Investigation of the changes in the stream sedimentation pattern by creating a loop in the Karun River in the Zargan-Farsyat region.

Background and Objectives: The present research was carried out with the aim of investigating and determining sediment changes in the Karun River, in the Zergan-Farsyat region, taking into account the existing conditions and the time of formation of the ring. The simulation of the river at a distance of about 75 km in Zargan-Farsyat region was done using HEC-RAS model. One of the main and most important issues in the water sector is the supply of required water and its transfer. One of the factors that reduce the transfer efficiency is sedimentation in water transfer channels and irrigation facilities. Knowledge of sediments and their control is essential for water engineering specialists. Therefore, in this study, the pattern of flow and transport of sediments in the network of open channels has been examined. Also, in this study, in order to determine the changes in the flow and sedimentation pattern of the Karun River in the existing conditions and when the loop was created. Gibson et al. (2010) modeled the COWLITZ River with respect to the moving bed and used the HEC-RAS model for simulation. They did the modeling for the last 20 miles of the river because flooding was more likely due to sedimentation in this area. The purpose of this study was to investigate the effects of sedimentation on the increase of flood risk in the long term. The Laursen- Copeland relation was used in the sediment calculations because the sediments ranged from very fine sand to large rubble. They also stated that the ability to add bed particle size in 20 different classes has been added to the HEC-RAS model in version 4.1. Materials and Methods: Karun River is one of the largest and longest rivers in Iran and collects the waters of vast areas of the country and finally leads to the Persian Gulf. The length of the Karun river is about 890 kilometers and the area of its watershed is 62570 square kilometers. The geographic location of this research is based on the UTM geographic system, around 32°22'33'' to 17°45'32'' north latitude and 48°22'40'' to 19°57'48'' east longitude. According to Figure 1, the total length of the studied route is about 75 km between Zargan and Farsiat and includes 155 cross sections, and the approximate length of the meander is 13.4 km. In order to simulate the hydraulic flow and sedimentation of the river by HEC-RAS model, geometric information (cross sections, distance of sections from the downstream control section, Manning's roughness coefficient and opening and narrowing coefficients), hydraulic information (flow information and water level level at the lower station) meteorological (water temperature) and sedimentary (related to flow-sediment discharge and bed sediments granularity) is necessary. To calculate the sediment transport rate, various relationships are predicted in the model (Young, Eckers-White, England-Hansen, Larsen (Copeland), Meer-Peter and Muller, Tofalti and Wilcock) so that the user can find the appropriate relationship for the studied surface. They choose. Gibson et al. (2010) stated that the ability to add bed particle size in 20 different classes was added to the HEC-RAS model in version 4.1. Results: Generally, it can be said that after the formation of the ring, the upstream distance (R1) of its sedimentation decreases from upstream to downstream until it undergoes erosion in the fork of the river and the middle distance (R2) changes from an erodible state to a sedimentary state, and the middle distance (R2) changes from the erodible state to the sedimentary state, and the distance downstream of the ring (R3) remains almost unchanged and shows very little tendency to erode. In section 21, if there is no ring, this section tends to erosion, and when the ring is created, it tends to sedimentation, which can be seen that at the end of the hydrograph, in the absence of a ring, there is approximately 1 cm of erosion on the bottom of the river. It has happened and in the case of the presence of a ring, 11 cm of sedimentation has taken place, and the amount of sedimentation reaches 23 cm almost in the middle of the hydrograph. Conclusion: The middle part of the Karun River (R2) turns from erodible to sediment by creating a ring, which is the reason for the river's tendency to deposit sediment in this part after creating the ring. Also, the decrease in flow rate and as a result the decrease in flow speed in this section is due to the division of the flow of the two branches of the river upstream of this section. In general, it can be said that after the formation of the upstream interval ring (R1), its sediment decreases from upstream to downstream until it undergoes erosion in the fork of the river, and the middle interval (R2) changes from erodible to sedimentary, and the interval of the base ring (R3) remains almost unchanged and has very little tendency to erode. [ABSTRACT FROM AUTHOR]