Levee system transformation and its impacts on the human-water system in the Kiso River Basin, Japan

Societies decide whether to try to protect themselves against floods (fight) or live with floods (adapt). Levees and levee systems are important factors in determining whether a society fights or adapts; however, these factors have been considered as fixed boundaries in previous studies. We analyse a levee system transformation process covering the past century, from the indigenous ring levee system to modern continuous levees, and the impacts of this changes on human-water dynamics in the Kiso River basin, Japan. In this study, historical maps were digitized to detect and analyze changes in the shape of the levee system over the past century in order to spatially and quantitatively analyze the characteristics of the historical landscape. In addition, we performed a quadratic trend and narrative analysis of several socio-hydrological variables.

The results reveal an interactive relationship between technologies and the human-flood system; the transformation of the levee system affects local communities and local culture, while social changes affect the local water management framework, including the levee system. With these interactions, Japanese society has shifted from adapting to and living with floods to fighting against them, thus characterizing the levee system transformation process. The relationship between a levee system transformation and the human-flood system can be represented as a causal loop diagram. First, the levee system transformation in the Kiso River basin shows that there is a trade-off between modern continuous levees and indigenous ring levees. The construction of continuous levees began with the opening of Japan and the westernization of society, and their development was accelerated by social and hydrological drivers/trends such as flooding, war-induced food shortages, industrialization, economic growth, and population growth/urbanization. The increased extension of continuous levees reduced the flooding frequency and reduces people’s memories of flooding . These changes led to an increased population within the floodplain, a decreased mitigation capacity for floods (decrease in the number of flood fighters), and decreased needs for ring levees. On the other hand, the 1976 flood ended this downwards trend in the extension of ring levees, and this reminder of floods potentially triggered a reevaluation of traditional technologies. In general, reevaluations of the versatility and flexibility of traditional or indigenous technologies have often emerged in the face of various water-related crises

This study provides a case of a paradigm shift in society from "adapting to" to "fighting" floods. In the process, these two different societal modes coexisted in one region, though the dominant society transitioned over time alongside technological transformations. These changes dramatically transformed underdeveloped societies, resulting in rapid economic growth while simultaneously causing extreme changes in the original dynamics of human-water interactions, thus generating different challenges. This study strongly suggests the need for water scientists to interdisciplinary observe the historical coevolution between human and water systems to accurately understand related dynamics and processes.