DEM-based morphometry of large-scale sand dune patterns in the Grand Erg Oriental (Northern Sahara Desert,Africa)

Formerly, sand dune patterns were investigated mostly by aerial and satellite images, but more recently, geomorphometric analysis based on digital elevation models (DEMs) has become an important approach. In this paper, sand dune patterns of the Grand Erg Oriental (Sahara) are studied using the De Ferranti (2014) DEM, which is a blending of SRTM (Shuttle Radar Topography Mission), ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) and other elevation datasets. In the Grand Erg Oriental, there are four large-scale dune pattern types with gradual transitions between them and with several subtypes, namely P1, consisting of large, branching linear dunes; P2, a complex pattern including small-size and widely spaced star and dome dunes; P3, a network type created mostly from crescentic dunes; and finally, P4, consisting of large and closely spaced star dunes. The largest dunes with 90–100-m mean height can be found in the southern parts of the Grand Erg Oriental, where P1 and P4 patterns dominate, and these areas are also characterised by the most intensive sand accumulation with 25–30-m equivalent sand thickness. In the present study, we use regression analysis to investigate the functional relationships between sand dune characteristics. Further on, we have elaborated a DEM-based method to delineate dunes and calculate sand volumes and dune orientations. Comparing wind rose data and sand dune axis rose diagrams, it is concluded that in some parts of the Grand Erg Oriental, the present dune types and patterns are in agreement with the actual wind regime, but in other cases, the present dune patterns are at least partially the results of former wind regimes.     

Determination of sand dune characteristics through geomorphometry and wind data analysis in central Iran (Kashan Erg)

The combination of wind measurements and remotely sensed geomorphometry indices provides a valuable resource in the study of desert landforms, because arduous desert environments are difficult to access. In this research, we couple wind data and geomorphometry to separate and classify different sand dunes in Kashan Erg in central Iran. Additionally, the effect of sand-fixing projects on sand dune morphology was assessed using geomorphometry indices (roughness, curvature, surface area, dune spacing and dune height). Results showed that a Digital Elevation Model of the National Cartographic Center of Iran (NCC DEM) with 10-m resolution and accuracy of 54% could discriminate geomorphometry parameters better than the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data with 30-m resolution and Shuttle Radar Topography Mission (SRTM) data with 90-m resolution and 45.2 and 1.6% accuracy, respectively. Low classification of SRTM DEM was associated with too many non-value points found in the DEM. Accuracy assessment of comparison ground control points revealed that ASTER DEM (RMSE = 4.25) has higher accuracy than SRTM and NCC DEMs in this region. Study of curvature showed that transverse and linear sand dunes were formed in concave topography rather than convex. Reduced slopes in fixed sand dunes were established due to wind erosion control projects. Measurements of dune height and spacing show that there is significant correlation in compound dunes (R ² = 0.546), linear dunes (R ² = 0.228) and fixed dunes (R ² = 0.129). In general, the height of dunes in Kashan Erg increases from the margin of the field to the center of the field with a maximum height of 120 m in star dunes. Analysis of wind data showed that sand drift potential is in low-medium class in Kashan Erg. Linear sand dunes in Kashan Erg show that they are following a global trend in forming of these. Finally, established of geomorphometry method in dune classification will help researchers to identify priority of land management and performance assessment of sand dunes fixing projects in arid arduous environment.     

Dynamic processes acting on a longitudinal (seif) sand dune

ABSTRACT Field measurements were made on a longitudinal dune in the Sinai Desert in order to understand its morphology and dynamics. The field measurements contradicted the wind structure indicated by the helicoidal flow theory. Rather, it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank.
The essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line. 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 velocity of the deflected wind is higher than on the crest line or the windward flank and longitudinal sand transport occurs. When the angle is less acute (> 40°) the velocity of the deflected wind drops and deposition takes place on the lee flank.
The angle of incidence in each wind storm is changed intermittently between 30° and 100° along the dune because the dune meanders and because of the sinuous outline of the crest line. In this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line. As a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month. Peaks and saddles along the crest line advance at an average rate of 0.7 m per month.
The lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition. This can explain the formation of peaks along the crest line of the dune.     

