نوع مقاله : مقاله کامل
نویسندگان
1 دانشجو- دانشگاه تهران- دانشکده جغرافیا
2 گروه جغرافیای طبیعی دانشکده جغرافیا-دانشگاه تهران-تهران- ایران
3 جغرافیای طبیعی دانشکده جغرافیا-دانشگاه تهران-تهران- ایران
چکیده
کلیدواژه‌ها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Extended Abstract
Introduction
Wind erosion is one of the active processes in arid and semi-arid areas, which has caused the formation of various landforms, including sand flats and sand dunes in these areas. Iran is located in the dry and desert belt, so that more than two-thirds of Iran's area is covered by dry and semi-arid lands, the lack of rainfall, climate changes and improper management have increased the growth rate of desert areas in the country. One of these areas is the Boralan Plain, which is located in the most northwestern part of Iran. This area has a dry climate, and sand storms and the movement of sand dunes are threatening factors in the area. The purpose of this research is to investigate the effect of erosive winds and to draw the graph of sand mud and sand mud and its indicators and to use the results of granulometry and microscopic and macroscopic examination of sediments in order to study the characteristics of the texture and origin of sediments in order to identify the sources of sediments, causes and effective factors in And in line with optimal management, he concentrated the necessary preparatory measures in these areas.
Methodology
In this study, in order to determine the wind regime, the wind direction index, the sand transport potential index, the resultant drift direction index of erosive winds, and the resultant transport potential index in the region were examined. Wind data with a 30-year statistical period from the synoptic station of Maku City were used, and the WR Plot and Sand RoseGraph software were employed to determine the dominant wind directions, analyze wind data, calculate sand transport potential, and draw sand roses.
Subsequently, to identify the source of the transported sediments, sand samples were collected. Granulometry and morphoscopy techniques were used to identify and determine their physical and chemical characteristics. For granulometric analysis, after transporting the sediments to the laboratory, 100 grams of each sample were used for a dry sieving test. Sieves with mesh sizes of 2000, 1000, 500, 250, 125, and 63 microns, along with a container for collecting particles smaller than 63 microns, were utilized.
The results of sediment grain-size distribution were then input into GRADISTAT software for statistical analyses, including mean grain size, sorting, and skewness, which are effective in determining the distance from the source of transported sands. Statistical parameters of the sediments were calculated based on the Folk and Ward method, and grain-size distribution curves were drawn.
To study the morphoscopy of the sediments, 25 grains were randomly selected from each sample and examined under a binocular microscope from two aspects. First, the grain surface conditions were analyzed, including characteristics such as dullness, matteness, speckling, brilliance, and signs of physical and chemical weathering.
Results and discussion
The results obtained from the wind rose and sand rose diagrams indicated that the dominant annual wind direction is from the southeast and east. The sand transport potential at the studied station, based on a threshold wind speed of 13 knots, exceeds 65 vector units per year. The general direction of the sand rose is toward the east, with sand being transported from the southwest, west, and northwest directions. The strongest winds in the study area occur during the winter and spring seasons, primarily from the southwest, west, and northwest sectors.
According to the granulometric analysis, the mean grain size of the sediment samples collected in the study area ranges between 2.126 and 2.939 phi. The standard deviation, or sorting, of sediments is a parameter that indicates the uniformity of the particles and how close or far apart their diameters are. The more uniform the grain sizes, the better the sorting. Based on Folk's method, the sorting values range from 0.465 phi to 1.309 phi, indicating that the sediments vary from moderately well-sorted to poorly sorted.
The skewness parameter reflects the asymmetry and the tendency of the grain-size distribution curve toward finer or coarser particles. If there is an abundance of fine particles, the curve tail extends to the right, indicating positive skewness. Conversely, if coarser particles dominate, the tail shifts to the left, indicating negative skewness. The results show that skewness values range between -0.541 phi and 0.449 phi, with most samples being nearly symmetrical.
Additionally, the kurtosis parameter, based on Folk’s classification, showed that most samples fall within the range of 0.9 to 1.11, indicating a mesokurtic (moderately peaked) distribution curve.
In the morphoscopic analysis, the first step examined the surface conditions of the grains, identifying characteristics such as dullness, matteness, speckling, brilliance, and signs of physical and chemical weathering. Results showed that approximately 45% of the grains were dull, while about 43% were brilliant.
In the second step, the grain shapes were analyzed for roundness and abrasion. The findings suggest that the more a grain is exposed to wind and water transport, the more rounded it becomes, which also allows inference about the transport distance. For classifying grain abrasion levels, four categories were used: (a) unabraded or angular grains, (b) slightly abraded grains, (c) abraded grains, and (d) well-rounded grains. The results indicated that more than 45% of the grains were angular, reflecting the strong influence of physical weathering processes on them.
Conclusion
The results of this study indicate that the alluvial deposits of the Aras River play a significant role in the supply of sand particles in the region. Additionally, the angular shape of most grains suggests mechanical destruction of the igneous rocks present in the area. This, along with overgrazing—which has led to sparse vegetation cover—has contributed to the activation of wind erosion in the region. Therefore, it can be concluded that sand transport in the area is a combined result of both water and wind processes
کلیدواژه‌ها [English]
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