Recent Flood Control Measures for Urban Rivers in Japan: Case Study of the Kanda River in Tokyo

Recently, floods occurred frequently almost everywhere in urban areas in Japan. These urban disasters became an important social problem. In the present study, we investigated floods on the Kanda River in Tokyo. The history of flooded areas, and flood control projects in progress, involving river improvements, expansion of sewerage projects, and urbanization, are reviewed. We also have presented our recent survey results for the flood of August 27, 1993 that occurred in the Kanda River Basin. Hydraulic and hydrologic impacts of historical floods are also given, including an investigation of the economic and social impacts. In urban areas, it is difficult to increase the width of the river. Thus, river improvement works essentially consist of the use of underground diversion channels. In Tokyo, Takadanobaba, Edogawabashi, and Suidobashi diversion channels have been completed. Regulating reservoirs along these rivercourses have been constructed in the Myoshoji River Basin and Zenpukuji River Basin. It is quite difficult to buy land along river courses in Japan. An interesting decision was recently made about the 75mm Plan, which introduces a new method for an underground river; the so-called Tokyo Loop 7. The underground river consists of a pipe having a diameter of 10.0-12.5 m. Loop 7 starts from the upstream section of the Shirako River; and draws flood water from ten different rivers, including the Myoshoji, Zenpukuji, and Kanda rivers. The total length of the pipe is 30 km. The flood water through Loop 7 is pumped out to Tokyo Bay.