Toward a model for airflow on the lee side of aeolian dunes

The interaction between dunes and the primary wind results in a complex pattern of secondary airflow on the lee side of dunes. From 15 dunes studied during transverse flow conditions at Padre Island in Texas, White Sands in New Mexico, and the Algodones in California, distinct flow regions can generally be recognized, with the overall flow structure comparing favourably to that proposed for subaqueous bedforms. Downwind of dunes with flow separation is a back-flow eddy that extends about four dune-brink heights downwind from the brink of the dune. Beyond the separation cell, the velocity profiles can be divided into regions based upon segments separated by ‘kinks’ in the velocity profiles. The interior is an area above the dunes of relative high wind speed but low velocity gradient. Beneath the interior is the wake, which consists of two layers. The upper wake exhibits an uppermost portion where the flow decelerates while the remainder exhibits accelerating flow, so that the overall velocity gradient decreases downwind. The lower wake exhibits low velocity gradients and wind speeds that accelerate downwind at all heights, but primarily near the top of the layer, thereby causing the velocity gradient to increase downwind. At about eight dune heights downwind, the upper and lower wakes equilibrate to a single profile with the kink between them no longer apparent. The lowest recognizable region is the internal boundary layer. It is recognized by a relatively steep velocity gradient below the wake, and never exceeds a few tens of centimetres in height for our data set. Because of acceleration and increasing shear stress within this layer, interdune flats are at least potentially erosional. Overall, the wake and internal boundary layer show a downward transfer of momentum from upper regions so that the flow recovers. Where flow separation does not occur, simple flow expansion down the lee-face causes flow deceleration.     

Temporal trends in grain-size measures on a linear sand dune

Within aeolian dune systems spatial patterns of grain-size variation have been recognized, but little has been said about temporal changes. Increasingly it is becoming clear that linear dunes are associated with bi-directional wind regimes which are often seasonal. In the Namib Sand Sea, where linear dunes are aligned roughly north-south, winds blow from the west in summer and from the east in winter. In response to this regime, sand is eroded from the west slopes and deposited on the east slopes in summer, and eroded from the east slopes and deposited on the west slopes in winter. Preliminary evidence from a study of a single Namib linear dune reported here confirms that this seasonal aeolian regime induces seasonal responses in some grain-size measurements due to the dynamics of sand transport on the dune, the characteristics of the sand source immediately upwind of the sample point and the nature of the deposit. Thus, time of sampling is crucial to the results obtained.     

Deflection of sand movement on a sinuous longitudinal (seif) dune: use of fluorescent dye as tracer

Sand was marked by fluorescent dye in order to trace sand movement and deposition on a longitudinal (seif) sand dune in the Sinai desert. The wind regime was monitored simultaneously. Tracing the dyed sand was possible after light to moderate sand storms and was graphically represented on maps.The dune was subjected to a seasonally bidirectional wind regime, with the wind hitting the dune obliquely on either side. On the windward flank the sand was transported parallel to the wind direction. On the lee flank sand movement was deflected towards parallelism with the crest line. Sand movement was deflected if the dune had a sharp profile which favored separation of wind flow on the lee flank. The deflection depended on the angle of incidence between the wind and the crest line: when the angle of incidence was < 40°, sand on the lee flank was transported parallel to the crest line; when the angle of incidence was nearly perpendicular to the crest, movement along the lee flank abated and deposition occurred. Where the dune was low, flat and blunt, as in a zibar dune, there was no boundary-layer separation and no deflection of sand movement on the lee flank. The deflected movement along the lee flank resulted in elongation of the longitudinal (seif) dune.     

