Dam upgrades for sediment sluicing in the Mimikawa river cascade following major typhoon events

The damage caused by Typhoon Nabi in the Mimikawa river basin, Japan, in 2005 resulted in significant sediment yield into mountainous areas, necessitating full-scale countermeasures to prevent sedimentation in reservoirs. In response, Miyazaki Prefecture formulated the Mimikawa River Basin Integrated Sediment Management Plan in 2011, covering the entire basin from the mountains to the sea. A core component of this plan is sediment sluicing operations at the dams along the main stream of the Mimikawa river basin, a project which began in 2017 at Saigou and Ouchibaru dams, and later at the Yamasubaru dam in 2022. This paper presents the rationale for sediment management at dams in the Mimikawa basin and evaluates sediment sluicing as a sediment management technique. It also discusses the advanced dam upgrades that have been implemented to enable sediment sluicing, the subsequent changes in riverbed geomorphology below the dams after sediment sluicing, and the associated environmental impact assessment.