Abstract: Erosion occurs naturally in riverbeds. The high cost and limitations of physical models have led to using mathematical and numerical models. In this research, the two-dimensional mathematical model CCHE2D was applied for the simulation of sediment transport in Saqqez River flows throughout the Kurdistan province to determine the rate of erosion and sedimentation. The Study area includes the main and sub-branches that intersect at the site of Tamogheh village. After analyzing the sensitivity of the model, the model proposed by Wu et al. with a Root Mean Square Error of 0.002 and Mean Absolute Error Percentage of 7.14 was used as the most appropriate relationship for calculating sediments. Finally, it was found that on average, in the study period, sediment deposition was about 82485 cubic meters, erosion was about 18708 cubic meters, and average bed level changes were 0.20 meters. The Khan tributary accounted for 63% of the total erosion of the study period.

Introduction: Sediment hydraulics is a science that discusses the movement, erosion, and deposition of sediment materials in channels and rivers. The sediment load is divided into two parts, the bed sediment load, and the suspended load, and the sum of these two forms the total sediment load.
Simulation or modeling techniques are used in order to predict complex hydraulic phenomena. Research results have shown that the CCHE2D model is a suitable tool in hydraulic engineering for studying turbulent flows, sediment transport, morphological changes, and free surface changes with time in sedimentary beds (Athow Jr & Trawle, 1976; Habibi et al., 2014; Khosravi, 2018; Maghrebi et al., 2010).
There are many applications for natural channels with complex flow conditions, topography, and hydraulic structures (Jia et al., 2002).
Wu (2001) simulated the flow and sediment transport of the downstream of the Yellow River in China. Based on this, it was concluded that all turbulence models provide accurate results about simple flows. But regarding complex flows, the two turbulence models, k-ε and RNG, show more realistic results than the Zero-equation models.

Methodology: The studied area includes an area of the Saqqez River, one of the important branches of Zarrineh Rud. The total length of this river is about 100 km, and its drainage basin is about 1350 km² (‎Fig. 1).
The CCHE2D model is a numerical model for the simulation of unsteady turbulent flow and sediment transport in open channels, which was developed at NCCHE of the Mississippi University of Technology, USA.
In the CCHE2D model, for the hydraulic analysis of the flow, the continuity equation ‏‏(1) and the Momentum equations ‏‏‏(2) and ‏‏(3) are used in the two-dimensional space (Zhang, 2005).
In the sediment section, the CCHE2D model uses the general Advection Diffusion equation (Eq.‏(4)) to calculate the sediment load and the continuity equation of the bed load (Eq. ‏(5)) to calculate the deformation of the bed (Zhang, 2005).
In the field visit of the studied area in separate selected sections, flow velocities, longitudinal profile of the water level, and sedimentary materials of the bed were collected to determine the gradation and the representative diameter of the particles, as well as DEM of the area for the preparation of the computational grid and simulation. (‎‎Fig. 4).
For the studied area of Saqqez River, part of the topographic information was collected by field surveying. The other part was obtained from a complete topographic map in the form of elevation curves in AutoCAD Land format.
In the current study, the study period is from 1998 to 2010.

Results and Discussion: In the model used in this research, there are 5 main parameters in order to calibrate the model. These parameters are the disturbance model, Coincidence Factor, Schmidt number, Roughness Coefficient, and Empirical Relationship.
The comparison of the results of bed sediment distribution showed that Wu's experimental relationship is more consistent with the observed values at Cheragh-Weiss station than the modified Englund-Hansen method. In order to check the model more accurately, two statistical methods,  and , were used (Eqs. ‏(6) and ‏‏‏(7)); Therefore, Wu's empirical relationship with a Correlation coefficient of 0.9268,  of 0.002, and  of 7.13 was selected for simulation.
The optimal values of the Schmidt number equal to 0.75, the Coincidence Factor equal to 0.5, and the Roughness Coefficient value equal to 0.045 were obtained and used in the simulation. Correlation diagrams of the average distribution of bed sediments are shown in ‎Figs. 7 and ‎8.
By separating the results, the amount of erosion and sediment deposition for the main and sub-branches were presented in ‎Table 4.
As a result, the total amount of erosion per year and also the total amount of sediment deposition per year in the study area of Saqqez River, with a length of 22 km, is determined.
The output results showed that Saqqez River would have an average bed level change of 0.20 m in the entire study area. ‎Figs. 9 and ‎10 show an example of the bed changes output in the studied area graphically.

Conclusion: In this research, the simulation of sediment in a section of the Saqqez River in Kurdistan province, which is limited to two data measuring stations, Chirag-Weis and Qabqablo, was carried out using the CCHE2D numerical model. The results showed that the annual amount of erosion is 18708 m³ on average and the deposition of sedimentary materials is 82485 m³, among which the Khan tributary river with an amount of 11748 m³ includes about 63% of the total erosion and also with an amount of 57186 m³, includes about 69% of the total deposition of sedimentary materials in the studied area. As a result, the Khan tributary river, which meets the main branch of the Saqqez River in Tamogheh village, is morphologically in the category of young rivers, and the tendency to erosion in this tributary is more than the more mature main branch.