Grain-size sorting in grainflows at the lee side of deltas

The sorting of sediment mixtures at the lee slope of deltas (at the angle of repose) is studied with experiments in a narrow, deep flume with subaqueous Gilbert-type deltas using varied flow conditions and different sediment mixtures. Sediment deposition and sorting on the lee slope of the delta is the result of (i) grains falling from suspension that is initiated at the top of the delta, (ii) kinematic sieving on the lee slope, (iii) grainflows, in which protruding large grains are dragged downslope by subsequent grainflows. The result is a fining upward vertical sorting in the delta. Systematic variations in the trend depend on the delta height, the migration celerity of the delta front and the flow conditions above the delta top. The dependence on delta height and migration celerity is explained by the sorting processes in the grainflows, and the dependence on flow conditions above the delta top is explained by suspension of fine sediment and settling on the lee side and toe of the delta. Large differences in sorting trends were found between various sediment mixtures. The relevance of these results with respect to sorting in dunes and bars in rivers and laboratory flumes is discussed and the elements for a future vertical sorting model are suggested.     

浑善达克沙地的光释光测年研究

光释光测年技术以光可以使在半导体结晶固体中被俘获到光敏陷阱中的电子释放出来这一原理为依据,以样品在被覆盖前,它们的光释光信号都已经晒退回零为假设前提进行测年的。而风成沉积物是最符合这一假设的,所以光释光测年对于风成沉积物非常适合。本文对采自浑善达克沙地样品中的石英粗颗粒进行光释光测年,试图重建沙地的古环境演化过程。本文研究的样品分别采自浑善达克沙地的中部SG点(42°40'N,115°57'E)和东部YS点(43°14'N,116°49'E)。采样时,先挖大约30cm深,然后用锤子将金属小筒打入,拔出后,小筒用黑色塑料和胶带封死。到实验室后,小筒顶端大约1cm厚的样品用来测量     

The statistical analysis of the sand grain size distribution of Al- Ubay-lah barchan dunes,Northwestern Ar-Rub-Alkhali desert,Saudi Arabia

The paper analyses the statistical parameters of the sand grain size distribution of Al-Ubaylah barchan dunes applying the sieving technique and Folk and Ward, and Trask's equation. The results of the sieving analysis divulged that nearly over 90% of the sand particles fall in the size ranging between 1.50ø and 3.ø. Insignificant amount of sand grains also fall in the size ranging from 0.0ø to 1.50ø and from 3.0ø to 4.0ø respectively. The statistical analysis reveals that the sand is characteristically fine grained, moderately to moderately well sorted with an average diameter of 2.4ø and a standard deviation of 0.64ø. The sand distribution is positively fine skewed and mesokurtic to leptokurtic in nature. No significant variation in the statistical parameters (median, mean, sorting, skewness and kurtosis) of the sand grains samples used in this research is detected. The sand grain size distribution of all the samples can also be considered symmetrical and unimodal in nature. Those abovementioned characteristics are found to be nearly similar to that of other Saudi dune deserts. The miscroscopic investigation undertaken by this researcher displays that quartz represents often more than 99% of the samples. Felsic, mafic and calcium carbonates minerals represent less than 1% of the mineralogical constituents. The sand grains are subrounded to subangular and iron and calcium carbonates coated.     

Remote-sensing data-based Archaeological Predictive Model (APM) for archaeological site mapping in desert area,South Morocco

Morocco hosts numerous archaeological sites, some of which are part of the UNESCO world heritage. Many of these sites, especially funerary mounds also called tumuli, or rock engravings and ceramics, are located in remote areas with limited access, particularly in the Saharan Morocco desert. We developed a remote sensing and GIS model to identify areas with high potential for hosting archaeological sites in the Awserd region of southern Morocco. A field campaign in a “reference site” zone of 21 km² has revealed 233 archaeological sites. Here we use satellite images and Digital Elevation Models to examine with various techniques (spatial analysis, statistical techniques, and fuzzy logic functions) the relations between the distribution of the archaeological sites and geo-environmental variables such as ground geology, topographic elevation and slope, orientation (aspect), and distance to water sources. We derive empirical relations that reveal that the distribution of archaeological sites depends on the above geo-environmental variables. We then use the empirical relations to anticipate the potential locations of archaeological sites in a region of 980 km² enclosing the reference site area. The model proves capable of predicting 582 sites in the larger region. Subsequent field observations there confirmed that about 80% of the model anticipations were correct. Our Archaeological Predictive Model (APM) can be scaled to larger areas and varied geographic settings, and hence can be a useful guide for archeological studies in desert regions.     

