Paradigm Shifts on Flood Risk Management in Japan: Detecting Triggers of Design Flood Revisions in the Modern Era

Flood risk management (FRM) has repeatedly evolved through paradigm shifts in 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. It has been historically revised and increased to reflect the sociohydrological situation of each era. Through this study we aimed to identify these changes to FRM and their reasons (after the advent of the modern society in Japan), focusing on the design flood revision triggers in 109 river basins. We extracted all these triggers through critical reviews of governmental reports (and other historical resources), then tried to identify and divide this time period into a few eras, based on these shifts. In addition, we performed a quantitative trend analysis of several sociohydrological variables that contributed to (or refracted) the shifts and conducted a qualitative analysis of their sociohydrological backgrounds. The revision triggers could be classified into five categories: “national policy change,” “mega flood,” “dam construction,” “economic growth,” and “others.” From the transition of triggers, 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.” The paradigms shifted in each era due to variations in both socio‐hydrological events and backgrounds. The results showed that the Japanese FRM paradigm has shifted from “green society” to “technological society,” with respect to human‐technology‐flood evolution.