بررسی فرایندهای تشکیل دهنده موانع طولی در رودخانه‌های کوهستانی، مطالعه موردی: البرز شمالی، حوضه آبریز لاویج‌رود

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.