عنوان مقاله [English]
The river route has different patterns, such as straight, meandering and braided, and these changes have always been of interest to geomorphologists. A river along its path can have different patterns; the Meandering rivers are part of a single-channel river, the river has only one flow. One of the important issues in geomorphology is to consider changes in the shape and pattern of flow channels in the course of the river. The morphology of the rivers depends on several factors such as water flow changes, bed slopes, river cross section, geology, tectonics or morphotectonics of the area, as well as the time needed for formation and change of river plan forms. The spatial displacements of the meanders cause changes in the dimensions of the bedding stream and the flood plain. Due to their many dynamics, this phenomenon also causes major problems. In many parts of the world, numerous studies have been done on the phenomenon of rivers 'meandering, and more than 30 theories have been presented on the process of rivers' meandering. [Maghsoudi et al., 2010, 276]. The meanders can be sinusoid, regular or compact, very irregular, crumpled, and even other forms. The shape of the meander is a function of the characteristics such as slope and flow rate, river width, and the material of the bed [Rezaei Moghadam et al., 2012, p. 86].
The Mahneshan County with a total area of 2,786 square kilometers is located at 47 degrees and 15 minutes to 48 degrees and 10 minutes of Greenwich and 36 degrees and 20 minutes to 37 degrees and 10 minutes north latitude, northwest of the province Zanjan. The entire length of the Ghezel Ozan route is from the Chehel Cheshme mountains of Kurdistan to the Manjil Dam Lake, 550 km, about 91 km lies within the Mahanshan county. The Mahanshan area comprises mountain ranges stretches the northwest-southeast that lies between them, plains and valleys.
In this study, topographic maps or scale of 1.50000 and geology of 1.100000 were investigated for the study of drainage network and geological formations of the region. A geological map was used to investigate the relationship between meanders with formations and geological structures. Using the GPS during fieldworks, probable meanders were positioned. IRIS satellite images with a resolution of 2.5 m in 2007, 1389, 1394 and Google Earth in 1373 were used to study and investigate the process of changing the direction of the Ghezel Ozan. By digitizing the route, the amount and position of the river changes were detected. Based on data on length of arc, valley length and radius of circles on the river arch, bending coefficient, central angle and sinusoid index, were calculated. Then, according to the calculation of the bending coefficient of each arch and the central angle of the arches, and the Korins classification, the percent of frequency of the central angle of the meander arches were determined.
Results and discussion
- River bending coefficient
According to calculations of the bending coefficient and the central angle of the arches of the Ghezel Ozan River in the Mahneshan County, the dominant river pattern over the course of four years is meandering and their coefficient is between 1.25 and 2. While the sinuosity of 55% of the meanders was more than 1.5, the bending coefficient of the arches 1 to 10 is high, indicating that they are in the category of highly developed meanders and the rest are underdeveloped to advanced meanders.
Using the Korins central angle index), the development rate of meandering was calculated for four periods. Calculated values shows that in 1996, about 60% of the course of the river had a developed meandering pattern. The central angles of the arch were in 1373, 136, which illustrates the highly developed meandering pattern. This amount was reduced to 131, 96.5, and 106.5 in 1386, 1389, 1394, respectively, reflecting a developed meandering pattern. The decline in the highly developed meandering pattern and the increase in the developed and undeveloped meandering pattern show that the numbers of arch rivets have gradually decreased.
Arc radius, wavelength and valley length
The mean radius of arches in the studied boundary was calculated for four years 1994, 2007, 2010, 2015, respectively, 506, 254.5, 166, 229 m. It is seen that the mean radius of the circles of 1994 is higher than in other years, which is often due to erodibility of the river banks, which causes the radial development of meandering rings. The geometric parameters of the studied sections were analyzed in four periods of 1994, 2007, 2010, and 2015. The distance between the two alternating meanders in 1994 is much higher than in 2007, 2010, and 2015. The increase in the longitudinal distance of two successive meanders is somehow indicative of an increase in the meandering conditions.
Longitudinal profile of the river
After plotting the longitudinal profile of the Ghezel Ozan in the Mahneshan County, the mean slope of the river was 0.07 percent. The longitudinal profile of Ghezel Ozen shows that the river at first has a slight slope and the meanders are formed. Down the river, increasing the number of effective faults, increases the slope of the bed at the end of the stream
Meanders are the main causes of erosion and changes in flood plains, and if these changes are high and severe, it leads to lateral erosion. Findings of the research indicate that in four periods, there is a large variation in the course of the river; by examining the parameters and field observations, it can be said that the Gezel Ozan River is a developed meander. The bending coefficient of years 1994, 2007, 2010, and 1394 was calculated between 2-1.25. Also, with respect to the values obtained, the center angle is in the range of 158-85 degrees, is related to the developed meanders. In this study, there was no evidence that tectonics affected the river route directly and, and thus led to a redirection of the river. Mahnashan County is active tectonically. The right part (look in the direction of the Gezel Ozan stream) is more uplifting. Consequently, the behavior of the small streams affects through the formation of alluvial fans and rapids in the Gezel Ozan River.