مئاندرها ازچشماندازهای پویا و ازاشکال مهم تأثیرگذار در ویژگیهای دشتهای سیلابی محسوب میشوند که به دلایل مختلف تشکیل وتحت شرایطی به صورتهای متنوع جابجا میگردند. با هر جابجایی، تغییراتی در پهنای دشتهای سیلابی و میزان رسوبات وارده به رودخانهها صورت میگیرد. مئاندرهای تشکیل شده در دشتهای سیلابی قرهآغاج (واقع در دامنههای شرقی سهند با مختصات جغرافیایی ،از '00 °37 تا '20 °37 عرض شمالی و از'45° 46 تا ' 15° 47 طول شرقی ) از نوع بینظم بوده و بسیارپویا هستند. در این پژوهش از تصاویر ماهوارهای و نقشههای توپوگرافی برای ترسیم مسیرهای جابجایی پیچانها استفاده شده و محل جابجاییها با مشاهدات میدانی درسطح زمین تثبیت شده است.
نتایج حاصل از بررسی این پدیدهها که درمحدودهی مورد مطالعه با استفاده ازاندازهگیریهای مسیرهای جابجایی قدیمی و نسبتهای R/W صورت گرفته و همچنین زمان لازم برای جابجایی با استفاده از شاخص ویژه (TΔ) محاسبه شده است، نشان میدهد که میزان جابجاییها در طول بخشهای مختلف مسیر بسیار متفاوت است (با تفاوت زمانی 13 سال). همچنین نتایج این بررسیها حاکی از این است که زمان لازم برای جابجاییها در بخشهایی از مسیرکوتاه و در بخشهای دیگر زیاد است. بررسیها نشان میدهد که در محدودهی مورد مطالعه، خطر فرسایش کناری در بخشهایی که میزان جابجاییها زیاد بوده، بسیار بالا است.
Investigation and Analysis of the Type and Time Length of Displacements at the Route of Meandering Rivers and Their Role on Lateral Erosion in Semi –Arid Areas
Case study: Garaagaj River
Dr. Maryam Bayati Khatibi
Associate Professor of Geomorphology
University of Tabriz
Meanders are of dynamic landscapes and important shapes affecting on the characteristics of flood plains, which are formed for different reasons and displaced under some conditions in various shapes. These geomorphologic and hydrological phenomenon are the most important factors of changing the flood plains and also are the main reason of increasing the entering sediments in to the rivers. By knowing this fact that the type of configuration at the rivers’ flow path especially the existence of meanders are not accidental at most of the times, therefore through the study of the configuration of the rivers’ path and recognition of the factors effective on changing their flow path, it is possible to recognize the dominant dynamism on the river’s flow in the time period of the study and also the manner and trend of changes’ occurrence in the current situation in the flood plains and manner of dominancy of possible conditions in the future . by the study of these phenomenon and referring to the existing heritages, which indicates the displacement of the rivers’ flow paths in the past, it is possible to comment on the manner of pervious changes and also about the features of the processes and the dominant dynamism on the rivers from the view point of erosion or sedimentation and settlement of deposits. Garaagaj is considered as the most meander rivers of Garangoo ( having geographical coordinates of 37 °00 ' to 37 20' of north latitude and from 46° 45 ' to 47 ° 15 ' of east longitude) . This river with having different meanders in its path and the changes has made in the path, has increased the sedimentation load of the river. regarded that a lot of dams and weirs have been established at the path of this river, the creation of meanders and consequently increase of the lateral erosion during the time, in addition to the entrance of lateral soils of large flood beds in to the river, the increase of resulting sediments has changed to a great and severe problem for the established dams at the route of these rivers.
Areal photos have been used for the review and analysis of displacement rate of active channels in Garaagaj flood plain ,and the new and old routes have been fixed and traced from areal photos , the rate of displacements have been measured from the remaining lines and signs and the required calculations have been made by the resulting data. Sinuosity rate of Garaagaj river has been calculated by using (S=T/L) S relation. In this paper, for the review of displacements made in meander route and along Garaagaj River, firstly the radius of meanders’ arch and then the bed width has been calculated and then by the use of the resulting data of these measurements, the ratio of these two parameters (R/W) has been obtained. For estimating the required time for displacements in active channels, the relation of (Δt=Wcb/V) has been used.
