Sediment replenishment effects on a gravel-bed river downstream of the Futatsuno Hydropower Dam in Japan

Sediment management of dam reservoirs is one of the most important issues to ensure the sustainability of hydropower. In the Kumano River basin, with abundant water and active sediment production even for Japanese standards, multiple dams were installed for the main purpose of power generation. Reservoir sedimentation is progressing, leading to localized riverbed aggradation or degradation. The Kumano River basin has various issues in sediment management. Futatsuno Dam, managed by J-POWER, was completed in 1962 and is a power generation dam located at the downstream end of a dam cascade. With a gross storage of 43 million m3 and an average annual sediment volume of about 0.5 million m3, sedimentation has reached about 40% of the total reservoir capacity. We have been trying to excavate sediments from the reservoirs to avoid upstream flood risk. However, this method is not sustainable due to limitations on the amount and location of excavation, difficulties in finding disposal sites, the high cost associated with sediment removal, and environmental issues. As a permanent solution, J-POWER is considering installing a sediment bypass tunnel. In order to evaluate the potential environmental impacts on the downstream river caused by the tunnel operation, trial sediment replenishment has been proposed. In this paper, we used one-dimensional (1D) riverbed variation analysis to predict long-term changes in sediment transportation in the river channel downstream due to sediment replenishment. Based on the results, we discussed the appropriate amount of sediment replenishment volume and proposed a monitoring plan before the sediment bypass project starts. In addition, environmental surveys on the current river channel were conducted based on the proposed monitoring plan. It was confirmed that the river reach downstream of the dam is clearly armored and features low biodiversity. It is expected that these conditions will be improved through sediment replenishment.