2018年排砂バイパス運用による小渋ダム下流における河岸侵食と流路変化

Abstract

Sediment bypass tunnel (SBT) of Koshibu Dam was constructed in 2016 to reduce reservoir sedimentation and to promote downstream sediment transport. In 2018, significant channel changes occurred in the 5-km downstream reach by three events of sediment transport through SBT. Several bank erosions including failures of a riverside maintenance road occurred during the first event in early July. Bank erosions were extended much inner or downstream side, and in an unconfined reach the main flow was laterally shifted to a previously vegetated floodplain during the second and third events in middle and late September. Total deposition and erosion of sediment in the reach in this year was estimated to be about 63, 000-73, 000 and 75, 000-76, 000 m3. Many of the bank erosions occurred closed to a developed gravel bar. Processes of sediment transport that lead to a multiple occurrence of bank erosion during a single event were discussed. These events provide us insight on how to rework of stabilized sediment storage in the river channel.