Study on the Koshibu Dam sediment bypass tunnel operation based on sediment transport monitoring in upstream reaches

Abstract

SBTs (sediment bypass tunnels) are a leading technique to mitigate reservoir sedimentation. SBTs consist of a tunnel connecting the upstream and downstream reaches of a dam and diverts sediment-laden flood directly during flood events. A difficulty in operating SBTs is to reduce sediment inflow into the reservoir effectively, and simultaneously sustaining dam functions such as flood mitigation and hydropower generation. To optimize the problem in controlling the timings required in opening and closing the gates in the SBTs, which are governed by the hydrograph and inflow sediment data. Despite many studies on SBT operations considering hydrograph, there are few of those on the temporal change of sediment inflow. Ultimately, this study aims at improving SBT operations by understanding sediment inflow from upstream reaches using an indirect bedload monitoring system called an impact sensor. For this study, the Koshibu River basin was chosen because impact sensors are already placed in several locations around the basin including inside of the SBT. This observation has revealed the amount of transported sediment during each season in a year, are shown with relations between hydrograph and sediment inflow, and the spatial transient of sediment transport peak levels. According to these observations, desirable rules for SBT operations are suggested.