Socio-Hydrological Transition of Flood Risk Management and Levee Systems during the Modern Era in Japan

Flood risk management (FRM), which has been practiced since ancient times, has repeatedly evolved throughout human history. With the emergence of the Anthropocene epoch (the early part of the twentieth century), FRM was not recognized as an engineering measure but also as a social endeavor. The evolution of FRM has been in the context of natural and social backgrounds in each region and country. The Japanese modern FRM started in the Meiji Era (1868-1912) with the opening of Japan to the world. Before the Meiji Era, FRM in each basin was conducted as community-based flood mitigation, using either discontinuous levee systems known as "Kasumi levees" and "Wajyu levees" (ring levees), or elevated houses; the developing communities were forced to correspond to be the "green society" by Di Baldassarre et al. (2015). However, the Japanese society and FRM regime turned into "technological society" with using continuous levee systems, dam reservoirs, and other modern FRM technologies through the modernization. This study aimed to capture the processes of regime shifts on FRM and the levee systems during the modern era (1868-present) in Japan with focusing on the design flood revisions and spatial configuration of levee systems. We extracted all historical triggers of design flood revisions with critical reviews of governmental reports and other historical resources to detect certain periods of FRM regime in the history. In addition, we developed a spatial database of levee system and land-use in the Kiso River basin to capture the changing processes. The results showed that the Japanese modern history of FRM shifted from "Era 1: 1910-1935, Changing society," to "Era 2: 1935-1970, Response to mega floods," and "Era 3: 1970-2010, Response to economic growth." At the same time, the traditional discontinuous levee systems, the "Wajyu levees," have been replaced with the modern continuous levee systems: the length of Wajyu levees decreased by half, and on the other hand, the continuous levee doubled during the last century. We will discuss the importance of capturing the changing process of FRM and its technologies with developing historic and spatial database for a further understanding of human-water (flood) interactions.