Introduction
Aeolian processes occur in environments such as deserts, agricultural fields, and coastal regions. The features of such environments include a nonexistent or sparse vegetation cover, strong winds, and a supply of fine sediments. In Egypt, the increase in human population has led to the lateral spread of human settlements from the Nile’s flood plains to various locations in the western desert land. Such areas include the Kharga Oasis, where the government has initiated land reclamation projects for agriculture and human settlements. However, the developmental plans have missed the issue of natural hazards such as dune movements. Hence, the success of such projects relies on the geomorphic understanding of the landscapes hosting the projects. The present study, therefore, investigates the aeolian processes in Kharga and devises a mitigation plan against the advancement of sand dunes into the town.
Origin and Geomorphology of Aeolian Bed-forms in Central Egypt
Usually, aeolian processes occur in deserts and coastal zones (Nordstrom, Psuty, and Carter, 1990). They also result in the mobilization and emission of dust that forms areas of sand dunes (Lancaster, 2009). However, the processes depend on geologic agents such as rivers for the supply of sediments. The aeolian transport of fine sediments may cause dust storms, which affect visibility and air quality in their vicinity (Lancaster, 2009). In addition, the deposition of dust has significant effects on the nature and composition of the soils in arid areas. Consequently, dust from distant sources may determine the nutrient status and the soil chemistry of the region (Lancaster, 2009).
The Western Desert in Egypt is about 700,000 km2 and has significant oases such as Dakhla, Siwa, and Kharga (Salman et al. 2010). The Eocene limestone plateau from the north and east bounds the Kharga oasis, with steep cliffs forming a sharp boundary to the depression floor. The flatland stretches along the upper and middle Egypt elevating up to about 550 meters above the sea level (Salman et al. 2010). Nevertheless, towards the west and south of the Kharga Oasis, the depression floor merges with the Nubia Sandstone desert gradually (Salman et al. 2010). The Kharga landscape is either a low-lying depression floor or a high plateau in the eastern and northern boundaries. However, the in-between pediment areas are badlands (Salman et al. 2010).
The satellite image in Figure 1 can help in the description of the sand dunes and other geological processes in central Egypt. The Figures 2, 3, and 4 show alternating dark and light chevron-shaped patterns, which occur downwind from dune fields and low scarps. The light chevrons in Figure 2 may represent thin accumulations of sand-sheet deposits in the bedforms’ lee. Such aeolian deposits may represent wind ripples. However, the dark chevrons in Figures 3 and 4 comprise of coarse-granule lag deposits admixture, which are winnowed continuously by aeolian erosion on the bedforms’ windward sides. Therefore, the aeolian deposits in Figures 3 and 4 may represent draa or complex dunes.
Figure 1. Regional base-map of central Egypt. The figure locates three high definition images 2, 3 and 4
Figure 2. Dune field, western district, central Egypt. The image shows linear sand dunes
Figure 3. Dune field, north-central district, central Egypt. The image shows simple crescentic sand dunes
Figure 4. Dune field, south-east central district, central Egypt. The image describes complex crescentic dunes
In Figures 2, 3 and 4, the areas occupied by sand dunes indicate a long-term sediment supply to the study site. However, the changes in the location and total area covered by dunes reflect the degree of the dune migration and the area’s long-term sediment budget (Lancaster, 2009). Therefore, decreases in the dune size and the area covered by the dunes show a negative sediment budget involving a high rate of sediment loss. Conversely, an increase in the area of dunes or dune size reflects a positive sediment budget, whose supply of sediments is higher than the losses. Dune fields accumulate at the down-wind’s zones, where wind direction and speed vary causing sand influx to exceed the out-flux As a result, the wind variability causes the deposition and growth of dune fields. Over centuries, the wind’s transport of sediments determines the dynamics of dune fields (Lancaster, 2009). However, vegetation cover determines sediment availability while wind strength influences the sediment mobility. The interactions of such variables are evaluated in relation to the condition of the aeolian system (Lancaster, 2009).
In an investigation carried out in the Kharga Oasis, a dune movement rate of nine meters per year was reported (Sparavigna, 2013). The study, therefore, proves that dune movements are encroaching rapidly into the Kharga Oasis (Sparavigna, 2013). However, a geomorphologic study of the Bir Safsaf region, which occurs in central Egypt, increases the understanding of dune systems in the landscape near the Kharga Oasis. In central Egypt, Quaternary soils and active aeolian deposits cover the sediments. The aeolian deposits and soils are also referred to as the Selima Sand Sheet (Issawi, 1972; Maxwell and Haynes, 2001). The Bir Safsaf area is part of the Uweinat Uplift, which extends from Bir Safsaf in the East to Gebel Uweinat in the West. Schandelmeier, Richter, and Franz (1983) observed that the Uweinat Uplift covers an area of 40,000 km and exposes crystalline rocks.
