بررسی عددی عوامل موثر بر توزیع غلظت رسوب معلق نامتعادل در رودخانه های طبیعی (مطالعه موردی: رودخانه قره سو، کرمانشاه)

چگونگی تغییرات غلظت رسوب معلق در طول مسیر رودخانه و بررسی عوامل تاثیرگذار بر آن همواره مورد توجه مهندسین علم هیدرولیک و محیط زیست می باشد. عدم وجود ایستگاه‌ های اندازه گیری کافی و مشکلات نمونه ‌برداری رسوب معلق، تهیه مدل‌ هایی که به درستی رسوب معلق را در طول مسیر رودخانه روندیابی نمایند ضروری می نماید. در طبیعت رسوب بصورت نامتعادل انتقال می‌یابد در حالی‌که خیلی مدل های تجاری حالت ظرفیت حمل یا متعادل را در نظر می‌گیرند. از این رو در تحقیق حاضر مدلی عددی تهیه شده با حل عددی معادله تک بعدی انتقال و پخش غیرماندگار روندیابی رسوب معلق در یک بازه رودخانه ای را در شرایط عدم تعادل انجام می دهد. پس از صحت سنجی مدل، تاثیر ده روش عددی منفصل سازی، پنج معادله انتقال رسوب، هشت رابطه ضریب پخشیدگی و هشت رابطه سرعت سقوط ذره بر تغییرات بار رسوب معلق در طول بازه مورد مطالعه بررسی شد. نتایج تحقیق نشان داد استفاده از رابطه تجربی وایف مقدار رسوب معلق بیشتری را نسبت به سایر روابط دیگر برآورد می کند. در میان روابط سرعت سقوط ذره رابطه استوکس سرعت سقوط بیشتری را برآورد می کند که باعث می شود احتمال معلق شدن ذرات رسوب کمتر و در نتیجه غلظت آن نسبت به سایر روش ها کمتر باشد. همچنین در بین روش های منفصل سازی روش وان لییر خطای کمتری را دارا است. از طرفی رابطه الدر کمترین و رابطه کاشفی پور– فالکونر بیشترین مقدار پخشیدگی را در هیدروگراف غلظت از خود بجا می گذارند. علاوه بر این نتایج تحقیق حاضر نشان داد غلظت رسوب برآورد شده در حالت عدم تعادل حدود 11.7 درصد بیشتر از ظرفیت حمل بار معلق محاسبه شده توسط روابط تجربی می باشد.

Numerical Study of Factors Affecting Distribution of None-Equilibrium Suspended Sediment in Natural Rivers (Case Study: Gharasoo River, Kermanshah)

Introduction: The concentration changes of suspended load along the river reach and the contributing factors are of importance for hydraulic and environmental engineers. The first step to calculate the concentration of suspended sediment load is determining the flow hydraulic characteristics along a river reach. Although most of flow in nature are unsteady, the quasisteady flow condition was considered to be simple in this study and the water surface profile along the river reach with irregular cross sections was calculated by standard stepbystep method. In order to calculate suspended sediment load under nonequilibrium condition, the advectiondiffusion equation with source term was numerically solved. In the present sediment model, ten discretization methods, five relations for calculating capacity of suspended sediment load, eight relations for diffusion coefficients and eight relations to calculate particle fall velocity were used and their effects on suspended sediment distribution along 18480 m of Gharasoo river were investigated.Results and Discussion: The HECRAS model output was used to calibrate the present hydraulic model. The models were run with the conditions as same as Manning roughness coefficient and river geometry conditions. The results showed that the calculated water surface profile along the river reach by two models are completely overlapped each other. In other words, the present model has a very good accuracy to predict the water surface profile in the river reach. As most commercial 1D models (same as HECRAS) only consider the equilibrium condition for sediment transport and the bed or total load sediment, comparing the results of present sediment model with them seems not to be reasonable. Therefore, to validate the present suspended sediment model and finding the best method of discretization, an especial shape concentration hydrograph was introduced to the present model as input hydrograph and the model was run when the source term has been deleted deliberately. The volume below the input concentration hydrograph and calculated hydrographs in different cross sections was compared to each other. Comparing the hydrographs showed that the maximum error in calculating the volume of concentration hydrograph with the input hydrograph was 0.029% implying that the model satisfies the conservation laws as well as reliable programing. Among ten discretization methods, the best method for discretization of the advectiondiffusion equation was Van Leer apos;s method with the least error compared to other methods. After validating the model, effect of five relations for calculating capacity of suspended sediment load was investigated. The results showed that using the Wife equation estimated the amount of suspended sediment higher than other equations. The Toffaletti equation also estimated suspended sediment load lower than other equation. Among eight particle fall velocity formulas, Stokes relationship estimated the fall velocity larger than other equations. Hence, the Stokes equation application decreases the possibility of suspending the sediment particles. However, employing Van Rijn and Zanke relationships resulted in a greater suspended sediment load distribution along the river reach. Among eight relationships for diffusion coefficients, Elder and the Kashifipour Falconer equations exhibited the lowest and the highest amount of diffusion in the concentration hydrograph, respectively. Furthermore, the calculated suspended sediment concentration under nonequilibrium conditions was 11.7 % higher than that under equilibrium conditions along the river reach.Conclusion: Most 1D numerical models only simulate the bed and total loads sediment transport under equilibrium condition while sediments are transported under nonequilibrium conditions in nature. Sediment transport under non equilibrium conditions may be greater or lower than the equilibrium condition known as the capacity of sediment transport. In this research, a numerical model was developed to simulate the suspended sediment transport in a river reach under nonequilibrium conditions. The amount of suspended sediment concentration was calculated for each sediment grain size. The results showed that the distribution of suspended load along the river reach is not significantly sensitive to the fall velocity relations while the type of sediment transport equation affected the suspended sediment transport concentration. The concentration of suspended sediments for nonequilibrium conditions was also 11.7% higher than the concentration of sediments in equilibrium condition.