Investigation and Analysis of the Type and Time Length of Displacements...
Discussion and Results
The produced meanders at the path of Garaagaj River have various configuration and magnitude along different parts of the river. Review of the river meanders with S index shows that out of 25 parts of the under study, 8 parts are of sinuosity type and 17 parts are of meander type. Sinuosity rate of Garaagaj is more at the river outlet and at the end of the path, but sinuosity change is relatively low at the path length, in fact it can be said that more than 70% of the path along the under study parts are of meander type path and considering that each arch at meander path is considered as the exciter and displacement motor for the next arch and finally for displacement of the flow path, therefore increase of meander type parts of the path indicates the high potential of the flow path for more and faster displacement. The studies show that there exists a relationship between meander radius (R) and width (W), type and size of sedimentation load and also lateral erosion. When the rate of R/W reaches to maximum, flow turbulence also increases which leads to the increase of erosion power of the flow at meanders’ part. Review and study of the rates obtained from estimating the ratio of canal radius (R/W) with displacement rate of flow path in Garaagaj River and canal width specifies maximum amount of displacements happened in the past at the river path. Displacement of the path is together with creating cesspits and bank sides and as a whole erosion of the bed, so such phenomenon can be seen in the path of the said river. Studying the type of displacements in the path of different parts of Garaagaj river shows that different types of displacements have been happened at the river path .study of Garaagaj river bed shows that the required time for canal displacement at Garaagaj path is various in different parts. The lowest time for displacement of channel is 4 years and maximum time required for displacements is more than 17 years. This river at the areas near to confluence place of Garangoo changes its path rapidly and in a short time , but in middle parts, this time decreases and again at the ending part of path, the time of displacement will increase. Type of soil at flow bed and flood bed has a great effect on the severity and weakness of lateral erosion. When the soil shows sensitivity to the lateral erosion, the amount of material left in to the water will increase at the same rate and the width of flood bed will increase. Studying the soil type of surface formations of Garaagaj path shows that the type of formations at the under study area is sensitive to lateral erosion.
Meanders are the main causes of lateral erosion and are considered as the changing factor of flood plains. In semi arid areas that the flood plain banks do not protected by vegetation cover, the occurrence of such changes is great and lateral erosion has been intensified. On eastern slope of Sahand Mountain, as a semi arid area, the most typical meanders have been formed on the river path. One of these meander paths is Garaagaj Chai, which by creating various and different arches along its path has great displacements which has intensified the lateral erosion. With respect to the type of surface formations at the flow path of this river, frequent displacements at the flow arch and meeting the meander part of the flow with uncovered sides made the flood plain to become wider and middle bank sides at the flow path to be displaced rapidly.
The results suggested that R/W on Graagaj River is not same in all parts of path, in some parts is high and in other parts is low .This range shows the change of type, features and displacement rate throughout river length. With respect to the results of performed studies, these changes can be related to the type of flow paths and the impact of tectonic factor. In the under study area, in parts meander and Sinuosity can be seen, except active tectonic, no other reason can be find for changing configuration of Garaagaj Chai river. In performing development operations in meander paths of semi arid areas , which lateral erosions resulting from displacements can produce major ad great problems, review and estimation of the required time for displacements has a great importance.
The obtained results of the studies at Garaagaj Chai river flow path shows that the required time for displacement of channel at Garaagaj Chai river is various in different parts. Minimum time required for displacements is 4 years and maximum time is more than 17 years. It can be said that 4 years is a short time for displacements and the river at this time can make major changes in its middle and lateral part.
Keywords:Meanders, movement of channel ,displacement time ,lateral erosion ,semi-arid ,Garaagaj,
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کلیدواژه ها [English]
Meanders, movement of channel, displacement time, lateral erosion, semi-arid, Garaagaj