Lithostratigraphy,biostratigraphy and sedimentary dynamics of the Lower Cretaceous deposits on the northern side of the western High Atlas (Morocco)

The analysis of a sedimentary succession on the northern side of the western High Atlas, from Essaouira to Amizmiz, has enabled us to identify, and describe, thirteen formations ranging in age from Tithonian to Clansayesian. The collection and determination of ammonite and echinoid faunas has allowed us to attribute a precise age to most of these units. Lower Cretaceous formations are organized into six main depositional sequences: Late Jurassic-Berriasian; Valanginian; Hauterivian-Barremian; Bedoulian; Gargasian; Clansayesian-Albian. This organization, the characteristics of the major discontinuities and the geometry of sedimentary succession allows recognition of the following events: a Late Berriasian distensive phase; a period of sea-level change and basin infilling during Valanginian, Hauterivian, Barremian and Bedoulian times under essentially eustatic control; an intra-Aptian tectonic phase; and a period of relative sea level rise and topographic levelling during Gargasian-Albian. This is essentially linked to the evolution of the North Atlantic ocean during the Early Cretaceous.     

Single quartz grain electron spin resonance (ESR) dating of a contemporary desert surface deposit,Eastern Desert,Egypt

Signal resetting by sunlight prior to burial is a crucial assumption in electron spin resonance (ESR) dating of sediments. This resetting process is expected to be completed to a greater extent in arid than in fluvial environments. The present paper investigates the natural and artificially irradiated signal intensity of Ti related centres in single quartz grains collected from the desert surface (Eastern Desert, Egypt) in order to test this hypothesis. The results suggest that in most grains both the Ti–Li and Ti–H signal are completely reset to zero. Additive dose curves based on the sum of both Ti centres show an anomaly in the low dose region. Possible causes for this behaviour are briefly discussed. Three fitting procedures are conducted and each of them shows a different palaeodose distribution with a rather large spread in D_E values. It is concluded that similar fossil deposits would be datable by single grain ESR using Q-band measurements of the Ti–Li or Ti–H signals in quartz.     

Vulnerability and fate of a coastal sand dune complex,Rosetta-Idku,northwestern Nile Delta,Egypt

Types, distribution, and origin of recent sand dunes between Rosetta and Idku, in the western sector of the Nile Delta, Egypt were investigated. Sand samples from the dunes, beach, and seafloor were studied for grain size distribution and mineralogical composition. It has been found that most of the dunes in the study area have been subjected to deterioration and removal due to the construction of buildings and the International Coastal Highway. The remnant constitutes a damaged belt of foredunes that extends from El Bouseily village to the west of Idku town. The dune’s origin is interpreted to be the result of coastal drifting and the subsequent transport of sediments of the former Canopic Nile branch eastward by the predominant longshore current and by aeolian processes. The blown sand grains accumulated to form a belt of coastal sand dunes of original longitudinal and crescentic forms. Urbanization of the coast has severely altered the landscape. The study area is considered vulnerable to the impacts of climate change and the expected rise in sea level. The outcome of potential sea level rise is serious; erosion problems are expected to be exacerbated and vast areas from land and property would be lost. Thus, protection and preservation the remaining dunes in the study area are vital requirements for shore protection.     

Variations in wind velocity and sand transport on the windward flanks of desert sand dunes

The magnitudes of increases in wind velocity, or speed-up factors, have been measured on the windward flanks of transverse and linear dunes of varying height. On transverse dunes, velocity speed-up varied with dune shape and height. For linear dunes, speed-up factors varied principally with wind direction relative to the dune, with dune shape and dune height. The main effect of velocity speed-up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas. This is greatest for large dunes, with winds of moderate velocity blowing at a large angle to the dune. Changing ratios of base to crest sand-transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase. On linear dunes, the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi-directional wind regime.     

