Monitoring of sediment runoff and observation basin for sediment movements focused on active sediment control in Jo-Gan-Ji River

Abstract

Continuously measuring sediment runoff along multiple sections of the Jo-Gan-Ji River is necessary to understand both the propagation of sediment as well as the changing of grain sizes in order to appropriately evaluate sediment yielding from debris flows temporally and spatially. The present study proposes a combination of sediment monitoring tools and appropriate equipment to identify various sediment transport modes from wash load to bedload in mountainous torrents. As a result of monitoring runoff volume and grain sizes, sediment management can be achieved. In the Jo-Gan-Ji River basin in Japan, temporal and longitudinal sediment runoff has been measured continuously since the 1990's. Previous studies help determine the proper instrumentation suite for this type of sediment runoff monitoring. Bedload is measured with a Reid-type bedload slot sampler and by use of the hydrophone to survey acoustic waves. In addition, hydrophones and a velocity meter (vertically installed on a side wall) are used to quantify suspended loads. A turbidity meter is also used to measure wash load. Propagation of sediment particles can be observed during flooding in mountainous torrents. Specifically, bedload discharge rates of each particle are evaluated using of the hydrophone. Monitoring of the Jo-Gan-Ji river also identifies inactive bedload movements such as large boulders. Previous installations of this type monitoring equipment make it clear that the destructive nature of bedload collisions indicate a need for robust instruments. Alternate instrumentation methods, that are robust, are explored here. Moreover, in order to actively control sediment runoff in flooding, we developed a sabo dam with shutter and pilot operations that activate during flooding. Differences of those sediment transport characteristics with/without the shutter also shown through the sediment monitoring along the Jo-Gan-Ji River.