The northern Bir Safsaf formation has magmatic rocks of granitic composition. Moreover, the granitic composition has complex and intrusive relationships of porphyritic granite, hornblende-bearing gneisses, and abundant dykes. However, the southern area is composed primarily of course to medium-grained Migmatites and gneisses. Further, the rock composition is mainly granitic.
The outcrops usually rise a few meters above the surrounding flatlands and are often difficult to observe on the Landsat images. Figure 1 shows the area, typical of Bir Safsaf’s landscape, floored by interbedded shale, quartzitic sandstones, and conglomerates of the late Cretaceous age. The landscape also comprises of granitic outcrops, as well as the granitic gneiss of Precambrian age (Schaber, McCauley, Breed, and Olhoeft, 1986). The sediment cover’s superficial sand sheet has about one centimeter to one-meter thick layer of aeolian deposit. A thin deposit of small pebbles or quartz granules covers the layer. Further, the sand sheet overlies granitic bedrock and an alluvium that contains medium to coarse sand, as well as calcified pebble gravel (Maxwell and Haynes, 2001).
However, the structural trends of the Bir Safsaf region comprise of east–west normal faults. The Bir Safsaf area in central Egypt is a desert site where researchers discovered sand-buried paleodrainage channels (McCauley et al., 1982). McCauley et al. (1982) noted that the channels age ranged between the middle Tertiary and the Quaternary. The L-band radar images from an SIR-A mission helped in the channels’ discovery (McCauley et al., 1986). The structures were later observed during the SIR-B and SIR-C missions (McCauley et al., 1986). Through field investigations, Schaber, McCauley, and Breed (1997) reported that L-band frequencies could penetrate the dry sand sheet covering the Bir Safsaf region down to one meter. The observation was confirmed through laboratory measurements of radar transmission (Williams and Greeley, 2001).
Regional Wind Regimes
The aeolian deposits identified in Figures 3 and 4 are complex and compound dunes while those in Figure 2 are wind ripples. Dunes occur in the presence of an adequate supply of sand-sized sediment, conditions that enhance sediment deposition and transport, as well as winds for transporting the sediments (Lancaster, 2009). The pattern of aeolian dunes depends on the wind regime, particularly its directional variability. The morphological dune patterns are linear, parabolic, star-star, and crescentic (Lancaster, 2009). Figure 2 shows linear dunes while Figures 3 and 4 show crescentic dunes. The sediments forming the dunes may result from playas or distant sources such as the far-traveled dust from Asia and the Sahara (Lancaster, 2009).
Apart from satellite imagery, researchers have used remote sensing techniques to relate sand dunes to the dominant wind regime (Warren and Allison, 1998). Consequently, remote sensing methods can effectively monitor dune movements by comparing multi-temporal satellite images (Hereher, 2000; Elbanna, 2004; Yin and Sarina, 2004). Researchers used the remote sensing approach to classify crescentic dunes as migrating dunes and linear dunes as elongating dunes (Embabi, 1995). Barchan (crescentic) dune movements dominate the Kharga area and correlate with effective winds (Ashri, 1997). At Kharga, wind data shows that 95% of the effective wind flows from the northern direction at a speed of 20km/hr (Ashri, 1997). Consequently, it moves the smaller dunes faster than larger dunes.
Areas of Risk at the Kharga Oasis
In the Kharga Oasis, the active dune belts comprise of Quaternary sediments. In addition, they pose a threat to human settlements because they occur in the mostly inhabited areas of the Kharga Oasis. The sand dunes are mainly barchan and measure up to 15 meters in height, 150 meters in width and 200 meters in length (Salman et al. 2010). In addition, smaller barchan dunes, sand heaps, and sand sheets are frequently observed in the inter-dune areas. The dunes result from the movement of northerly to northwesterly prevailing winds (Salman et al. 2010). Consequently, the moving sand dunes encroach on the cultivated lands in the Oasis. Thus, the barchan dune movement represents a primary threat to farming activities.
Attempts to clear issues of dune migration have included the erecting of reed fences in parallel groups (Hilmy et al., 1980 in Salman et al. 2010). The dune’s rate of movement depends primarily on dune size and the moisture content of surrounding areas. The dune belts advance through the oasis’ depression resulting in the formation of multiple barchan belts. Next, the dunes extend southward a considerable distance beyond the depression (Salman, 2010).
The Kharga Oasis’ structural elements result from the stable shelf tectonics. Therefore, faults and gentle folds dominate the surface of the area; hence, signify differential block movements. The thickness and lithology of the overlying sedimentary cover govern the resultant deformations. Structural analyses have indicated a dominance of nearly north-south treading faults (Salman, 2010). The faults extend through the depression and along the northern and eastern scarps, as well. Primarily, the Taref-Teir and the Qarn Ginah-Boulaq faults are encountered in the area (Salman, 2010).
