بررسی اثرات احداث سد سهند بر شرایط هیدرولوژیکی رودخانه و تحلیل تغییرات شکل هندسی مجرای قرانقو‌چای (از بازه پایین‌دست سد سهند تا روستای خراسانک)

رودخانه و فرایندهای رودخانه ‌ای در سطح زمین، مهم ترین سیستم های ژئومورفیک فعال است که در اثر کنترل ‌کننده‌ های محیطی و دستکاری ‌های انسانی در سیستم‌ های طبیعی، طی مقاطع زمانی و مکانی مختلف، تغییرات متفاوتی را از خود نشان می ‌دهند. این تغییرات و جابه‌ جایی ‌ها مشکلاتی را برای کاربری ‌های انسانی و اکولوژیکی ایجاد می ‌کند و از مباحث مهم در ژئومورفولوژی رودخانه‌ ای محسوب می ‌شود. هدف پژوهش حاضر، بررسی اثرات احداث سد سهند بر شرایط هیدرولوژیکی رودخانه قرانقو چای از بازه پایین دست سد سهند تا روستای خراسانک و تحلیل تغییر شکل هندسی مجرا، در بازه زمانی 1374 تا 1397 است. این پژوهش متکی بر کار های میدانی، استفاده از تصاویر ماهواره‌ای، داده ‌های هیدرومتری، نقشه ‌های توپوگرافی و زمین ‌شناسی است. شرایط هیدرولوژیکی رودخانه با استفاده از 26 شاخص هیدرولوژیکی، در دو دوره قبل از احداث سد و بعد از احداث آن محاسبه شد. سپس برای بررسی رابطه معناداری شاخص‌ ها، از آزمون آماری t جفتی استفاده شد. به منظور بررسی تغییرات شکل مجرا نیز از پارامتر های بررسی شکل هندسی رودخانه شامل ضریب خمیدگی و زاویه مرکزی استفاده و میزان مهاجرت مجرا با استفاده از روش ترانسکت محاسبه شد. با توجه به نتایج مطالعه، از لحاظ شاخص ‌های هیدرولوژیکی q5، q10، q15، دبی حداقل، دبی حداکثر و دبی مد، بین دو دوره قبل و بعد از احداث سد تفاوت معناداری دیده می ‌شود. میانگین ضریب خمیدگی در سال 1374، 15/1 بود و در سال 1397 به مقدار 18/1 افزایش یافت و الگوی رودخانه از پیچان ‌رودی بسیار توسعه‌ یافته به توسعه یافته تبدیل شد. میانگین آهنگ مهاجرت مجرای رودخانه در طی بازه زمانی 24 ساله مورد مطالعه، در حدود 143/0 متر در سال بود. بیشترین مقدار جابه ‌جایی عرضی مجرا، در ترانسکت یک به میزان 41/0 متر در سال و کمترین جایه ‌جایی در ترانسکت یازده، به مقدار 022/0 متر در سال است. در نتیجه، مشخص شد که رودخانه قرانقو چای در بازه مطالعاتی، تغییرات مورفولوژیکی و جابه جایی مجرا داشته است و عوامل موثر این تغییرات به طور عمده شامل تغییرات دبی به ویژه در زمینه احداث سد، فرسایش پذیر بودن سازند های بستر و کنار رودخانه و عوامل مختلف انسانی است.

Investigation of the effects of construction of Sahand dam on the hydrological conditions of the river and analysis of changes in the geometric shape of Qaranqoochay canal (from the lower reaches of Sahand dam to Khorasanak village)

Extended abstract1 IntroductionRiver and river processes are considered the most significant geomorphic systems active on the earth rsquo;s surface. Over time, many changes in the morphology and dynamics of the river system can occur. The effects of river adjustment caused by the natural factors require a much longer period to reveal. However, there are few exceptions that natural factors such as river floods, landslides, or earthquakes can induce channel adjustments in a concise time. In contrast, human activities can significantly impact natural processes and trends, resulting in a compressed time scale for river adjustments. Consequently, the flow regime of many rivers has been dramatically altered from their natural flow regime.Redirection, deformation of the canal, destruction of the banks and lands along the river are observed in Qaranqoochay river. Therefore, the purpose of this study is to investigate the effects of Sahand Dam construction on the hydrological conditions of the Qaranqoochay River, determine the geometric components of the river, and investigate the displacement and shape changes of the Qaranqoochay River Canal.2 MethodologyIn this research, the topography map with a scale of 1:50000, geology map with a scale of 1:100000, google earth and Landsat images, hydrological data from and field data are used. ENVI 5.3, Arc GIS 10.5, Excel, and SPSS software were also used for image processing and data analysis. To study the hydrological conditions of the study area, the data of the Chapini station were studied using 26 hydrological indices. Then, a paired Ttest was used for statistical analysis of indices to study the significance of indices changes and, consequently, the effect of dam construction. The geomorphological parameters of the river and their variations, including bending coefficient and central angle, were measured. The curvature coefficient is one of the few criteria used in river shape segmentation using s = 1/ (y.2), i.e., dividing the valley length by wavelength for each arc (Pittchr('39')s coefficient). It is calculated. The central angle of the arcs on each of the intervals was calculated using the relation A = 180L / R pi;, where A is the central angle, R, of the fitted circle radius (Kornias coefficient). The lateral changes of the canal were investigated using the transect method and calculation of river migration rate. According to the Transect method, lines with distinct distances from both sides of the duct are depicted as baselines. These lines are constant for the periods studied and can be calculated quantitatively for duct movements relative to these lines.3 Results According to the results of calculating hydrological indices and Ttest analysis, in terms of hydrology Q5, Q10, Q15, minimum flow, maximum flow, and Mode flow, between the two periods before and after the construction of the dam, a significant difference was seen. It can be stated that the construction of Sahand Dam and the changes in the flow by the dam can affect the amount, timing and it affects the duration of upstream and downstream currents. Also, it can provide suitable bedding for intensifying hydrological changes and, subsequently, river morphological changes. Downstream changes include sedimentation and erosion in parts of the river, displacement of meanders, flooring, and wall demolition. The average curvature coefficient had increased from 1.15 in 1995 to 1.18 in 2018, and in 1995, 21.43 of the river patterns were straight (curvature coefficient 11/05), but in 2018, there is no direct pattern in the path. The plan of the duct in both periods is sinusoidal (curvature coefficient 1/51.05). According to the values ​​of the central angle, in 1995, the studied route from Qaranqoo river was Highly developed as a meandering river (central angle 296158), and in 2018 it was extended to the meandering river (central angle 15885), it arrives. With the decrease of the average central angle in 2018, the average radius of the circles has also decreased. The average wavelength and valley length in 2018 compared to 1995, and the decrease in wavelength indicates a decrease in the distance of successive turns.The average rate of river canal migration during the 24 years studied was about 0.143 m per year. The ductchr('39')s maximum amount of transverse displacement in transect one is 0.41 m per year, and the lowest amount of displacement in transect eleven is 0.022 m per year. 4 Discussion Conclusions In general, the changes in the plan of the Qaranqoo river in the studied time and place period have been in the form of expansion of existing meanders, shifting of the river course, and formation of small meanders. Formation and morphological changes of the Qaranqoo river in the studied time and place period are mostly influenced by hydrological processes caused by dam construction (significant reduction of runoff in the river and accumulation of sediment load in the bed path) and lithological conditions of the bed and side. Of the river. Also, human beings have changed the pattern of the Qaranqoo canal in the studied area by harvesting river materials, encroaching on the river area in the form of landuse change, farming in river lands, and pumping water for agricultural lands along the rivers, construction of bridges.