بررسی آزمایشگاهی و عددی تبادلات هایپریک در حفره برداشت شن و ماسه

مصالح رودخانه‏ای به علت دسترسی آسان و اندازه ذرات مناسب همواره نقش مهمی در صنعت ساخت‏وساز ایفا می‏کند. استخراج شن و ماسه غیراصولی می‌تواند باعث از بین رفتن محیط‏زیست آبی و اختلال در اکوسیستم رودخانه شود، لذا ضروریست که از رویکردهای پایدار برای کاهش تاثیرات منفی بر محیط‏زیست استفاده شود. یکی از تاثیرات برداشت شن و ماسه، تاثیر آن بر ناحیه هایپریک است. ناحیه هایپریک، منطقه‏ای اشباع بلافاصله زیر بستر رودخانه است که تبادل آب سطحی و زیرزمینی در آن رخ می‏دهد و تاثیر مستقیم بر اکوسیستم آبی دارد. با توجه به فرسایش بستر رودخانه در اثر برداشت غیراصولی، الگوی هیدرودینامیکی جریان درون حفره تغییر کرده و می‌تواند بر تبادلات هایپریک تاثیر داشته باشد. پژوهش‏های زیادی در زمینه تاثیر شکل بستر رودخانه بر روی الگوی جریان هایپریک و مشخصات آن انجام شده است در حالی‏که در خصوص تاثیر حفره برداشت بر تبادلات هایپریک مطالعه‏ای صورت نگرفته است. در این تحقیق اثر طول حفره و عمق آب بر مشخصات ناحیه هایپریک شامل میزان تبادلات هایپریک، زمان ماند و حداکثر عمق نفوذ مورد مطالعه قرار گرفت. برای شبیه‌سازی جریان سطحی و زیرسطحی به‏ترتیب از نرم‏افزارهای فلوئنت و کامسول استفاده شد. تجزیه و تحلیل نتایج نشان داد که افزایش طول حفره برداشت، موجب افزایش زمان ماند، عمق نفوذ و کاهش درصد دبی تبادلی می‏گردد. هم‏چنین افزایش عمق آب باعث افزایش دبی تبادلی می‏شود و توزیع فراوانی زمان ماند از الگوی لوگ‏نرمال و مقادیر حدی تعمیم یافته پیروی می‏کند.

laboratory and numerical investigation of hyporheic exchanges in sand mining pit

introduction: river materials such as sand are widely used in the construction industry due to their accessibility, texture, and suitable particle size. while sand mining is essential for economic development and infrastructure projects, it can have significant adverse effects on river ecosystems. therefore, it is crucial to implement sustainable mining practices and enforce regulations to mitigate these effects. when a river is diverted for sand mining, it can result in the loss of natural river features that promote hyporheic exchange. these features include meanders, riffles, and pools, which create flow patterns that allow water to infiltrate into the sediment and exchange with the groundwater. the hyporheic zone is the area just beneath a river where water and nutrients exchange between the river and groundwater. sand mining pit can disrupt the hyporheic zone by altering the channel morphology and reducing the connectivity between the river and groundwater. as a result, the exchange rates of oxygen, nutrients, and other substances between the river and groundwater can be reduced, affecting the overall health and functionality of the river ecosystem. in this study, the effect of different sand mining pit lengths and upstream water depths on the characteristics of the hyporheic zone is investigated. additionally, the numerical results of surface and subsurface models are calibrated with laboratory observations. methodology: experiments were conducted in a flume with a length of 7 meters, width of 1 meter and height of 1 meter. the velocity of the water was measured using an electromagnetic current velocity meter with an accuracy of 0.5 cm/s. sediments with an average diameter of 2.3 mm, falling within the recommended range of previous studies, were filled in the channel (rovira et al., 2005; wu and wang, 2008; mori et al., 2011). trapezoidal-shaped sand mining pits with a height of 0.1 m were constructed in the middle of the flume, and their lengths varied. the range of dimensions for the mining pits was determined based on previous studies conducted by lee et al. (1993), barman et al. (2019), jang et al. (2015), and haghnazar et al. (2019). the objective of the study was to investigate the impact of the length of the mining pits, the water depth, and different discharges on the hyporheic exchanges. the experiments were carried out in eight scenarios. in scenarios e1 to e4, the upstream water depth was 0.061 m, and the pit lengths were 0.25 m, 0.5 m, 0.75 m, and 1 m, respectively. in scenarios e5 to e8, the pit lengths were the same as before, but the water depth was increased to 0.101 m. to simulate the surface flow on the sand mining pits and the subsurface flow in the sediment, computational fluid dynamics software was utilized (cardenas and wilson, 2007a, 2007b, 2007c; chen et al., 2015). anasys fluent software was used for simulating the surface flow, while comsol software was used for simulating the subsurface flow (bear, 1972; cardenas and wilson, 2007a, 2007b; trauth et al., 2013).results and discussion: for the calibration of the surface model, the observed and simulated water surface elevation and the surface flow velocity were compared. rmse for the free surface elevation in the e4 scenario was found to be 0.002 m, indicating a good agreement between the laboratory and numerical model. the comparison of vertical velocity profiles also showed a close match between the simulated and experimental velocities. it demonstrates the model’s capability to simulate flow behavior and can be utilized for simulations related to similar scenarios. additionally, injecting dye into the bed and comparing the simulated streamlines with the laboratory results were done to assess the accuracy of the subsurface model.