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. Analysis of dye paths in the laboratory demonstrated that the simulated streamline pattern closely follows the pattern observed in the laboratory, it further indicates that the numerical model is capable of accurately representing the flow dynamics. It appears that increasing water depth has a significant impact on pressure values and the maximum pressure gradients. Additionally, the length of the pit also affects hyporheic exchange, with longer pits resulting in decreased exchange. the percent of hyporheic exchanges for the depth of 0.061 meters ranged from a maximum of 9.612% in E1 scenario to a minimum of 6.133% in E4 scenario. Also, the percentage of hyporheic exchange decreases with increasing discharge. Similarly, for the depth of 0.101 meters, the maximum percent of hyporheic exchanges was 10.003% in E5 scenario, while the minimum was 6.171% in E8 scenario. The E4 and E8 scenarios exhibited the highest dimensionless penetration depth and residence time, while E1 and E5 had the lowest values. The increase in water depth also led to an increase in the dimensionless penetration depth. It shows that at shallower depths, turbulent eddies may influence the flow field, resulting in shorter residence times and particle penetration. Regarding residence time distribution, the histogram analysis revealed a log-normal distribution for the E1 and E5 scenarios, while a Generalized Extreme Value Distribution was obtained for the other scenarios.
Conclusion: In this paper, the effect of different lengths of sand pit mining and different depths of water was studied by analysis of surface flow and exchange with the subsurface flow. The results showed that increasing the length of the pit was found to decrease hyporheic exchange, indicating an inverse relationship between pit length and dimensionless hyporheic exchange. This suggests that longer pits may have reduced interaction between surface and subsurface flows. On the other hand, increasing the depth of water in the tested pits was seen to increase the dimensionless hyporheic exchange. This means that deeper water levels enhance the exchange between surface water and subsurface flow. The average maximum dimensionless penetration depth decreased as the pit length decreased, ranging from 1.66 to 2.66. This indicates that shorter pits may have limited penetration of particles into the subsurface. The range of hydraulic gradient values at a depth of 0.061m was observed to be between 0.10386 and 0.10644 meters, while at a depth of 0.101m, the range was between 0.10517 and 0.10645 meters.