Ministry sets 100mm rain standard for urban flood defenses, leaves funding to local governments

The land ministry's new policy targets inland flooding from cloudbursts like August's in Chiba, but the cost of drainage and storage facilities falls on municipalities.

By Newsroom13 November 2026, 08:45 JST1 min read

Ministry sets 100mm rain standard for urban flood defenses, leaves funding to local governments — Land, Infrastructure, Transport and Tourism Minister Ibayashi at a press conference at the ministry on the morning of the 10th. (x)

The land ministry published a policy on Tuesday to reduce damage from inland flooding, urging local governments to build drainage and storage facilities to handle rainfall of up to 100 millimeters an hour, Kyodo News reported.

内水氾濫 (naisui hanran), or inland flooding, is what happens when storm water overwhelms city drainage and spills into streets and low-lying ground rather than over a riverbank.

The policy follows a string of flood damage around the country, including heavy rain in Chiba in August.

Under current guidelines for local governments, districts where urban functions are concentrated are asked to build drainage able to withstand roughly 60 to 70 millimeters of rain an hour.

The ministry will set a new standard of 100 millimeters an hour for hazardous spots such as hollows in residential areas. It cited the growing severity of heavy-rain disasters as the climate changes.

The ministry also plans to step up the release of information on flood risk.