Japan to review flood plans as panel warns of more torrential rain from global warming

TOKYO -- The amount of torrential rain is likely to increase 1.1 times on average across Japan in the future due to global warming, and the rise should be factored into flood-control plans for state-managed rivers, a land ministry panel recommended on May 31.

Up until now, river management plans had been created for individual river systems based on the maximum rainfall each region had received during past downpours. But following the recommendation in a proposal drafted by a panel of experts at the Ministry of Land, Infrastructure, Transport and Tourism, the ministry is poised to switch to a method that incorporates future predictions for climate change.

In the wake of major water damage in recent years, including torrential rain that hit western Japan in July 2018, the expert panel has advanced discussions so that measures be adopted to ensure people's safety even if the effects of climate change are seen. The ministry will review its river management plans based on a proposal by the expert panel due to be compiled as early as this summer.

The government is currently working to reduce greenhouse gas emissions under the Paris Agreement, an international framework to counter global warning, aiming to hold the increase in the global average temperature to under 2 degrees Celsius above pre-Industrial Revolution levels. The average temperature worldwide has already risen by about 1 degree Celsius, resulting in heavy downpours causing serious damage.

The expert panel considered the amount of rain that could hit Japan during torrential downpours of a scale occurring only about once a century, and calculated that if the average global temperature were to rise 2 C, rainfall during such downpours could rise by 1.1 times on average across Japan. The panel additionally calculated what would happen if no global warming countermeasures were adopted and the temperature rose 4 C above pre-industrial levels. In this scenario, the rainfall would increase by 1.3 times on average, or between 1.1 times and 1.4 times depending on the region, the panel found.

The panel's draft proposal recommended that river management plans be altered after calculating river flow volumes based on rainfall under a scenario in which the temperature rises 2 C or more. They additionally recommended that the scenario of an increase of 4 C be considered, and that floodgates and other infrastructure be designed based on that scenario.

If river management plans are revised, then it would become necessary to alter the design of levees, dam projects and drainage infrastructure.

Tomohito Yamada, an associate professor at Hokkaido University specializing in river engineering and hydrology who is serving as a member of the expert panel, commented, "From here on we need flood-control plans that include future predictions."

(Japanese original by Yuka Saito, Science & Environment News Department)