Japan Raises Drainage Standard to 100mm an Hour to Curb Inland Flooding

The land ministry's revised benchmark tells municipalities to build sewers and storage facilities that can handle the kind of downpour that submerged cars in Chiba this summer.

By Newsroom13 November 2026, 06:30 JST1 min read

Japan Raises Drainage Standard to 100mm an Hour to Curb Inland Flooding — New national standards for "inland flooding" countermeasures: strengthening sewer and drainage development for 100 mm of rain per hour (x)

The land ministry has set a new benchmark for municipal flood defenses, calling for sewers and rainwater storage facilities strong enough to withstand about 100 millimeters of rain an hour, as cities face downpours beyond what their drainage was built for.

The Ministry of Land, Infrastructure, Transport and Tourism revised the standards after heavy rain in Chiba in August caused inland flooding, known in Japanese as 内水氾濫 (naisui hanran) — water that backs up inside a city when drains and rivers cannot carry it away. Cars were submerged and homes were flooded above the floor.

Under the new benchmark, municipalities are to strengthen sewers and rainwater storage facilities so they can cope with roughly 100 millimeters of rain an hour. The ministry also plans to revise its manual for drawing up flood-hazard maps.

The 100mm figure marks a shift from drainage systems designed for gentler rainfall. Inland flooding has accounted for a growing share of urban rain damage as downpours intensify.