بررسي تغييرات بستر و ويژگي‎هاي ژيومورفولوژيكي رودخانه‎ي گيلان‎غرب در سال‎هاي 1344 تا 1381

Investigation of Bed Changes and Geomorphologic Characteristics of Gilanegharb River in 1965-2002

سيستم‎هاي رودخانه‎اي به‎شدّت به تغييرات زمين‎ساخت حسّاس هستند و در واكنش به حركات زمين ساختي، دچار تغييراتي در ويژگي‎هاي ژيومورفولوژيكي بستر و تغييرات بستر خود مي‎شوند. اين تغييرات موجب پيدايش آثار مخرّبي روي زمين‎هاي كشاورزي، نواحي مسكوني و سازه‎هاي اطراف رودخانه مي‎شود. در اين پژوهش رودخانه‎ي گيلان‎غرب به طول 20 كيلومتر از لحاظ آثار نوزمين ساخت مورد بررسي قرار مي‎گيرد. با استفاده از عكس‎هايي هوايي سال 1344، تصاوير ماهواره‎اي TM سال 1365 و تصاوير IRS سال 1381 مسير رودخانه، در سه دوره‎ي زماني در محيط نرم افزارArcGIS رقومي شده و تغييرات بستر، تعداد مياندرها و تغييرات طول رودخانه در اين سه دوره محاسبه شده است. در ادامه براي اثبات تاثير فعّاليّت‎هاي نوزمين ساختي، شاخص‎هاي ژيومورفيك SL و S محاسبه شد و نيمرخ طولي رودخانه مورد تحليل قرار گرفت. درنهايت با بازديدهاي ميداني از منطقه، شواهد ژيومورفولوژيكي آثار نوزمين ساخت بر رودخانه‎ي گيلان‎غرب شناسايي شد. نتايج پژوهش حاكي از آن است كه طيّ دوره‎ي زماني 37 ساله، نوزمين ساخت عامل اصلي تغييرات رودخانه‎ي گيلان‎غرب بوده است. به‎دليل شدّت فعّاليّت‎هاي نوزمين ساختي، ميزان تغييرات رخ داده در رودخانه طيّ دوره‎ي زماني 1365 تا 1381 شديدتر از دوره‎ي زماني 1344 تا 1365 بوده است و اين امر توسّط جابه‎جاي بيشتر بستر، تغييرات تعداد مياندرها، طول رودخانه و ميزان حدّاكثر جابه‎جايي رخ داده در اين دوره و همچنين مقادير كمّي شاخص‎هاي ژيومورفيك و شواهد ژيومورفولوژيك و تفسير نيمرخ طولي رودخانه تاييد مي‎شود. الگوي رودخانه در قسمت‎هاي كم‎شيب‎تر از نوع مياندري و در مناطق پرشيب‎تر از نوع الگوي شرياني است.

Introduction One of the most important of river’s geomorphologic studies is Investigation of bed changes and geomorphologic characteristics of them. River systems are highly sensitive to land construction changes and in order to response to tectonic movements, some changes happen in bed and also in geomorphologic characteristics of bed. The Gilan-gharb River is located in northwest of Zagros in south of Kermanshah province. The geomorphologic documents of the it’s basin involve continuing neotectonic activities in pleu-quaterners period. Changes in the bed of Gilan-gharb River result in adverse impacts on agricultural lands, residential areas and structures around Gilan-gharb River. Goal of this research is evaluating of bed changes and geomorpholic characteristics in Gilan-gharb River. Methodology In this article, a case study range which is 20 kilometers of Gilanegharb River has been studied and neo construct impacts on the river have been examined. The images of the river in three times period have been digitized by using aerial photos of 1946, TM satellite images of 1986 and IRS images of the 2003. Then changes in bed, meander number and river length have been calculated in Arc GIS. Next for proving the impacts of neotectonic activites, S and SL geomorphologic indicators have been calculated and river longitudinal profile has been analyzed. For the survey impact of neo construct on river’s trust DEM 10 meter has used and 3 profiles in first, middle and end of the region provided. Finally geomorphologic evidence for neo tectonic imapcts on Gilan-gharb River has been identified by field visits. Results and Discussion Because of the decline in the neotectonic construct activities, the length and number of meanders of Gilan-gharb River have decreased in period 1946-1986. The slope if river’s bed has increased because of increase in neotectonic construction activities that cause increase in the sinusitis of the river and increase in the number of meanders in period 1946-2002. Maximum of movement had been about 27 meters in period 1946-1986, in the period 1986-2002 about 45 and in the period 1946-2002 about 73 meters. The amount of S and SL shows that the rivers of this region don’t have equilibrium still and neotectonic construction activities are strong. The geomorphologic indicators like river young becoming, vertical walls rising, broad meandering, multiple trusts, movement and rift of meanders show the impact of neo construction activities on Gilanegharb River. Conclusion The results of the research indicate that neotectonic construction is the main factor of Gilanegharb river changes during 37 years. Due to intensity of the neo tectonic activites, the level of the changes have been more sever during the years of 1986-2002 in comparison with the years of 1965-1986 and this is because of the bed displacement, changes in meander numbers, river length and maximum occurred displacement in this time period and it is approved by quantitative amount of geomorphologic indicators, geomorphologic evidence and river longitudinal profile interpretation. River pattern is meanders in low-gradient parts and it has arterial pattern in steep areas. So the neotectonic construction is active in this region and that is the main factor of changes in the rivers.