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EN
The article describes evolution of a dune field located in a vicinity of Douz on south-eastern margin of the Chott Jerid. The aim of the research was to assess the migration rate of dunes and to find determining factors. Between 1997 and 2000 location of selected dunes was measured every month. Textural features of dune sediments were analysed in order to determine dynamics of aeolian processes and a source of the sediment. The dunes, mostly barchans in a youthful stage of development, 1,5-2m high and 40 m wide, migrated generally in two directions: E and NE in spring and SW and W in autumn. In winter and in summer the migration rate was much lower which is explained by seasonal changes in wind direction and speed. The dunes migrated with an average rate of 80m/yr eastwards with a maximum rate of 40m/month and 2-3m/day. The dune sediment mostly consists of fine and very well to well sorted sand which testifies for a long-lasting aeolian process. A share of quartz, very resistant to mechanical abrasion, is high and a share of gypsum is low which is a result of multiple and long-lasting process of redeposition. This is also supported by an analysis of heavy minerals among which minerals highly resistant to mechanical abrasion dominate (mostly garnet and epidotes). Results of sedimentological analysis combined with a broad phypico-geographical view of the area allow to conclude that, the migration rate of the researched dunes is determined by textural features of dune sediments. Such a high migration rate, much higher comparing with other dune fields in the world, is an effect of finer and better sorted sand which can be easily transported by weaker winds.
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EN
The article includes physico-geographical characteristics of the Chott Jerid which was selected for detailed studies presented in following articles. The analysed region is in southern Tunisia and lies within the Great Chotts region. Surface lithology is dominated by Quaternary clayey sediments of fluvial and lacustrine origin and aeolian sands. Crataceous and calcareous rocks can be found along margins of the chott. The climate can be described as very arid continental subtropical. Surface drainage and lakes appear only seasonally in lowest parts of the Chott Jerid and the water is rich in salt. Underground aquifers are very water efficient therefore there are many artesian or subartesian wells allowing for irrigation. Soils in the region belong to Aridisols and are rich in gypsum or calcite. Salt-rich soils dominate in the region. Natural vegetation is sparse (mainly shrubs and halophites) because of arid climate, lack of surface waters and salty soils. Dactyl palms dominate on irrigated land.
EN
The aim of the study was to determine the influence of textural features of sand on aeolian transport in six designated fields in northern Sahara. An analysis of textural features such as: mineral composition, grain shape, mean grain size diameter and sorting were performed during a multi-year research on dune sediments. Information on the movement of small landforms (1.5m) in areas of similar physico-geographical environments was taken under consideration. It was revealed that the fastest movement, of about 90 my⁻¹, is characteristic for barchans on dune field no. 4. (where gypsum sand dominates). Barchans on dune field no. 1 are slower, with 50 my⁻¹ (where limestone dominates), and the slowest are those on dune field 6 (where quartz dominates), with about 30 my⁻¹. The result of the study proves that textural features have a significant influence on aeolian transport.
EN
The article describes morphology and dynamics of 2 small wadis (gullies) in the vicinity of Gabes, tributaries of the Wadi Gabes, which cut a loess cover of sea-side region. Mapping in 1995 and 1998 was aimed to recognize contemporary gullying resulting from precipitation 807 mm with maximal 51 and 92 mm per day. Slope gradients and length of the thalweg as well as depth and width in cross sections were measured. The gullies are characterized by many knickpoints, but only few are up to 1 m. The changes in morphology of the gullies indicate slow retreat of the knickpoints (sporadicaly to 60 m) or their cutting and the incision along most of the gully length (5-10 cm locally more), as well as lateral erosion, particularly in lower parts of the gullies. The deposition occurred only locally and was very irregular. Changes in morphology of the gullies were very limited due to highly calcareous loessial and sandy-gravel bedrock. The length of the gullies increased due to the development of deep rills above heads of the gullies and their sinuosity in the lower part. The rate of gully heads advance is about 1,6 and 2,5m m per year.
