نوع مقاله : مقاله پژوهشی
1 استادیار ژئومورفولوژی، گروه جغرافیا، دانشگاه یزد.
2 دانشجوی کارشناسی ارشد ژئومورفولوژی، گروه جغرافیا ، دانشگاه یزد.
عنوان مقاله [English]
Investigation of River Meanders Geomorphologic Changes Using Satellite Imagery During the Period from 1990 to 2018 (Case Study: Bashar River)
Identification of the shape and structure of the river is called the river morphology so that it can be obtained by knowing the river morphology. The morphology of a river is affected by different factors such as the rate of erosion and sedimentation. In river morphology studies, information such as geometric shape of the river, shape of bed and longitudinal river profiles can be obtained. The most common criteria related to river morphology are geometric or River Plan Rivers are one of the main sources of water and energy for human beings
Bashar River is one of the branches of the Khorsan River and flows in the city of Yasuj and part of the city of Sepidan in Fars province. This basin, with geographical coordinates of 30 °, 15 ° to 30 °, 50 ° north, and 51 °, and 25 ° to 51 ° and 48 ° east, collecting waters of this highland area and entering the Khersan River. The Bashar River, about 150 km long, originates from the western slopes of the Zagros Mountains, whose early branches are located in the northwest mountains of Sepidan, Fars province, and flows along the southwest to the northwest. The area of Bashar basin is 3099 square kilometers and its maximum height is from the height of Yasouj 4283 meters and the minimum height is 1320 meters.
To identify the hydraulic and morphological characteristics of the rivers, the quantitative geometric parameters of the river are measured and calculated. In the study of Meanders to determine the behavioral pattern of the river and how it changes over time, the Cornish method and river pattern recognition through satellite images have been used. In order to obtain the river wavelength on the river plan, the ArcGIS software first defined the curvature routing points of the river, and then the two points indicated by a single segment are connected, the length of this segment can be calculated in the ArcGIS environment.
Remote sensing technology has a higher position in tracking river changes than other methods of study, because in most cases it is not possible to study the field for us, or it is too time consuming or costly, or to have authentic historical documents available to us. In order to determine the variation of the Bashar River route by using distances, the satellite images of the area were first obtained and processed using ENVI software. For this purpose, after the digitization, the study area was cut and the Flaash method of atmospheric correction was applied to the images. In the next step, for the overall understanding of the unregistered classification area, the method was K-means or C-means. Then, for the classification of the oversight, using the classes constructed by ROI, Maximum Likelihood is one of the most accurate and most used monitoring method. ArcGIS software was called to determine the amount of image changes. A binary method was used to isolate water pixels in the images. To determine the maximum river change, the buffer was 100 meters along the route. The Thaning method was then used to separate the river from other parts of the water.
By measuring the central angles of the Meanders in AutoCAD software, their information and specifications were calculated and the number of river meanders from the source to the end was determined. Then, using the Cornish method, the central angle of each meander is calculated and by predicting the extent of development of each grunge, the menders that have a high potential for interruption in the future can be predicted.
According to the information obtained from the Meanders of the Bashar River, the Meanders are categorized and identified using the GIS software, according to their type of route. They are divided into four simple, direct, composite, and arterial meanders.
Euclidean distance was measured in different parts of the river that was more distorted and more variable in order to determine the extent of the river route changes and which period of erosion occurred. So, to calculate the Euclidean size, at first, where the river route had a large difference, two points were measured and the distance difference was measured, and then the coordinate system was determined. Then, in Excel, the difference between the two points was calculated and the corresponding graph is drawn which shows the distance and the amount of path changes in two years. In this area, from both directions of the river in 1990 and 2018, about 40 points were taken at an approximate distance of 3.5 km, and the difference between these two points was measured and plotted.
One of the main goals of this study was to investigate the changes in the bedding of the Bashar River. The results show the potential of the river to produce morphological changes in different sections of the river route, so that Figure 14 shows the river route changes during the period from 1990 to 2018, indicating that about 62% of the route is displaced and Changes have been made. According to the results, the role of human in the creation of Meander was determined by analyzing the river route. Meanders, along with human activities, such as agriculture with change in use and construction or harvesting of sand, had the most displacement. It can almost be said that along the river's route, it is rarely seen that it has not been used by humans. The results show that the extraction of the Bashar River route using satellite imagery to determine the river boundary has a high potential in analyzing geomorphology and identifying changes and the river is moving. The study of river bed changes using remote sensing analysis revealed that the basin of the Bashar River during the statistical period (2018-1990) was about 38% unchanged and 62% affected by various factors.