موانع طولی یکی از اشکال ژئومورفیک درون کانالی هستند که در قسمت مرکزی کانال‌های رودخانه‌ای تشکیل می‌شوند و به علت اینکه نقش مهمی در فرایند شریانی شدن رود دارند دارای اهمیت هستند. از این رو در این مقاله تشکیل موانع طولی و نقش آن در تغییر الگوی کانال مورد تجزیه و تحلیل قرار گرفته است. محدوده مورد مطالعه حوضه آبریز لاویج رود نام دارد که در استان مازندران و جنوب شهر نور در دامنه‌های شمالی البرزمرکزی قرار گرفته است. روش تحقیق بدین صورت بوده است که 3 بازه از مسیر رود که دارای موانع طولی بوده‌اند مورد بررسی قرارگرفتند. نقشه‌برداری از مسیر رود با استفاده از GPS انجام گرفت و موقعیت موانع طولی در کانال رود مشخص گردید. سپس از چندین مقطع عرضی از هربازه نقشه برداری شد و اندازه ذرات رسوبی با استفاده از روش شمارش پبل تعیین گردید. حد دبی لبالبی با استفاده از شواهد میدانی شناسایی گردید. سپس با استفاده از روابط تنش برشی مرزی (کل)، تنش برشی بحرانی و پایداری نسبی بستر، توانایی رود مورد تجزیه و تحلیل قرار گرفت. نتایج مقدار کم تنش برشی شکل بستر نشان می‌دهد که قسمت زیادی از انرژی رودخانه صرف غلبه بر مقاومت اشکال بستری می‌شود. نسبت کمتر از 1 پایداری نسبی بستر در بازه‌های مورد مطالعه نشان‌دهندة پایداری کم ذرات رسوبی در بستر رود می‌باشد. از این رو موانع طولی در قسمت‌های عریض کانال رود در جریان‌های کمتر از لبالبی و در نتیجه تغذیه زیاد رسوب و کاهش قدرت رود انباشته شده‌اند. تعداد این موانع در بعضی از بازه‌ها نشان دهند? تغییر تدریجی الگوی رود از حالت تقریبا مستقیم به الگوی تقریبا شریانی می‌باشد که نتیجه آن ناپایداری بیشتر کانال می‌باشد.
Process Measurement of Longitudinal Bar Forming in Mountainous Streams, Case Study: Northern Alborz, Lavij Roud Catchments
نویسندگان [English]
R Esmaili؛ M.M Hosseinzadeh
چکیده [English]
Introduction
Longitudinal bars are stream geomorphic features formed in the central part of river channels. Usually they are classified and range from simple to compound the obstacles based on the shape and position of the river which showed absence of multiple phases and deposition of sediment transport during the various flows are included. The form of these sedimentary bars is long and drop form. Sediment particle size varies from pebble to fine-grained particles such as gravel and sand. Sediment particle size in the upstream part of these bars is larger and downstream side is smaller. These deposits are usually formed the imbricate that shows the flow of water is during the flood period. Longitudinal bars play a significant role in the process of braiding. Hence, in this research, we analyzed longitudinal bars formation and its role in river pattern change. The study area is located in Lavij roud catchments in Mazandaran province, south of Noor city and northern central Alborz. This basin located from 36° 16' to 36° 27' 30" degree of north latitude and from 51° 58' to 52° 05' degree of east longitude. The area of basin is about 116 square kilometers.
Materials and methods
Methodologically, three reaches of stream channel were selected which contain longitudinal bars.
Reach 1: In this reach, between 50 and 90 percent of cases, the channel is located in the valley margins. The river channel from one side of the valley is located on the sidelines and on the other hand is a floodplain.
Reach 2: This reach of river is entirely alluvial and valleys to a depth of approximately 15 meters are filled from alluvial deposits. This reach was limited to the southern part of the fault North Alborz with a rapid and then it is subsidence. Hence is filled with alluvial sediments.
Reach 3: In this reach, between 10 and 50 percent of cases, the channel is located in the valley margins. Gradient is less than 3 percent. Bankfull channel discharge is estimated 13.5 cubic meters per second.
Survey of channel performed with the use of GPS and positions of longitudinal bar are determined in channel stream. Then, some cross sections are mapped and size of sediment determined with Pebble Count Method in the reaches. Bankfull discharge identified using field evidence. Then entrainment analyzed with boundary shear stress, critical shear stress and bed relative stability functions.
Results and discussion
In the studied reach, the total shear stress at all levels is more than the critical stress (the bondary) when the current bankfull is flown. Hence, the river has the ability to carry sediments flown existing longitudinal bars and channel bed is the current bankfull. If the number bed relative stability is larger of one, shows bed stability and everything is much more to show that sedimentary particles in the bed on the floor will remain stable. In the studied reach, this ratio is less than one. Hence this relationship shows that in the bottom sediments of streams in the current bankfull or less are removable and the bed in case of sediment movement is unstable.
There is no doubt that existing longitudinal bars in river channel change could lead to direct and meander pattern to be braided. Therefore in sections of reach 2 because of existed midpoint bars channel is divided into several parts.
Conclusion
Low values of bed form shear stress show that a large part of the river's energy is used to overcome the resistance of bed forms. The bed relative stability ratio less than 1 in this reaches represent low stability of sediments in the river's bed. Hence, longitudinal bars form in wide channels and the flow lower than bankfull lead to much sediment supply and decrease of competence limit. Number of longitudinal bars in some reaches indicated that river pattern shifts from straight to meandering which leads to more instability of the channel.
کلیدواژه‌ها [English]
Lavij roud, Longitudinal bars, Mountainous Streams, Relative stability, River pattern, Stream bed
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