ABSTRACT
Since the 20th century, the flood risk in river cities worldwide has increased significantly due to economic development and population growth. For instance, Tokyo (the capital of Japan) has grown from about 3.7 million inhabitants in 1920 to more than 12.8 million people in 2008 (Statistics Bureau of Japan, 2010), and the population in the Dutch city of Rotterdam has grown from 0.3 million people in 1900 to 0.6 million people in 2010 (COS, 2010). These developments have increased the potential consequences of a flood event because more people and property are at risk of flooding. As a result, economic damage and social disruption caused especially by storm surge and fluvial floods are higher. Moreover, the transformation of rural land into urban land has caused an increase in the probability of pluvial floods. Since the end of the 19th century, in many cities canals have been transformed into roads and train tracks and the construction of residential districts has caused the transformation of green areas into paved areas that contain dwellings and apartment blocks. These land-use changes have reduced the ability of the subsoil to absorb rainwater.
