Risk Analysis for Functional Loss of a Bottom Outlet Gate due to the Passage of Sediment and Submerged Wood Debris

Abstract

In 2017, the gate of the bottom outlet at the Susobana Dam in Nagano Prefecture became inoperable during flood control due to sediment and submerged wood debris that reached the level of the bottom outlet. This accident is an unacceptable risk for the function of the dam, and it is necessary to identify the causes and take countermeasures to prevent recurrence, including at other dams. In this study, a fault tree for gate inoperability due to sediment and submerged wood debris was developed for the high-pressure roller gate of the bottom outlet, which does not design for the passage of sediment and submerged wood debris. Furthermore, we organized each risk factor systematically and quantitatively.