تحلیل مورفومتریک مخروط افکنه ها در مناطق پایکوهی البرز شمالی، مازندران (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
مخروط افکنه ها یکی از مهم ترین لندفرم های رسوبی رودخانه ای هستند که عمدتاً در محل خروج از کوهستان تشکیل می شوند. عوامل متعددی در شکل گیری و تکامل این اشکال نقش دارند. تحقیق حاضر، با هدف ارزیابی مورفومتری مخروط افکنه ها در پایکوههای البرز شمالی (مازندران)، بررسی عوامل مؤثر در تشکیل آنها و مقایسه آنها با مخروط افکنه های سایر مناطق ایران انجام شدهاست. در ابتدا محدوده مخروط افکنه ها با استفاده از شاخص MRVBF شناسایی شد و سپس با استفاده از شکل منحنی تراز حاصل از مدل ارتفاعی رقومی با قدرت تفکیک 5/12 متر تعیین حدود شدند. پارامترهای مساحت، طول، شیب، حجم و زاویه جاروب مخروط ها اندازه گیری شدند. مقادیر خصوصیات هندسی حوضه های آبریز بالادست، درصد پوشش جنگلی و مقاومت لیتولوژیکی آنها با استفاده از آنالیز مؤلفه های اصلی (PCA) مورد تحلیل قرار گرفتند. تعداد 45 مخروط افکنه با مساحت های 2/0 تا 370 کیلومتر مربع مورد ارزیابی قرار گرفتند. تحلیل رگرسیونی متغیرهای مختلف بیانگر نقش پارامترهای مساحت، طول و درصد پوشش جنگلی حوضه در تخمین ابعاد مخروط افکنه شامل مساحت، طول، شیب و حجم با ضریب تبیین بالا است. بر اساس این ویژگیها، مخروط ها در سه دسته گروه بندی شدند. مقایسه ویژگیهای هندسی مخروط افکنه های البرز شمالی با سایر مناطق کشور نشانمیدهد که مساحت این مخروط ها نسبت به سایر مناطق کشور 3 تا 4 برابر کوچکتر و شیب مخروط ها 2/1 تا 6/1 برابر کمتر است. علاوه بر خصوصیات هندسی حوضه های آبریز، درصد پوشش جنگلی و پهنای دره در قسمتهای انتهایی حوضه هم در ابعاد مخروط افکنه ها نقش داشته اند.Morphometric analysis of alluvial fans in northern Alborz foothills, Mazandaran
Introduction Alluvial fans are cone-shaped alluvial landforms created at the outlet of mountain basins and the transition from relatively steep slopes to relatively flat areas such as valleys and plains. Many urban and rural areas of Iran are located in the foothills and on alluvial fans. Much research has been performed on alluvial fans in arid and semi-arid regions of Iran. However, little research has been done on alluvial fans in the foothills of the northern Alborz Mountains, which are located in a relatively humid region. Mazandaran Coastal Plain is a part of the Caspian Coastal Plain is located between the Caspian Sea and the Alborz Mountains. This zone is covered with Quaternary sediments. In the southern parts of the Mazandaran Plain, many large and small alluvial cones have been formed, which have been studied in this research. Therefore, this research has been carried out with the following objectives, which are: 1) Analysis of the morphometric characteristics of alluvial fans and their catchment areas in the study area, 2) Study of the influencing factors in alluvial fans morphometry, 3) Comparing the results of alluvial fans morphometric the foothill of Northern Alborz mountains with other regions of Iran. Methodology The morphometry of the catchments and alluvial fans is performed based on the digital elevation model (DEM) with a resolution of 12.5 meters ALOS PALSAR. Multi-resolution Valley Floor Flatness (MRVBF) index was first used to determine the boundary of alluvial fans and the approximate range of alluvial fans was identified. Then, using the shape of contour lines, the boundary of the alluvial fans was drawn. The morphometry of alluvial fans was performed by measuring the variables of area, length, volume, gradient, and sweeping angle. Also, morphometric variables of catchments including: area, perimeter, length, Melton roughness index, basin compaction coefficient, and elongation form were measured. In addition, the percentage of forested areas of the catchments and the map of rock resistance to erosion were also prepared as the controlling factors of the basin. The morphometric parameters of the catchments were evaluated using the Principal Component Analysis (PCA) method. The alluvial fans of the study area were grouped based on the variables of area, length, slope, and volume. To better understand the relationships, regression analysis of morphometric variables of watersheds and alluvial fans was performed in SPSS software. Results and Discussion A total of 45 alluvial fans were studied in the Northern Alborz mountains. These fans were classified into three groups based on their morphometric characteristics. Group 1, which includes 91% of the fans in the region, has an average area of 8.5 km2, a length of 2.7 km, a volume of 193 million cubic meters(Mm3), and a longitudinal gradient of 2.47%. In the second group, 6.7% of the fans were placed, which had a larger average area, length, and volume than the first group, but their longitudinal gradient was lower than the other groups, with a value of 0.56. The third group included only one alluvial fan (alluvial fan of Haraz River) which has created a significant difference with the other groups with higher values of the main variables. The analysis of the PCA shows that the first component is mainly based on the geometric characteristics of the catchments. The slope and roughness variables of the basin are placed in the second component and the basin compaction coefficient and elongation form in the third component. These three components include a total of 82.8 percent of the variance. Relationships between catchment and alluvial fan parameters have often been in the form of power regression. Basin area as an important parameter has had significant regression relationships with the parameters of area, length and volume of the fans. R2 values for these regression relationships were calculated as 0.8, 0.78, and 0.8, respectively. In the power regression of the basin and fan area, the value of coefficient b, which is the power of the equation, usually varies between 0.6 and 1.1. The studies conducted in Iran are also in the same range of changes. This coefficient (b) was 0.77 in the study area. The slope of the fan is a variable that has an inverse relationship with the area of the basin and the alluvial fan. The regression of basin area and fan area variables with the slope of the fan has coefficient b equal to -0.18 and -0.22, respectively. Melton's number has a direct relationship with the slope of the fans with an R2 of 0.47. There is a logarithmic regression between the percentage of forest cover and the area of Alluvial fans, a relationship with an R2 of 0.67. In addition to the physical characteristics of catchments, the percentage of tree cover, climate difference in larger basins, and the width of the valley at the end part of the basin have been effective factors in the dimensions of alluvial fans of groups 2 and 3. Conclusion Examining the morphometric variables of alluvial fans the northern Alborz Mountains and their upstream basins shows significant relationships. Despite the complications caused by the changes in the level of the Caspian Sea in the late Pleistocene and Holocene and the tectonic effects of the Caspian fault, these relationships exist between the characteristics of the basin and the morphometry of alluvial fans in the region. The morphometric comparison of the alluvial fans of the northern Alborz mountain range with other regions of Iran shows that the area of alluvial fans in the region is three to four times smaller than in other regions of Iran. The regression relationships show that the slopes of alluvial cones in the study area are 1.2 to 1.6 times lower than other country regions. Therefore, it seems that the controlling role of the catchment, such as the presence of dense forest cover, has reduced the amount of transferred sediments from the catchment. As a result smaller alluvial fans with a gentler slope than other regions of the country have been formed.
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