Dune belts occupy about 20% of the Kharga’s depression floor (Salman, 2010). The dunes are sources of active wind-blown sediments containing erosive quartz grains. The quartz grains abrade roads and human settlements. Sand dunes, therefore, are a real hazard to monumental sites and human structures. Stokes et al. 1999 (in Salman et al. 2010) reported that the dunes have a fast downwind migration rate of 9.3 m/year. Consequently, they accumulate on monumental sites, destroy recent villages, and encroach into cultivated lands. In addition, the dunes provide sediments to the sandy-wind storms that use sand blasting to erode buildings and damage crops.
Figure 5 shows some of the primary sites that are at risk of the advancing sand dunes. Since the wind that moves the sand dunes flows from the northern direction (Ashri, 1997), the main roads (Figure 5) are at the highest risk of sand blasting and dune encroachment. However, the agricultural lands and the airport (Figure 5) are at a medium risk while the urban settlements are at the lowest risk of the impacts of dune movements.
Figure 5. Outline map of northern Kharga Oasis. The map shows the sites at risk of dune encroachment
Mitigation Plan
In Kharga Oasis, the interaction between the town structures and the environmental geology is clear. Sand dunes pose a serious hazard to significant sites in the town. The dunes are advancing into the town at a rapid rate. For example, they accumulate on the main roads, feed sandstorms that erode buildings, and encroach on cultivated lands. The sandy-wind storms are also a threat to the airport.
The Kharga area is subjected to an arid environment and a wind system that promotes the dune movements. In addition, the precipitation received in the region is too low to stabilize the dune movements. Therefore, the present mitigation strategy examine the aeolian processes, which drive sand dunes, and attempt to control them. Various management practices will be formulated and implemented in order to achieve the objectives.
Moreover, the mitigation plan will involve agricultural management practices aimed at reducing wind erosion. The methods will include the planting of windbreaks to modify wind flow patterns and the retaining of plant residue in the farm to conserve soil moisture. The stabilizing of the soil surface using applied chemicals and water will help to reduce wind transport of the sediments. In addition, the fields near the main road and the airport will be tilled in order to bury erodible particles. Consequently, the roughness of the soil surface will be increased through the reduction of erodible aggregates. Nordstrom and Hotta (2004) suggested that establishing bed patterns that are perpendicular to the predominant winds could reduce wind erosion significantly. Moreover, shelterbelts and windbreaks are effective methods of decreasing the wind’s erosive force. They comprise of rows of shrubs and trees planted along a farmstead’s margin. However, the plants are prone to mortality due to drought or poor windbreak management. Hence, the current mitigation plane will utilize windbreaks that are more reliable. The windbreaks will comprise of rock walls, fences, and earth berms.
The mitigation plan will also promote the maintenance of crop residues in the farms. The crop residue will protect the soil surface by increasing the roughness height and maintaining the soil moisture. Nevertheless, Skidmore (1994) noted that growing crops and standing residues offer greater protection when compared to the flat residues because they alter the wind profile. Therefore, the proposed management practice will maintain an efficient cover of residues, such as crop residue, in order to limit the wind’s contact with the soil surface.
On the bare soils in Kharga’s agricultural areas, the tillage technique will be the predominant method of preventing wind erosion. The technique will involve raising beds perpendicular to the prevailing wind’s direction. The strategy will increase aerodynamic roughness of the soil surface and, hence, prevent cascading saltation. In addition, a soil surface comprising of non-erodible aggregates will create a random roughness that will create sheltered areas. As a result, the use of non-erodible aggregates will also prevent cascading saltation. Fragile soils comprising of low aggregate stability may promote wind erosion in the localized areas. Moreover, the erosion may occur at the downwind ends of the unpaved and frequently traveled roads. In the areas along the main road, therefore, fences will be erected to promote deposition and reduce saltation.
During implementation of the management strategy, the soil surface characteristics and vegetation canopy-based indicators, such as the bare patch index, will be monitored constantly. Such monitoring will help to determine the area’s susceptibility to wind erosion (Whitford et al., 1998; Herrick et al., 2005; Herrick, 2000) and assess the effectiveness of the mitigation plan.
Since the use of effective agricultural practices is a significant mitigation strategy against wind erosion, the adverse effects of sandy-storms and the encroachment of dunes into the Kharga Oasis can be controlled. The windbreaks that will be erected along the margins of the main road and the airport will include drought-resistant trees and shrubs. As a result, the accumulation of wind-induced sediments on the main road and other urban structures will be reduced.
Conclusion
The view emerging from the present case study is that aeolian processes are a major threat to farming activities, monumental sites, and the urban settlements in the Kharga Oasis. However, the investigation presents an effective mitigation plan that can reduce the exacerbating effects of sand dunes and sandstorms. The scheme involves reducing the wind’s contact with sediments, hence, preventing wind erosion. It employs agricultural practices that have successfully reduced wind erosion in many arid areas. Therefore, the implementation of the strategy will effectively reduce the encroachment of sand dunes into the Kharga.
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