Repair and maintenance plan for invert concrete of Koshibu Dam sediment bypass tunnel

Koshibu Dam was built in 1969. In autumn 2016, a sediment bypass tunnel (SBT) which is to prevent sedimentation of the reservoir and to increase sediment supply to downstream reaches was completed. During a large flood event in 2020, four years after the completion of the SBT, the prolonged operation of the SBT of Koshibu Dam resulted in the release of approximately 1.88 million cubic meters of sediment. This operation led to severe abrasion on the tunnel invert, causing the loss of the 450 mm thick invert and resulting in critical damage that necessitated extensive repairs. Based on the damage conditions of the invert, the mechanism of damage expansion is assumed, and based on the damage assessment of the facility, the restoration policy was established. The SBT resumed operation in 2023, three years after the 2020 disaster. To prevent catastrophic damage similar to that experienced in 2020, the resumption of operations was accompanied by the implementation of SBT operations based on abrasion predictions using the model proposed by Auel and the formulation of a continuous maintenance plan.