EN
The article includes description of nebkas - the smallest accumulation forms of aeolian origin. They originate from accumulation of wind-blown material accumulated on a lee side of a vegetation obstacle. The research was undertaken in the eastern part of the Chott Jerid, within its Holocene floor. Such forms are thought to indicate erosion of arid regions. Their appearance is usually linked to lowering of ground water table and related change in vegetation density. The researched was focused on two selected fields of nekbas. Morphometry and sedimentological features of building material were analysed. It was found that the shape of nebkas depends on a type of vegetation which constitutes the transport obstacles and forces accumulation. Lower forms (up to 0,5 m high) are created on a lee side of arfej species and are characterised by a high to length ratio 1:7 and so they have a typical shape of nebkas. Higher forms (up to 1 m) are created on a lee side of tamarix species and their high to length ratio is 1:4. Such a shape is due to a width of the plant and intensive erosion of a distal part of the form because wind frequently changes direction in this field. Sedimentological features of the forms show that material building the higher forms is better sorted comparing with lower forms and therefore it was probably transported for a longer distance. Mineral-lithological composition of the forms show a high share of quartz as well as relatively high share of gypsum (not resistant for abrasion). High content of gypsum in nebkas, seasonal capillarity with intensive evaporation lead to crusting of especially lower forms. Such a crusting strengthens the forms, protects from erosion of their distal parts and therefore explains differences in shapes of nebkas.
EN
Barchans are characterized by a symmetric shape independently of their stage of development and the axis of symmetry parallels to the dominant wind direction. However, shape of the dunes can be modified by several factors, among which most important are: change of wind regime, dune collisions or interaction of closely spaced dunes, ground surface relief, asymmetric sediment supply. The aim of the study was to analyze the impact of short-term (diurnal) and long-term (seasonal) changes in wind direction and velocity on shape of barchan dunes. The study was conducted in southern Moroccan area of Western Sahara, between Tarfaya and Laâyoune. This area is characterised by significant variability of active wind (RDP/DP=0,66) which is unfavourable for barchans development. Measurements were conducted in two spring seasons of 2011 and 2012 and two autumn seasons of 2007 and 2011. 52 isolated barchans were selected for detailed studies. Based on GPS-RTK measurements, precise 3D models of the landforms were obtained allowing the detailed analysis of dune shapes. Changes of basie morphological parameters were analyzed as well as shape changes occurring the windward slope and within dunes horns. The shapes of the barchans were correlated with the short-term and the long-term wind regime. It was found that basic morphological parameters are not good indicators of wind variability, with the exception of barchan horns lengths which change according to the seasonal wind regime. Short-term wind regime changes are well registered by: i) "collars" developing along the dune crestlines, ii) "tongues" developing at the ends of dune horns and iii) location of the dune apex. On the other hand, beaded horns testify for very strong wind. I was found, that mentioned features develop and undergo modification during first days or even hours of changed wind regime. The analyzes of barchan shapes, performed in different seasons, proved that even a several-month long period of highly diversified wind regime does not change basie shape of the barchans. Therefore, it can be inferred that barchans development does not depend exclusively on wind regime.
EN
The aim of the research was to describe origin of aeolian residual hills located in the eastern part of the Chott Jerid. Two areas were selected to the research, first in a vicinity of Bechri (within the Pliopleistocene range of the chott), second 10km north of El Faouar (within the Holoce floor of the Chott). Morphometry of the forms and analysis of prevailing wind directions in the first research area proved, that the hills are erosion residual hills and can be called yardangs. Aeolian erosion as a shaping factor is questioned in the second research area, because of lack of longest axis of the forms as well as lack of the yardang-like shapes. It is put forward that the forms in the second research area originated mainly from salt weathering (exudation). Analysis of internal structure of the forms showed that the sediments were accumulated in aeolian environment. In the case of the first research area the accumulation produced barchans and barchanoidal dunes while in the second researched area – longitudinal dunes (seif-dunes). Textural features of the sediments building the residual hills, particularly medium sorting (σ₁= 0,6-0,8) and dominant share of gypsum (60-80%) which is not resistant to mechanical abrasion, show that aeolian transport was short and the material source was quite near. The researched residual hills are of different age. The forms in the vicinity of Bechri were created during the 20th c. TL dating of the material constituting the hills near El Faouar shows that accumulation of the material took place over 30 ka BP.
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