Study on Flood Risk Assessment Caused by Functional Deterioration of Dam Flood Control Gates

In recent years, dysfunction or malfunction of dam spillways or bottom outlets have been reported because of aging and sedimentation in existing dams worldwide. They affect discharge capacity of dams which may decrease suitable flood control functions. Therefore, in this research, risk factors for Japanese dams that cause functional deterioration of flood control ability have been discussed in order to extract high-risk dams. The risk factors for bottom outlets include inlet elevation over sedimentation level, number of outlet structures and existence of sunken large wood debris. In the risk assessment procedure, "n-1 rule" (n: number of gate) which is design criteria for securing the safety of dams in Switzerland is considered. We conducted case studies on the Susobana Dam in Nagano Prefecture to clarify actual risk after losing flood control capacity as well as numerical modelling to assess flow conditions acting on sunken large wood debris to be dragged into outlet gate