Document Type : پژوهشی
Authors
1 Yazd University, Department of Geography
2 M.A. of Geomorphology, Yazd University
Abstract
Introduction
Rivers' channels tend to change due to different factors such as lithology, discharges, floods, sedimentation, and humans. Of all factors, understanding the morphological features of a river and its major controlling factors is importance in its control and regulation. The morphological features and geometric characteristics such as change in the length and width of the river, wavelength, arc length, arc angle, and sinuosity are the most important factors that should be considered to study and mange a river. Many investigations have been done to study the changes in the morphology of the rivers and the geometrical parameters. For example, Leopold and Wolman (1957), based on structural viewpoint, divided rivers to three groups including braiding, meandering, and relatively straight channels. Singh (2014) studied morphological changes of the Ganga River during 10 years using GIS. Iranian researchers (Ahmadian,2001; Ghafari et al., 2004; Hafezi Moghadas et al., 2012 ; Maghsoudi et al., 2010; Shahbazi et al., 2009) have also investigated the morphological and geometrical parameters of different rivers. In the current research, the changes in the geometrical parameters of the Qaresou River between 1959 and 2015 were evaluated. This river is located between latitude of 34° 30’ N to 34° 54’ and Longtitude of 46° 22’ E to 47° 22’ within Kermanshah’ Province in the west of Iran. It is one of the sub catchments of the Karkhe basin. It has a land extended about 5278 km2, with a maximum of 3360 m and a minimum of 1270 m height. The average precipitation of the basin fluctuates between 300 to 800 mm in a year. Three rivers which flow into the Qaresou includes Mereg, Qaresou, and Razavar rivers.
Methodology
In this research, to study the morphology of the Qaresou River and its geometric parameters and changes, the aerial photos (1955) and satellite images IRS (2015) were used. In fact, the photos were the best tools to compare the rate of changes in the morphology of the channel for about 60 years. First, the photos and images were scanned and georeferenced. Then, the river was digitized with high exactness. Next, using AutoCAD Software, the geometric parameters such as wavelength, arc length, arc angle, amplitude, sinuosity, central angle, and meandering were evaluated. Finally, through comparing geometric parameters between two periods, the rate of changes for about 60 years was calculated.
Results and Discussion
Evaluation of the geometric parameters of the Qaresou River was based on using drawing circles tangent with meanders of the river for both periods. The findings showed that the pattern of the river and some geomorphological and geometrical characteristics were changed during the time. For example, the number of meanders had reduced from 535 in 1959 to 379 in 2014. Also, the central angle, as a criteria to divide and determine the development of the meander in a river, was evaluated. This criteria showed a reduction in the average of the central angles in all segments of the river except its first segment with an increase in its width because of higher erosion. In addition, during this period, 60 years, the river has tended to a straight pattern. The changes in the radius of the meanders also reduced in all segments except the first one. However, at first, the pattern of the river changed to develop in the meanders. Besides, the curvature coefficient of the river reduced in all segments except the third one. Finally, there was an increase in the wavelength and the length of the channel between the years 1959 and 2015.
Conclusion
The result showed that although the frequencies of the arc in all segments reduced during the statistical periods, the geometric parameters fluctuated. For example, in the first segment, the wavelength increased 96 m and the length of the channel increased 31 m. Also, the curvature coefficient reached from 1/83 to 2/38. The radius of the meander increased about 38%. The central angle increased 40%. The increase in the geometric parameters showed grubbing in the river bed and increasing in the curvature of the river. In the fifth segment, in contrast, all geometric parameters declined. For example, the wavelength and the length of the channel respectively declined 31 and 15 m. In addition, the curvature coefficient was also decreased 3%. There was also a decline in the flow water of the channel. Therefore, the river follows a natural process of digging in the upstream and sedimentation in the downstream. Near Kermanshah city, the slope of the channel tends to the least and the river starts to leave all sediments, which leads to displacement in the channel.
Highlights
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Keywords
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