مورفومتري و مقايسه تغييرات عرضي رودخانه ارس طي سه دهه اخير مطالعه موردي : پايين‌دست سد ميل مغان

Morphometry and Comparison of lateral changes in the Aras river channel over late three decades (Case Study: Downstream of the Milmoghan dam)

تغييرات و جابجايي‌هاي كلي و حتي جزيي الگوي رودخانه از عمده‌ترين مباحث مربوط به ژيومورفولوژي رودخانه‌اي به شمار مي رود. به همين علت تحقيقات و مطالعات زيادي توسط ژيومورفولوژيست ها در اين زمينه انجام‌گرفته است. با بررسي الگوي يك رودخانه مي‌توان شرايط كنوني و پتانسيل تغييرات احتمالي آن را در آينده بهتر درك كرد و همچنين مي‌توان پاسخ رودخانه را نسبت به تغييرات طبيعي و انساني پيش‌بيني كرد. رودخانه ارس يكي از مهم‌ترين رودخانه‌هاي مرزي ايران در حوضه آبريز درياي خزر مي‌باشد كه بنا به دلايل مختلف زمين‌هاي ساحلي خود را به‌واسطه تغييرات جانبي موردتهاجم قرار داده و نهايتاً موجب تخريب اراضي زراعي و مسكوني و جاده ساحلي شده است كه خصوصاً در مناطق دشتي حمله رودخانه به سواحل تشديد يافته و باعث تغييرات عرضي شديد شده است. وجود تاسيسات زيربنايي مهم كشاورزي، پروژه‌هاي عمراني و صنعتي در حاشيه رودخانه ارس موجب شده كه تغيير مسير اين رودخانه از‌ اهميت بالايي برخوردار باشد. در اين مطالعه به‌منظور بررسي تغييرات زماني و مكاني بستر رودخانه ارس از تصاوير ماهواره‌اي Landsatسنجنده OLIو TMدر بازه ي زماني 1987 الي 2013 و همچنين از تصاوير ماهواره IRS سنجندهP5 هم براي تفسير دقيق تر عوارض كناره هاي رودخانه استفاده گرديد. ضرايب هندسي رودخانه براي دو دوره زماني فوق مورد تجزيه‌وتحليل قرار گرفت. نتايج حاصله تغييرات و جابجايي عرضي زيادي را طي 26 سال اخير (در بعضي نقاط 7/1 كيلومتر) نشان داد. بيش‌ترين ميزان اين تغييرات در قسمت پايين‌دست بازه انتخابي صورت گرفته است. مطالعه عوامل موثر در تغييرات عرضي رودخانه ارس نيز نشان داد كه پايين بودن شيب رودخانه و همچنين جريان رودخانه بر روي بستر آبرفتي جوان فرسايش پذير از مهم‌ترين عوامل تغييرات عرضي اين رودخانه بوده است و ساير عوامل تاثيرگذار در مرتبه بعدي قرارگرفته‌اند.

Extended abstract Introduction Changes and general movement and even minor change of river pattern are the main topics related to fluvial geomorphology. For this reason geomorphologists have been many studies in this field. It may be possible to better understand current situation and potential changes in future with studying of the river pattern. Aras river is one of the most important border rivers of Iran is located in Caspian Sea basin. The study area with geographic coordinates between 39 17 59 to 39 51 41 N latitude and 47 29 11 to 47 53 58 E longitude is located in Ardabil province. The river for various reasons has been invaded coastal areas due to lateral changes. There are important agricultural and industrial infrastructure in sides of Aras river caused to lateral migration of the river be most importance. Methodology Bed changes of Aras river were analyzed in two time periods (1987 and 2013). To extract bed changes, were used satellite images, which Landsat 5 TM for the year 1987 and Landsat 8 OLI for the year 2013. Then Landsat satellite images imported to ENVI software and then the correlation coefficient between all bands of images was calculated. Then using OIF factor three bands with minimal dependence were selected and false color image of the region was produced. In order to better detections river, high-pass sober filter was used. In the next step for contrast enhance of image was used method of histogram equalization in ENVI software. Then images imported to Arc GIS software and waterpath river was digitized from satellite images. With overlaying plans extracted from Satellite images rate of river bed movement over late 26 years determined. Then the plans into Auto CAD software and the geometric coefficients of river plans determined. Discussion The investigation of longitudinal river profile of Aras river shows that river from height approximately 156 meter to 14 meter is flowing and The slope of the downstream river decreased.To better and more accurate evaluating 70 kilometers water path of Aras river divided into three reaches. That Aras river over 26 years In the first reach has low lateral migration, In the second reach mediocre lateral migration and The third reach high lateral migration. The maximum lateral migration in third reach is about 1.70 m, in the second reach is 902 m and first reach is 583 m. Amount of lateral migration during 26 years is indicative of rapid changes in river. The minimum radius in both 1987 and 2013 can be observed in first reach (Oltan village to boundary point). The low average of radius of the arcs in third reach compared with second and first reaches indicated that in the third reach meandering is developed. In first reach and second reach for both 1987 and 2013 river planform are sinuosity and in second reach planform is meandering. Conclusion The study of geometric parameters for two periods (1987 and 2013) showed that over late 26 years in the study reaches, number of river arcs with undeveloped meandering patterns reduced, while number of river arcs with developed meandering pattern is increased. Study of lateral migration of Aras river showing high change in late 26 years. Even lateral migration in some parts of the third reach has been 1.7 km. The results shows that in studied parameters, slop and lithology associated with anthropogenic factors were the most important factors in the lateral migration of Aras river.