اثرات احداث سد مخزني گيلان‎غرب بر مورفولوژي بستر رود گيلان‎غرب (در محدوده بالادست و مخزن سد)

The Effects of Constructing Gilan-e- Ggharb Reservoir Dam on Bed Morphology of Gilan-e-Gharb River (in Upstream of the Dam and Reservoir Area)

در اين پژوهش به بررسي اثرات احداث سد مخزني گيلان‎غرب بر ژيومورفولوژي جرياني رود گيلان‎غرب در بخش بالادست و محدوده مخزن آن پرداخته شده است. اهداف اين پژوهش، بررسي تغييرات ژيومورفولوژيكي ناشي از احداث اين سد در بستر، كناره‌ها و مقاطع رود گيلان‎غرب؛ تجزيه‎وتحليل تغييرات مورفولوژيكي قبل و بعد از ساخت سد و نمايش نوع و جهت تغييرات مورفولوژيكي، به‎دليل آبگيري سد در بالادست و محدوده مخزن است. داده‌هاي اسنادي، داده‌هاي هيدرو ـ اقليمي، نقشه‌هاي موضوعي و داده‌هاي حاصل از پيمايش ميداني، داده‌هاي مورد استفاده در اين پژوهش هستند. روش پژوهش، پايش تغييرات است كه با استفاده از تصاوير ماهواره‌اي و عكس‌هاي هوايي و ترسيم و تفسير نيمرخ‌هاي توپوگرافي در سه دوره زماني قبل، حين و بعد از ساخت سد و در بخش‌هاي بالادست و محدوده مخزن سد انجام شده است. نتايج نشان مي‌دهد كه شيب بستر رود در بخش بالادست و محدوده مخزن، پس از ساخت سد كاهش يافته است. محاسبه ضريب پيچشي رود حاكي از افزايش سينوزيته مجرا در بالادست سد است كه تغيير تدريجي الگوي رود در اين بخش به‎سمت پيچانرودي شديد را نشان مي‎دهد. همچنين احداث سد، سطح اساس جديدي در منطقه ايجاد كرده كه موجب گسترش فرسايش شياري و خندقي در دامنه‌هاي مشرف به درياچه سد و نيز، بروز فرسايش قهقرايي در مجاري فرعي شده است. تابع تغييرات سطح آب و فعاليت‌هاي نيوتكتونيكي گسل راندگي گيلان‎غرب، تراس‌هاي آبرفتي جديدي در حال تشكيل هستند. علاوه‎بر اين، نيمرخ‌هاي طولي و عرضي رود در دوره بعد از ساخت نسبت به دوره قبل از آن تغيير يافته و نيمرخ عرضي در بالادست از شكل وي (V) به شكل يو (U) تغيير يافته كه دال بر كاهش فرسايش در بستر و افرايش فرسايش كناره‌هاست.

Introduction Rivers are open systems which establish a new equilibrium over time by transfer basin sediments to the shoal and generally considered to be systems in equilibrium, where the system responds to input changes by negative feedback, or quasi equilibrium by adding a time lag between the change in the input processvariable and the internal morphological adjustment of the system. To optimize use of rivers for water supply needed for different purposes, dams are constructed on rivers which will create significant changes in the fluvial geomorphology. In this research we considered and investigated the effects of construction of Gilan-e- Gharb reservoir dam on fluvial morphology of the Gilan- e- Gharb River. Investigation of geomorphologic changes caused by constructingGilan-e-Gharb reservoir dam on bed and banks and cross-sections of Gilan-e-Gharb River, analyzing morphological changes before and after dam construction, and demonstration of types and directions of morphological changes due to dam impounding are objectives of this research. Methodology In this research we used documentary data, gathering hydro-climatic data, thematic maps and data gatheredthrough field surveys. We used methods of change detection using remote sensing data including satellite imagery (ETM+obtained during construction of the dam and IRS P5 obtained after its construction) and aerial photography(obtained before construction of the dam) in the Erdas Imaging and Leica Photogrammetry Suite softwares and drawing maps and cross-sections in the Arc GIS software environment.Thenthe obtained data was interpretedusing topographic maps and drawing width and longitudinal profiles at three time periods; before, during and after construction of the dam and two intervals betweenupstream of the dam and reservoir area. Results and Discussion In the dammedriver not only the discharge ofwater isaffected by the dam construction, but also the sediment transportis changed. In upstream of dam the rate and capacity of sediment load increases, especially in the first years of dam construction. Dams cause a break in the natural discharge regime of rivers. This occurs in the reservoir area geomorphology, up-dam geomorphology and down-dam geomorphology. In the up-dam section, the more conspicuous change occurs at the upper section of the reservoir, where themost massivesedimentation occurs. Any tributaries to the main river with a short distance from the reservoir’s upper entrance tend to develop delta systems in response to the break in water velocity and sediment deposition. Thus, the dam traps almost all of the bed and suspense loads and forms the delta in the reservoir of dam, which lessens the capacity of water storage. The slope of the river bed in upstream and within the dam reservoir area declines. Calculation of sinuosity coefficient by Petts’ method indicates that an increase in river sinuosity is created in upstream of the dam so that the range of sinuosity in upstream of the dam increases from 1.28 to 1.33 after dam construction, which shows that the river pattern in the upper rangechanges to hard meandering. This increase is a result of the sedimentation in the lake of dam and river bed in the upstream of dam caused by creating a new water table in the area. Conclusion The study results show that dam construction causesa new water table in the area and changes the slope of river bed, then the tributaries response to changes in the main stream channel and develop their beds during the head ward erosion. Also changes in the water table during the year causes growth of rill erosion and creates gully erosion on slopes nearlake ofdams and causeshead ward erosion in the tributaries which will cause changes in water level during the year. According to water level changes and neo-tectonic activities of Gilan -e- Gharb thrust fault, new alluvial terraces on the left bank of the river are being formed. In addition, the mainstream and width cross-sections of river after dam construction changescompared to those before construction of the dam.Also, theU shapedwidth profile of river in upstream dam changes to V shaped profile after dam construction, which implicates reducing of erosion in theriver bed and increasing of erosion at the banks of channel.