Sand encroachment in the Saharan Algeria; the not declared disaster - Case study: In-Salah region in the Tidikelt

BOULGHOBRA, Nouara,b, SAIFI, Merdasa and FATTOUM, Lakhdaria

a Scientific and technical research center on the arid regions CRSTRA, Biskra 07000, Algeria b Earth sciences department, sciences faculty, university of El Hadj Lakhdar, Batna 05000, Algeria E-mails: boulghobra.n@gmail.com; saifieco@gmail.com; crstra_biskra@yahoo.fr

Abstract — As a major natural risk, sand encroachment in the Tidikelt is attributable to the physical and climatic context of the region, but the continued population pressure on this arid system already subjected to climatic extremes, has increased the vulnerability to risks. This led to heavy damages on human, agricultural and other socio-economic activities. By use of remote sensing, mobile dunes were monitored over 19 years, and this showed continued migration of the dunes that complicates the resilience capacity of the local communities, calling for imperative awareness activities towards the populations and the authorities in order to enable an efficient and sustainable development.

Keywords — remote sensing, Landsat, sand encroachment, mobile dunes, Tidikelt, Algeria

1 Introduction

According to the United Nations (UNCCD, 2004), more than one billion people worldwide, most of them among the poorest are affected by drought and desertification. These people who occupy about one quarter of the planet are facing major problems, including soil degradation and vegetation loss, that contribute to chronic food insecurity (Ismat, 2004: 1). Algerian territory includes large arid and semi-arid areas under to desertification threat; While 20 million ha land area is being threatened by wind erosion, some other 5 million ha are already in an advanced state of degradation (Bensaid, 2006: 6). These spaces are parts of the 200 million ha wide Algerian Sahara that makes 80 % of the Algerian territory and which highly prone to desertification and land degradation; the core causes of increased wind erosion and risk to sand encroachment. A consequence of desertification, sand encroachment is said to take place when grains of sand are carried by winds until they collect on the coastal area, along water streams or over cultivated or uncultivated lands. As the sand dunes move, they bury agglomerations, roads, oases, crops, market gardens, irrigation channels and dams, thus causing major material and socioeconomic damages (FAO, 2010: 15).

The physical and climate context of In-Salah (southern Algerian Sahara) region favors wind erosion, considerably (figure 1). As all human and socio-economic installations in the region are located downwind of dynamic dune system, they stand as the only barriers against the dune movement and so, are frequently sanded (Boulghobra et all, 2014: 1).

This article highlights the physical, climatic and anthropogenic causes of sand encroachment in the region, along with the diachronic remote sensing monitoring of the mobile dunes in the study area. The experimental approach is based on the bi-temporal analysis of two optical medium resolution images: the Thematic Mapper (TM) image of 1987 and the Enhanced Thematic Mapper Plus (ETM+) of 2005. The latter were used to map the dynamics of mobile dunes and its impacts on the agglomerations, the agricultural land and the socio economic infrastructures.

Figure 1: Location map of the study area based on the TM Landsat image of september 2011. Green color in the trichromie RGB:541 corresponds to date palms and yellow color to sand cover

2 Causes of Sand Encroachment in Salah

Factors leading to sand encroachment in the Tidikelt can be divided into two categories: natural and anthropogenic.

2.1 Physical and climatic context

2.1.1 Topography

In-Salah and neighboring agglomerations belong to the Tidikelt dune system; this uniform topographic depression is located in a natural corridor and is dominated north and south by the Tademaït plateau. This position favors wind concentration and increases the wind velocity.

2.1.2 Lithology and soil state

Highly prone to wind erosion, existing lithological formations (Quaternary, dunes and late Cretaceous) have given rise to mobile, dry and crushed soils (coarse textured, rich in fine sands and poor in clay and organic matter), increasing wind corrosiveness and sand load capacity.

2.1.3 Climate

Characteristically, Tidikelt plain endures a hyper arid climate, owed to continental position of the Algerian Sahara and to low air humidity. Rainfall remains very low with increased spatiotemporal variations. Inter-annual and intra-annual maximums are 31.7 mm and 8 mm, respectively. Also, the Tidikelt plain is among the hottest regions of Algeria, the warm season lasting beyond 6 months with temperatures exceeding 45 °C in July. As insolation may extend over 3000 hours/year, while relative humidity is low (26.1 %), contributing to lands degradation and increased wind erosive efficiency.

Central Algerian Sahara (Adrar, In-Salah and Timimoune) is the windiest area in Algeria (Kasbadji Merzouk, 1999: 4). Wind speed averages 5 m/s to above 8 m/s with about 7% of the winds exceeding 11 m/s, a threshold at which wind can perform such actions as abrading, transportation and accumulation. Especially as north-east prevailing and effective wind is being the same as the direction of the plain elongation, the agglomerations located downstream are greatly endangered (figure 2).

Figure 2: Wind rose of In-Salah station based on 34 years data range (1980-2013); directional frequencies highlight the unimodal wind regime of the region, as well the trade wind (north-east) is the main factor of dunes migration (figure taken from the ncdc noaa website, 2013)

2.1.4 Vegetation state

Vegetation cover preserves the soil from wind erosion as it retains soil particles, and reduces wind impact on the soil. Therefore, reduced soil vegetation cover (sparse, stunted or non-existent woodland, bushland or grassland) in arid and semi-arid regions greatly exposes the soil to wind erosion (FAO, 2010: 17).

