Introduction

Currently, using discharge-Eschle curves and measuring discharge in natural rivers is generally difficult and error-prone. Therefore, the use of new models plays an effective role in reducing the time and cost associated with hydrometry (Ahmad et al. 2021). Due to the importance of this phenomenon and because the use of physical models requires a large space, high cost, and long time to conduct experiments, many river engineering problems are investigated with mathematical models (Azizi et al., 2019). For this reason, to better understand these phenomena, numerical and computational fluid dynamics models should be used to simulate flow, which are more accurate than other simulation methods (Ildoromi et al. 2020).

Bagheri (2023) evaluated the methods for calculating the discharge-Eschle curve based on bedform resistance relations at the Ablo Nekarud hydrometric station in Mazandaran and showed that the Einstein-Barbarossa method has the lowest absolute error compared to other methods. Mohammadi et al. (2023) showed in estimating the discharge-echel curve in natural rivers with unsteady flow that with a new method based on constant velocity meters, the discharge-echel curve of unsteady flow and depth and discharge hydrographs can be tracked.Kim et al. (2023) evaluated the sedimentation and erosion of riverbanks and channels in the Mekong Delta using the MK-BEHI index and showed that differences in bank stress and riverbank structures cause changes in the erosion rate and riverbank load.Boustani et al. (2023) studied the distribution of bed shear stress using numerical simulation and compared the results of Ansys Fluent and Flow-3D, showing that the numerical results of the Fluent model were closer to the experimental data compared to the Flow 3D model.



Methodology

The Yalfan River is located in the Ekbatan Dam watershed in the northern part of Alvand, 10 kilometers southeast of Hamedan city. In this study, open channel flow in a one-dimensional stable has been investigated.manner in channels with conventional rectangular and trapezoidal cross-sections in the Yalfan River of Ekbatan Dam, Hamedan, Given that the shape of the sides of the channel cross-section has an important effect on the discharge-Ecshle relationships, and the distribution of runoff in channels with different cross-section shapes and surfaces has an important effect on causing erosion and instability of the riverbed.Therefore, numerical values of runoff advance in rectangular and trapezoidal sections were calculated at different times and compared with experimental results for different turbulence models and percentage of error of each turbulence model was calculated and compared with each other.In this research, the second-order discretization method of the Fluent model was used to separate the load and the effect of rectangular and trapezoidal sections to investigate the propagation of runoff flow in scour and edge erosion, and the Reynolds stress numbers were used to determine the movement threshold and to analyze the movement pattern and settling of sediment particles in different flow tubes in a laboratory glass channel, using Fluent software.



Results and Discussion

The results showed that in a rectangular cross section, runoff distribution and pressure are distributed almost equally across the channel surface, and the flow rates do not show significant changes.In a trapezoidal channel with a closed cross-section, discharge - Eschle's measurements showed that the spread of runoff with higher velocity and pressure at the bottom of the channel causes increased erosion at the bottom.The results of the study in a trapezoidal channel with an open cross-section, the values of the discharge - Eschle and the histograms of pressure and diffusion velocity, showed that the pressure and diffusion velocity are higher on the sides of the channel and the erosion rate is higher on the sides.The results of the assessment of the discharge-Ecshle relationships showed that the more closed the trapezoidal channel opening is, the higher the pressure at the bottom and the lower the pressure at the channel wall, and as a result, the greater the erosion rate at the bed.Conversely, the more the trapezoid opening is opened, the greater the runoff pressure on the sides of the bed and, consequently, the greater the erosion rate.The results showed that due to the change in the relationship between the discharge-Ecshle and the flow velocity and pressure at different sections, the rate of erosion and sedimentation changes drastically.The study of runoff distribution showed that the channel shape has an important effect on creating riverbed instability and causes meandering and shifting of the river course over time, which should be considered in river engineering works.



Conclusion

Comparison of the output results of the Fluent hydraulic model shows that this model has good ability to estimate changes in river cross sections.The results show that changes in the channel sections of the Yalfan River along the longitudinal profile of the river cause changes in bank erosion and bank erosion.So that at the beginning of the flow with a trapezoidal cross-section, due to changes in flow propagation speed and pressure, erosion and foaming are greater than at the sides.While in rectangular sections in the middle and end of the stream, lateral erosion and bed sedimentation are observed, which must be carefully considered in the location and design of appropriate river engineering control and protection structures.The results showed that modeling the runoff propagation phenomenon in the Fluent environment has acceptable accuracy, but generating the geometry file and solution field network in Gambit software for study using topographic maps requires a lot of time.Therefore, it is recommended that other software be used to create geometry and modeling for modeling in large watersheds, and that the Fluent model be used for modeling in small watersheds that require high accuracy.

Keywords: Erosion, Cross Sections, Model, Flow Velocity, Fluent