Flood Disaster Mitigation Strategy in the Early Modern Age in Japan (The Beginning of 17th to the Mid-19th Century)

Rain runoff exceeding river channel capacities has been increasing in Japan. Although excess runoff events are still local and not very frequent, this trend is thought to be growing due to global climate change, and the government is discussing the need for flexible disaster mitigation measures by allowing river overflow to enter floodplains. However, the framework for formulating facility design based on this policy has not yet been established. Nevertheless, civil engineers of the early modern age developed flood control systems to avoid catastrophic flooding in important areas by inducing deliberate and safe river overflows without knowledge of modern hydraulics. This paper discusses the flood control strategy common in early modern times using numerical case studies with a shallow water model on three typical types of flood control systems. The results suggest that civil engineers at that time understood the natural flow tendency during flooding based on floodplain topography measurements together with flood trace inspections, allowing them to deal with excess runoff.