Adapting the city to heavier rainfall and increased flood risk

In small and medium-sized rivers managed by the Tokyo Metropolitan Government, it is necessary to prevent flooding damage up to a rainfall of 60 mm/hour, to keep the risk of house inundation at maximum 1/20, and to ensure the safety of life for the excess rainfall. There is concern that the risk of flooding will increase as the magnitude and frequency of heavy rain will increase. Flood risk scenarios and decision support is required to maintain flood safety.


Issue to be addressed

Extreme rainfall has increased in various places recently due to the influence of global warming, and the relationship between the magnitude and frequency will change in the future. In this case study, climate change indicators that the stakeholders in Tokyo can use for climate adaptation of flood risk are created. Among CIIs, 1) precipitation, 2) river flow/water level, and 3) inundation frequency are included. CII is planning to create rainfall obtained through climate scenario by inputting it into the storm runoff and inundation model.

Decision support to client

The final result will be used by Tokyo Metropolitan Government / to decide the policy on how to adapt small and medium-sized urban areas in Tokyo to increased flood risk. Furthermore, the results will help to make information on the expected impacts of climate change easier and more accessible to various stakeholders (e.g. policy makers in Tokyo).

Temporal and spatial scale

Adaptations will be implemented from 30 years at the small and medium sized urban river basin in Tokyo. The spatial scale for urban storm runoff and inundation analysis is about 10km2.

Knowledge brokering

Tokyo metropolitan university is founded by Tokyo Metropolitan Government. There is agreement between Department of Civil and Environmental Engineering of Tokyo Metropolitan University and Civil Engineering Support and Training Center under Bureau of Construction, TOKYO METROPOLITAN GOVERNMENT for research partnership on the social infrastructure related problems. We regularly have a meeting with each other at least once a month so far. Under the agreement, it is possible for us to promote this case study as cooperative research in near future. We had meeting with TMG. We have discussed on IDF design rainfall. We also decided to use event based rainfall from NHRCM.


Department of Civil and Environmental Engineering, Tokyo Metropolitan University (TMU)

Civil Engineering Support and Training CenterBureau of Construction of Tokyo Metropolitan Government

Poster displayed at the Kick–Off meeting, 7/8 September 2017, Norrköping, Sweden

Distribution of times for hourly rainfall of 50 mm