تحليل وضعيّت پايداري مجراي رودخانه‎ي قزل اوزن با استفاده از روش‎هاي تنش برشي، شاخص مقاومت نسبي بستر و مطالعات صحرايي

Analysis of the Stability of River Channels Using Methods of Bed Shear Stress and the Relative Strength Index (Ranging Between 30 km Miyaneh City to the Zanjan Political Boundaries)

ناپايداري مجراي رودخانه، نقش به‎سزايي در ايجاد فرسايش، تخريب سواحل و تاسيسات كناره‎ي رودخانه دارد. اين نقش به‎ويژه هنگامي تشديد مي‎شود ‎كه مجرا و بستر رودخانه آبرفتي باشد. رودخانه‎ي قزل اوزن يكي از مهم‎ترين رودخانه‎هاي ايران است. اين رودخانه اثريافته از عوامل مختلفي چون، زمين‎شناسي منطقه، ويژگي‎هاي تشكيلات آبرفتي و شرايط هيدروليكي جريان، رفتار مورفولوژيكي پويايي دارد. هدف اين پژوهش، بررسي وضعيّت پايداري رودخانه با استفاده از روش‎هاي تجربي و رياضي و مقايسه‎ي نتايج اين روش‎ها با نتايج مطالعات صحرايي و ويژگي‎هاي مورفولوژيك هر كدام از بازه‎هاي مورد مطالعه در طبيعت است. براي رسيدن به اين هدف، در اين پژوهش با استفاده از روش‎هاي تنش برشي و شاخص مقاومت نسبي بستر، وضعيّت پايداري رودخانه تحليل و سپس نتايج روش‎هاي گفته‎شده با مطالعات و اندازه‎گيري‎هاي صحرايي مقايسه شدند. نتايج نشان داد كه در بازه‎ي اوّل محدوده‎ي مورد مطالعه، نتايج روش‎هاي تجربي با مطالعات صحرايي هم‎خواني دارد؛ ولي در بازه‎هاي دوم و سوم نتايج روش‎هاي تجربي با مطالعات صحرايي هم‎خواني ندارد. اين بررسي نشان داد كه روش‎هاي تنش برشي و شاخص مقاومت نسبي بستر، تنها در مجراهاي آبرفتي اهمّيّت داشته و به نتايج آنها مي‎توان استناد كرد. به‎كارگيري اين روش‎ها در بازهاي غيرآبرفتي، نتايج غيرواقعي و دور از انتظار خواهد داشت و با ويژگي‎هاي مورفولوژيك رودخانه‎ي مورد مطالعه هم‎خواني نخواهد داشت.

Introduction Instability of the river channel has significant role in creating erosion, degradation of coastal and Facilities around the river. This role will be intensified especially when the channel of river bed is alluvial. By monitoring changes in river morphology and specific measures can be prevented from the occurrence of Instability River. The purpose of this study is investigation the stability of the river, using experimental and mathematical methods and comparing the results of these methods with results of field studies and morphological characteristics of each interval in nature. In this research, the study area is Qezel Ozan River in longitudes 47? 48ʹ to 48? 27ʹ and latitudes 37? 12? to 37? 25?. In this study for the separation of mountain and plain intervals, the studied route was divided into three periods. Methodology At first the studied area was investigated by Topographic maps 1:50000, geological maps 1:100000 and satellite images IRS 2007 to check the status of the stability river in the studied intervals and plan map of the river were obtained from satellite images. Then, during the field Performed sampling of the sides and bed materials the river for determine particles size River and River depth was measured. Particles size of the bed sediments and its sides was performed using standard methods Ashto 80-77. To calculate the return periods of flood flow used from distribution of Gambl. The total shear stress and critical stress was calculated using the relationship Swami and Mital. Results and Discussion This study showed that the methods of bed shear stress and the relative strength were significant only in alluvial channels and results are reliable. Results using these techniques in non-alluvial intervals will be unrealistic and unexpected and will not match with morphological characteristics of the river. Therefore first interval as the combination of arterial and Meandering River in alluvial river bed is unstable interval and Second and third interval Introduced As the stable interval that river has been limited by Rock bed and sides. Conclusion Comparing the existing stresses and critical stress indicated that in the all studied stations the shear stress was greater than the critical stress and so these areas are unstable. Based on this index, highly erosion and low shear strength in the studied stations have been caused increasing of changes studied intervals. Also investigation of RBS index showed that this index is less than unite in all stations so theirs are unstable. Survey of results of field studies, topographic maps and satellite images showed that the results of experimental methods in the second interval and very little for the third interval is different. The results of mentioned methods are quite consistent with the characteristics of the first interval that is due to morphological characteristics studied intervals.