Erosion modeling of Jajrud river banks Between Letyan and Mamlu dams Jajrud, Tehran

ABSTRACT
Rivers in their path are always struggling with a phenomenon called erosion, which on the one hand causes many changes in the geometric shape of the river section, morphology and hydraulic characteristics of its flow, and on the other hand, it brings irreparable effects to the lands adjacent to the channel. One of the main sources of sediment production is the erosion of river banks. In this regard, investigating the amount of coastal erosion is one of the management strategies. The area under study is the banks of the Jajroud River, between Letian Dam and Mamlo Dam, with a total of seven sections. In this study, for the erosion of the river bank and estimation of the amount of sediment from the method or model of estimation of the bank and foot of the bank (BSTEM) in this model, the geometrical parameters of the channel (angle of the wall and height of the bank and the distance of the toe of the bank and its angle), the height of the layers And their species, flow information and vegetation cover and other side cover materials have been extracted and used. In this research, the depth of the flow in a spiral discharge mode and the flow duration of 12 hours were used to model the bank erosion. The model models the amount of bank destruction by calculating the shear stress and soil resistance. The results of the research showed that all sections except section 6 have high erosion. The difference in erosion values in different stages was mostly due to the type of bank sediments and bank slope angle. In terms of bank stability and safety factor (FS), the most unstable bank was in section 5 and the most stable bank was in section 6 of the river
 
Extended Abstract
Introduction
Rivers always struggle with a phenomenon called erosion on the one hand, it makes many changes in the geometric shape of the river section, morphology, and hydraulic characteristics of its flow, and on the other hand, it brings irreparable effects to the lands adjacent to the channel. There are different types of erosion phenomena, one of the most important and common erosion mechanisms in rivers is the mass erosion of river banks. extensive research has been done in the field of mass erosion and factors affecting it that lead to soil erosion and loss of land adjacent to the river one of the main sources of sediment production is the erosion of river banks. There are various methods and models to estimate the amount of river bank erosion, and the Bank Stability and Toe Erosion Model (BSTEM) is one of the numerical simulation models. This model has been developed to predicting lateral retreat streambanks (caused by river erosion and geotechnical rupture). This model estimates the erosion rate by taking into account the soil resistance forces and driving forces along the surface prone to failure (rupture).
 
Methodology
This study was carried out between the Letyan and Mamlu dams in the Jajrud River in the east of Tehran city. The effects of erosion are evident along the entire length of the channel despite the dam and its controlling role. The maximum discharge of Letyan dam was 206 cubic meters per second in the water year 1994-95, and its average discharge was recorded as 1.67 cubic meters per second in the statistical period from 1988 to 2018. In this research, the cross-sections under study were selected, based on aerial photos and satellite images and then based on field visits, the selected sections (seven sections) were examined to study bank erosion by BSTEM model.
The BSTEM model is one of the most widely used and advanced models regarding the stability of the river bank. This model was developed by the National Sediment Laboratory in Oxford-Mississippi in the United States. This model estimates the erosion rate by considering the soil resistance forces and driving forces along the failure-prone surface. The required parameters of the model include the following 1- Geometric parameters of the channel 2- The thickness of the layers and their materials 3- flow data (flow rate) 4- Vegetation and other side covering materials. After entering the mentioned data into the model, can be seen bank erosion modeling (bank geometry, angle, and height of failure surface occurrence) and bank toe erosion modeling, for specific flow periods. The bank safety factor (FS) is calculated at the end of the modeling. In this section, you can see the results of the model, including the calculated shear stress, the amount of bank retreat, the amount of sediments transported from the bank and the bank toe, the new profile of the bank, and the amount of erosion.
 
Results and Discussion
In this research, the bank erosion has been simulated in the BSTEM model to investigate the amount of bank retreat and the amount of sediment produced in 7 cross-sections of the Jajrud River. This research was used the scenario of flow depth in the case of bankfull and 12-hour flow duration to simulate the bank and the bank toe. Based on the simulation results, the amount of hydraulic erosion and the change in the geometry of the bank toe should be determined. The amount of erosion for the cross-sections was as follows. cross-section 1 is 21m^3, back length is 0.57 m and safety factor is 0.38, cross-section 2 is 4 m^3, back length is 0.63 m and safety factor is 0.05, cross-section 3 is m^3, back length is 0.57 m and Safety factor 0.69, cross-section 4 6 m^3, rear length 0.66 m and safety factor 0.66, cross-section 5 is 21 m^3, rear length 1.28 m and safety factor 1.3, cross-section 6 is m^3, Back length - m and safety factor 3.34, cross-section 7 is 9 m^3, back length 0.65 m and safety factor 0.82. This model was carried to know the bank erosion and the amount of sediment production due to bank failure and erosion of the channel bank in seven cross-sections of the channel and the results of all sections except cross-section 6 show high erosion. In cross-section 6, the top of the wall was in a low-risk state, and the foot of the wall brought an acceptable amount of sediment into the channel. The bank angle is most important and effective parameter.
 
Conclusion
In all cross-sections, there is a large amount of retreat, the highest of which was related to cross-section 5 with an amount of 1.28 meters, and the lowest was related to cross-section 6 (almost zero). Other cross-sections are in the range of 57 cm to 66 cm.
In terms of bank stability and safety factor (FS), the most unsafe cross-section is number 5 to the amount of 0.05 and the safest section is number 6 to the amount of 3.34. Of course, the safety number of 1.3 for cross-section 5 with the condition of vegetation is also high safety. The highest weight of the fallen mass is for cross-sections 5, 1, and 7, respectively and after these sections, there is cross-section number 4 and cross-section number 2. Field observations after one year showed that results of cross-sections No. 2, 3, and 4 are very close to reality and the walls have collapsed, which shows the high compatibility of this model with the natural conditions of the region.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.