River Sediment Replenishment: Investigation of Stockpile Erosion and Downstream Geomorphological Responses

Sediment replenishment (SR) is a widely used sediment management strategy worldwide. The long‐term implementation of SR in Japan differs significantly from that in other countries, and its geomorphological responses require in‐depth investigation. This study focused on the erosion of the SR stockpile and its impact on the Naka River in Japan. A detailed study and a two‐dimensional (2D) depth‐averaged hydromorphodynamic numerical model were used to analyze the effects of flushing flow, replenished sediment volume and grain size distributions, and the influence of the SR on stockpile erosion and river morphology. The study revealed that a higher magnitude of flushing flow does not efficiently promote the erosion of SR sediment. Instead, an optimal flushing discharge of approximately 40% of the original flow is effective with a typical duration and frequency. Compared with single flood pulse, introducing a second flood pulse can accelerate SR erosion, leading to a 1.5% increase in the eroded volume and sediment transport ratio (TR). Furthermore, adding a percentage of fine sediment to the SR material notably increased SR erosion and promoted the formation of riffles and bars near the SR site. Additionally, arranging double stockpiles at the SR site can increase the TR by 3% and the downstream bed change indicator by 0.01 m. The study findings are valuable for predicting the erosion process of SR stockpiles, resulting in geomorphological changes in a longer reach downstream in the Naka River. They also provide useful recommendations for designing a successful SR system for other river basins worldwide.