Study on the mechanism of high turbidity flow during sediment flushing operation and its environmental impact assessment

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In the Kurobe River, coordinated sediment flushing operations of Dashidaira and Unazuki dams have been executed since 2001. Sediment flushing is the most economically effective reservoir sediment management method and it can contribute to sediment supply to the downstream from the viewpoint of the comprehensive sediment management in a sediment routing system. On the other hand, environmental damages may occur if we do not care biological acceptance downstream since huge amount of sediment will be discharged in a short period.This study focuses on the mechanism of high turbidity flow during sediment flushing operation and the assessment method of the environmental impacts. Study topics are as follows.1)Inflow and deposition process of fine sediment in a reservoir, and the mechanism of high turbidity flow during sediment flushing operationSediment-traps are installed in the Unazuki reservoir, and fine sediments inflow and deposited in the reservoir have been analyzed in an every month by grain size distribution, TOC, concentration of iron oxide (II) and so on which may influence on water quality changes during sediment flushing operations. Sediment and water mixture surface profile which eroded during sediment flushing operation have been measured by the 3D Laser scanner. Surface velocities have been measured by PIV method. Based on these data, relationship between reservoir water level and sediment erosion phenomena, and related parameters such as water discharge, tractive force and drawdown speed is discussed.2)Impact assessment method on the water quality changes caused by the operationWhen, how high and how long the high turbidity will occur are the main topics and other water quality changes such as DO decrease are also discussed. Assessment method to compare environmental impacts by these water quality changes is proposed in regard to SS, DO and grain size distribution.