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

به دلیل پیچیدگی‌های هیدرولیکی، بررسی محل تلاقی که از اجزا مهم مورفولوژیکی سیستم‌های رودخانه بوده از جنبه‌های رسوبگذاری، فرسایش و ملاحظات زیست محیطی دارای اهمیت زیادی است. با توجه به وجود محدودیت‌های آزمایشگاهی و عدم کاربرد مدل‌های سه بعدی در سطح گسترده، امکان بررسی جامع این پدیده تاکنون میسر نشده است. هدف از این تحقیق شبیه‌سازی تاثیر تغییر مورفولوژِی محل تلاقی شبکه زهکشی بر الگوی فرسایش و رسوب‌گذاری رودخانه سیمینه‌رود همدان با استفاده از مدل عددی فلوئنت است. نتایج شبیه سازی مدل عددی نشان داد که به علت انحراف جریان در ساحل سمت چپ، ابتدا میکرو گردابه‌ها در ساحل سمت راست و در انتهای جریان ، میکرو گردابه‌ها با وسعت بزرگتری تشکیل می‌شود. بررسی تنش رینولدز 130 در محل تغییر مورفولوژی مقطع عرضی رودخانه، از همگرا به و اگرا نشان می‌دهد که به علت تغییر و افزایش سرعت جریان، تشکیل میکرو گردابه‌ها به سرعت و شدت بیشتر در ساحل سمت راست باعث انحراف جریان و افزایش رسوب‌گذاری در ساحل چپ و افزایش سرعت و فرسایش در ساحل راست رودخانه می‌شود. تداوم چنین فرایندی موجب تغییر در الگوی فرسایش و رسوب‌گذاری در بستر و کناره‌ها، تشکیل جزایر رسوبی، تحول در مسیر و مورفولوژی مقاطع عرضی رودخانه به شکل شریانی، و در نهایت انحراف و مئاندری شدن آن می‌شود. در حالی که در محل تلاقی همگرا به واگرا، به دلیل افزایش ناگهانی سرعت جریان و ثابت ماندن فشار در قسمت معبر تنگ شدگی تشکیل میکروبه گردابه‌ها بر خلاف مقاطع دیگر به صورت قرینه در دو ساحل سمت چپ و راست تشکیل می‌شود که این تغییر وضعیت ها باید در طرحهای ساماندهی و حفاظت از سواحل حاشیه رودخانه ها با دقت بیشتری مورد توجه قرار گیرد.

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

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 modelAbstract (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 threedimensional 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, microvortices 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 microvortices 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: Microvortices, Convergent and divergent flow, Siminehrood, HamedanIntroductionSince 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 geometrichydraulic relationships of river crosssections using the inverse solution of the SaintVenant equations. The outcomes confirmed that hydraulichydrological 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.MethodologySiminehroud basin is the head of the main tributary of the Qarachachai river and surface and groundwater drainage of HamedanBahar 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 crosssection to a crosssection 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 velocitypressure 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 crosssections of the river are discussed.Results and Discussion The simulation results prove that the formation of erosive holes (microvortices) 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 crosssections change, the microvortices join more strongly and more rapidly, forming larger vortices on the left bank. Examination of Reynolds numbers also shows that at divergenttoconvergent junctions, most microbial sediments form on the right bank. Whereas, in rivers with convergentdivergent bedrocks, the creation of microvortices 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.ConclusionAt 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 divergenttoconvergent currents, owing to variations and increases in flow velocities, microvortices form more rapidly and intensely on the right bank, with a bed less wide than the right bank. In rivers with divergent confluence, microvortices 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 microvortices 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.