In the Tidikelt, the climatic and edaphic conditions do not encourage plant growth and development; however, spontaneous plants such as Tamarix Gallica, Tamarix Articulata, Zygophyllum Album and Aristida Pyngens, may be found in the wadi’s beds, yet in densities too insignificant to help mitigate wind erosive activity.

2.2 Anthropogenic effect and the man-made disaster

The availability of fresh water sources on the Tidikelt explains the human colonization of the region even before of sanding risk. Over time, human activities favored the establishment of the dune system, and therefore, aggravated the vulnerability, as the region was already subjected to an important wind dynamics. Human intervention can be summarized as:

  • Urban extension into the high-risk zones;
  • Construction of buildings perpendicular to the wind direction and creation of barriers (figure 3, 4);
  • Use of inappropriate techniques of sand dune fixation;
  • Selection of inappropriate sites for settlement (accumulation sectors instead of the sand source sectors) (figure 5).
Figure 3: Impermeable construction, perpendicular to the wind direction (Photo by Boulghobra, 2013)
Figure 4: Permeable construction: Even across the wind direction, permeable constructions prevent sand accumulation; 1, 2 and 3 on the photograph corresponds to vacuums. They allow winds loaded with sand to pass freely and thus, prevent accumulation, Adrar, Algeria (Photo by CRSTRA, 2012)
Figure 5: An unsuccessful greenbelt resulting from lack of irrigation (Photo by Boulghobra, 2013)

3 Sand Encroachment Monitoring Using Remote Sensing Technology

The diachronic monitoring of sand encroachment in the tidikelt required the processing of multitemporal Landsat 5 and 7 images, produced on March 3, 1987 and April 13, 2005, respectively. These images were remotely sensed with TM and ETM + sensors over a period of 19 years. Landsat satellite was used because of its medium spatial resolution of images (30 m) that enables analysis on a regional scale. Images were selected in regard of their availability, their quality and their spectral resolution. After a preprocessing, these images were classified, using both the supervised mode, and the maximum likelihood method. After the various stages of post classification (combining, clumping and sieving land-cover classes, and validation of the classification using the confusion matrix), a static map and a dynamic map (by overlaying) of mobile dunes layer vectors were developed for each Landsat image, to show the evolution of the mobile dunes from 1987 to 2005. Results show that mobile dunes of Erg Sidi Moussa migrate in the direction of the effective winds and settle on the eastern sides of all oases and agglomerations, covering large areas in the region that increased from 22.9 km² in 1987, to 25.8 km² in 2005 (figure 6). This is particularly observable in In-Salah which is the most vulnerable agglomeration. This phenomenon results mainly from anterior climate factors that favored the sand dynamics, the decrease in relative humidity (26.9 to 25.9 %) and the increase in temperature (25.9 to 26.5 °C), making the soils drier and more erodible. Additionally, the increase maximum wind speed (6.4 to 9.4 m/s) accentuated the wind sand transport capacity, considerably (Table 1).

Table 1: Parameters for measuring in In-Salah mobile dune dynamics (Boulghobra, 2014: 8)
Parameters 1973 to 1987 1988 to 2005
Mobile dunes area (km²) 22.9 (1987) 25.8 (2005)
Relative humidity (%) 26.9 25.9
Temperatures (oC) 25.9 26.5
Maximum wind speed (m/s) 6.4 9.4
Figure 6: Progressive dynamics of the mobile dunes in In-Salah region, 1987 to 2005 (map by Boulghobra, 2014)

4 Consequences of the Sand Encroachment Disaster in the Tidikelt Plain

Data on loss estimate as cause by sand encroachment in In-Salah are not available. However, our field investigations provide indications of the extent the disaster. These include losses of several hectares of date palm plantations, roads damages, dozens of buried residences and dislodged families, and many abandoned schools and administrative buildings (figure 7 - figure 10). Preventive efforts have not been successful, as many contributed to even aggravate the problem because of a lack of technical support for the establishment appropriate protective barriers and for human settlement.

Figure 7: Sanding of a date palm plantation in In-Salah (Photo by Boulghobra, 2013)
Figure 8: Sanded school in In-Salah (Photo by Boulghobra, 2013)
Figure 9: Sanding of a house under-construction in In-Salah (Photo by Boulghobra, 2013)
Figure 10: Sanded road in Igostene (Photo by Boulghobra, 2013)

5 Conclusion

Sand encroachment in Algeria and especially the Tidikelt is due to the natural operation of the dune system. However, urbanization with the construction of industrial structures, housing, and various infrastructures add to agricultural activities to exacerbate vulnerability, particularly in high-risk areas. The monitoring of sand dynamics reveals the spatiotemporal progression of sand dunes towards the agglomerations and socio-economic installations. Undoubted, this will lead to a serious disaster in the future. Efforts to control the phenomenon still remain ineffective, as aeolian processes are complex and poorly documented, and sand encroachment projects are implemented in non- participatory ways.

6 Recommendations

We would recommend that:

  • An emphasis be put on awareness improvement of both the populations and authorities;
  • Existing control installations be maintained while encouraging the application of effective control techniques upstream of the dune system;
  • The population be integrated into the implementation of all encroachment programs;
  • Land use be legislated in a way to facilitate the planning of local development and to mitigate damages.

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Citation

Boulghobra, N., Merdas, S. and Lakhdari, F. (2015): Sand encroachment in the Saharan Algeria; the not declared disaster - Case study: In-Salah region in the Tidikelt. In: Planet@Risk, 3(1): 72-76, Davos: Global Risk Forum GRF Davos.