Evaluation of infilling and replenishment of river sand mining pits

Rivers are one of the main sources to supply sand and gravel for construction projects. Depending on river morphology and hydraulic characteristics, its sediment transport capacity, and mining operation method, the extraction of river bed materials may affect its ecosystem through bank and bed erosion. To advance the mechanisms of river pit infilling, the effects of various parameters (i.e., the distance between pits, the pit plan shape, the pit depth, sediment size, and approaching flow velocity) on pit infilling volume are investigated in this research. The results of this research show that infilling volume of upstream pit is insignificantly affected by the distance between the pits, and it is completely refilled for different distances. However, the infilling volume of downstream pit decreases by increasing the distance between the pits. In addition, by reducing the ratio of pit length to its width (pit shape extension in spanwise direction), the pits can be excavated in a shorter distance from each other; when this ratio decreases by 15%, the infilling volume increases up to 30%. Subsequently, as a cost-effective option, the pit distance can be reduced up to 50% in these conditions. According to the obtained results, although the sediment size has negligible effect on infilling volume in the studied range, the infilling volume increases up to 20% by an increase of 8% in the approaching flow velocity. Increasing the ratio of pit length to its width (pit shape extension in streamwise direction) highlights the effectiveness of smaller depths, so that the infilling volume increases up to 20% by a decrease of 20% in the pit depth. In this regard, it is recommended that the pit depth be restricted to 70% of the channel flow depth to have a complete pit refilling.