Floods and Probability: A Technological and Social History of Design Flood in Modern Japan

The target discharge for a flood prevention plan is called the “design flood”. In modern flood prevention planning, a design flood volume (m3/s) is defined at one or more points along a river, and all engineering works are designed to safely carry that flood volume within the river channel. This flood volume indicates the degree of safety against flooding in an area and is a crucial value from a social and environmental context. Therefore, this volume and setting methods are often the subject of attention and criticism in opposition movements for the mega river construction projects such as dam reservoirs.

The current design flood in Japan is based on a probability method: the scale of design floods is expressed in terms of return periods, such as once every 200 years or once every 100 years. However, the methods are diverse, with some countries, the United States and China, using the past or maximum possible flood discharge as the design flood, while others, the Netherlands and Japan, use different return periods. Why does Japan currently use a probabilistic approach? What social and scientific historical backgrounds exist there?
This book clarifies the historical transformation process of design flood setting methods in Japan since the beginning of the modern era.

In the modern era, Japan introduced modern river engineering from the Netherlands, and the initial design floods were set using the historical maximum method (same), which was suddenly changed to a probabilistic method after World War II. This process illustrates the historical interaction process among water science technologies and society in Japan over the design flood setting method.