Paradigm Shifts on Flood Risk Management in Japan and Synchronism of Socio-hydrological Histories

"Paradigm," well known as Kuhn's theory, has been applied not only to history and many other sociological fields, but also policy analyses including water resource management. Flood risk management (FRM), which has been practiced since ancient times, has repeatedly evolved throughout human history. In modern FRM, a design flood, which reflects the protection standard or safety level of the FRM, is one of the most important elements. The design flood has been historically revised and increased to reflect the socio-hydrological situation of each era. This study aimed to capture the paradigm shifts on FRM and its reasons in Japanese modern society with focusing on the triggers of the design flood revisions in 109 river basins. We extracted all historical triggers of design flood revisions with critical reviews of governmental reports and other historical resources, and detect and divide into certain periods of paradigm in the history. In addition, we conducted quantitative trend analysis of several socio-hydrological variables that contributed to the shifts, and a qualitative analysis of their socio-hydrological backgrounds. The results showed that the Japanese modern history of FRM was divided into three eras: "Era 1: 1910-1935, Changing society," "Era 2: 1935-1970, Response to mega floods," and "Era 3: 1970-2010, Response to economic growth". These categories of each era can be shown as driving-forces for paradigm shifts in FRM: change in the governing structure and society, catastrophic floods, and rapid economic growth. Moreover, through these paradigm shifts, the technologies and measures of FRM in Japan have also evolved to address socio-hydrological changes. In this presentation, we will especially discuss a "synchronism" of FRM histories and paradigm shifts between regions and counties based on this results. Because this kind of paradigm shifts on FRM can be found in other centuries and regions during the same periods: Netherlands, U.K, and U.S.. This discussion will lead to a perspective of global historical study in the field of comparative socio-hydrology.