne Radar Laboratory in L-band HV data (Fig. 3.19) enhances the trace of Nahal Nizzana all the way to the Eastern Mediterranean coast. Similarly to Nahal Nizzana, several smaller wadis become better visible in the Shunra and Haluza sand fields. These wadis connect between Hal' Keren and the Evha Ridge (Figs. 3.16 and 3.17). These can be seen in the centre of Fig. 3.17 as areas that appear only when the L-band BY data are used, explaining the older connection between the southern drainage network and the older, more northerly one. 46 H. Tsoar et al.

3.7 Conclusions

At some of the upper reaches of the wadis in the dune areas of the north-western Negev, "playa"-type sediments composed of silt and clay from the late Quaternary can be found. These sediments were deposited by flooding of the interdune areas of Sde Hallamish, caused by the blocking of Nahal Nizzana by sand dunes (Harrison and Yair 1998). Other areas display disruptions and deterioration of the drainage patterns that can be attributed to the cycles of vegetation removal in the dunefields. Every such cycle causes the dunes to progress to the east and blocks existing drainage features, resulting in more buried fluvial sediments and the meandering of the current fluvial feature.

Acknowledgements We wish to thank the Minerva Arid Ecosystems Research Centre (AERC) of the Hebrew University of Jerusalem for supplying the required climate data for the Sde Hallamish sand dunes. We also thank Tali Neta, Nir Margalit and other "remote-sensing" students that contributed to the processing of some of the imagery, and the Jet Propulsion Laboratory of NASA for providing the SIR-C data.

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