Towards a better understanding of the Oulmes hydrogeological system (Mid-Atlas,Morocco)

Located in the Mid-Atlas (Morocco), the Oulmes plateau is famous for its mineral water springs “Sidi Ali” and “Lalla Haya” commercialised by the company “Les Eaux minérales d’Oulmès S.A”. Additionally, groundwater of the Oulmes plateau is intensively exploited for irrigation. The objective of this study, essentially performed from data collected during isotopic (summer 2004) and piezometric and hydrogeochemical field campaigns (spring 2007), is to improve the understanding of the Oulmes hydrogeological system. Analyses and interpretation of these data lead to the statement that this system is constituted by a main deep aquifer of large extension and by minor aquifers in a perched position. However, these aquifers interact enough to be in total equilibrium during the cold and wet period. As highlighted by isotopes, the origin of groundwater is mainly infiltration water except a small part of old groundwater with dissolved gas rising up from the granite through the schists.     

塔克拉玛干沙漠砂静力触探试验研究

在控制相对密度和含水量的条件下进行了系列室内大型槽和中型槽原位测试模拟试验及相关土工试验,取得了塔克拉玛干沙漠砂静力触探指标锥尖阻力qc与容许承载力pa、内摩擦角? 、相对密度Dr和变形模量E0.01的一系列有效数对,建立了qc- pa,qc-? ,qc-Dr,qc-E0.01等相关关系,并给出了适合在沙漠地区应用的数字化表格,为在沙漠地基勘察中推广应用静力触探技术积累了经验。     

Environmental impact of an urban landfill on a coastal aquifer (El Jadida, Morocco)

The El Jadida landfill is one among many uncontrolled dumping sites in Morocco with no bottom liner. About 150 tons/day of solid wastes from mixed urban and industrial origins are placed directly on the ground. At the site of this landfill, the groundwaters circulate deeply (10–15 m) in the Cenomanian rock (calcareous–marl), which is characterised by an important permeability from cracks. The soil is sand–clay characterized by a weak coefficient of retention.The phreatic water ascends to the bottom of three quarries, which are located within the landfill. These circumstances, along with the lack of a leachate collection system, worsen the risks for a potential deterioration of the aquifer.To evaluate groundwater pollution due to this urban landfill, piezometric level and geochemical analyses have been monitored since 1999 on 60 wells. The landfill leachate has been collected from the three quarries that are located within the landfill. The average results of geochemical analyses show an important polluant charge vehiculed by landfill leachate (chloride = 5680 mg l⁻¹, chemical oxygen demand = 1000 mg l⁻¹, iron = 23 000 μg l⁻¹). They show also an important qualitative degradation of the groundwater, especially in the parts situated in the down gradient area and in direct proximity to the landfill. In these polluted zones, we have observed the following values: higher than 4.5 mS cm⁻¹ in electric conductivity, 1620 and 1000 mg l⁻¹ respectively in chlorides and sulfate (), 15–25 μg l⁻¹ in cadmium, and 60–100 μg l⁻¹ in chromium. These concentrations widely exceed the standard values for potable water.Several determining factors in the evolution of groundwater contamination have been highlighted, such as (1) depth of the water table, (2) permeability of soil and unsaturated zone, (3) effective infiltration, (4) humidity and (5) absence of a system for leachate drainage. So, to reduce the pollution risks of the groundwater, it is necessary to set a system of collection, drainage and treatment of landfill leachates and to emplace an impermeable surface at the site of landfill, in order to limit the infiltration of leachate.     

巴丹吉林高大沙山表层孔隙水现象的疑义

1997-2003年对巴丹吉林沙漠中不同位置共7个高大沙山和1个流动沙丘表层2m内的孔隙水情况用常规方法作了观测,每年均在相近日期进行。孔隙水量随深度而增大,历年观测期间2m附近的体积含水量约3%,达到相应实测最大持水量的约65%或更高,25cm至1m孔隙水δ18O的正值表明它经过了反复补给-蒸发过程。观测了雨后入渗深度及其在沙丘120m坡面上的分布,另由历年短期能量平衡测验获得沙山陆面蒸发和凝结概念。认为当地年降水量和凝结量不足以对沙山2m表层内的孔隙水现象作出解释,疑另有与大尺度地下水系统相关的补给源。