شبیه سازی تأثیر تغییر مورفولوژِی محل تلاقی شبکه زهکشی بر الگوی فرسایش و رسوبگذاری رودخانه سیمینه رود همدان با استفاده از مدل عددی فلوئنت

Simulation of the effect of morphology change at the confluence of the drainage network on the erosion and sedimentation pattern of the Siminehrood River in
Hamedan using the Fluent numerical model
Abstract (extended)
Because of hydraulic complexities, it has not been plausible to examine the confluence of river systems factoring in the laboratory limitations and the lack of use of three-dimensional models. The objective of this research is to simulate the impact of site morphology change on the erosion and sedimentation pattern of Siminehrood in Hamedan using the Fluent numerical model. The results pointed out that owing to the deviation of the current on the left bank, at first, micro-vortices form on the right bank, and with a larger size at the end of the stream. Besides, the Reynolds 130 stress study confirms that changing and increasing the flow velocity causes micro-vortices to form more rapidly and intensely on the right bank, and increases sedimentation on the left bank of the river. Continuation of this process changes the pattern of erosion and sedimentation and transforms the river into the form of arteries, prompts its deviation, and meandering.
Keywords: Micro-vortices, Convergent and divergent flow, Siminehrood, Hamedan
Introduction
Since the use of physical models demands a large space, high expense, and a long time, several river engineering problems are studied with mathematical models (Azizi et al., 2019). In this respect, the use of the fluent mathematical model with minimal field information and computational volume has been extensively used in studies of bed change and river organization (Yasi et al., 2017).
Kalami et al. (2019) assessed the geometric-hydraulic relationships of river cross-sections using the inverse solution of the Saint-Venant equations. The outcomes confirmed that hydraulic-hydrological routing methods have great accuracy in river flood simulation. Oda (2019), in modeling sediment transport and bed erosion and riverbank variations, revealed that the multiphase numerical model has a reliable performance in simulating sediment transport and erosion. Also, with this model, the limitations of experimental data can be mastered.
Methodology
Siminehroud basin is the head of the main tributary of the Qarachachai river and surface and groundwater drainage of Hamedan-Bahar plains in Hamedan. In the present research, the outcomes of the simulation model of the flow and sediment pattern at the confluence of the canal from a narrow cross-section to a cross-section for an average yearly discharge of 5 cubic meters per second of the Simineh River from divergent to convergent and vice versa is used. To evaluate the accuracy of the Fluent model, water level profiles were predicted and longitudinal sedimentation and erosion profiles and the maximum sedimentation depth at the intersection at transverse sections were simulated by velocity-pressure evaluation. Also, using the Fluent numerical method, using the finite volume method, which is an accepted separation method and is efficient in solving the governing equations of the flow, the patterns of erosion and sediment transfer at the confluence of the two cross-sections of the river are discussed.
Results and Discussion
The simulation results prove that the formation of "erosive holes" (micro-vortices) first on the right bank causes the current to deviate to the left bank. This increases sedimentation on the left bank. Maintenance of such a process not only alters the manner and pattern of erosion and sedimentation in the bed and sides but also develops the river from a straight pattern to an arterial or alluvial pattern. Continuation of this process will ultimately result in the diversion and meandering of the river through the formation of sedimentary islands. Also, parallel with the increase of the distance from the watercourse inlet as the cross-sections change, the micro-vortices join more strongly and more rapidly, forming larger vortices on the left bank. Examination of Reynolds numbers also shows that at divergent-to-convergent junctions, most microbial sediments form on the right bank. Whereas, in rivers with convergent-divergent bedrocks, the creation of micro-vortices due to a swift increase in velocity and constant pressure in the narrowing of the bed, forms sedimentary ridges symmetrically on the left and right banks of the river.
Conclusion
At the confluence of the flow, the flow pattern is such that deep erosion holes in the bed, shore erosion, sedimentation, and finally strong vortices form. This, in itself, alters the morphology of the river. Studies show that at the junction of divergent-to-convergent currents, owing to variations and increases in flow velocities, micro-vortices form more rapidly and intensely on the right bank, with a bed less wide than the right bank. In rivers with divergent confluence, micro-vortices first form on the bank of a side of the river whose bed width is somewhat smaller than the opposite side. The constant formation of sediment ridges near the river yields the transformation of the river into an arterial form. Knowledge of how these micro-vortices are formed, by identifying the location and formation of sedimentary ridges on the river bank, will lead to the more successful design and implementation of river management plans.