01 · The problem
What is happening
Eastern Tokyo's lowlands between the Arakawa and Edogawa rivers include zero-metre areas lying at or below sea level, partly due to historical subsidence from groundwater pumping. Typhoons such as Hagibis in 2019 brought heavy rainfall that tested the region's defences. Infrastructure such as the Metropolitan Area Outer Underground Discharge Channel in Saitama, with its vast underground tunnels and tanks, diverts floodwater from smaller rivers into the Edogawa.
Moderate severity. Material effects on people or ecosystems; actively managed. Severity is descriptive editorial metadata summarising documented effects. It is not a measured index or a score.
02 · Causes
Why it is happening
- 01
Low-lying zero-metre areas
Parts of the city lie below the water level of nearby rivers during floods.
- 02
Typhoon rainfall
Tropical cyclones can bring intense rainfall across river catchments.
- 03
Historical subsidence
Groundwater pumping in the twentieth century lowered land levels before restrictions were imposed.
03 · Impacts
Who and what it affects
- 01
Potential for widespread inundation
A major levee failure could flood large residential areas for extended periods.
- 02
Evacuation challenges
Evacuating millions of residents from lowlands is a major planning challenge.
04 · Solutions
Documented approaches
Underground discharge channels and reservoirs
Large tunnels and storage beneath the city divert and hold floodwater.
Expected effectReduces flooding along smaller rivers during heavy rain.
Super levees
Very wide levees designed to resist overtopping and breaching.
Expected effectImproves resilience of the highest-risk river sections.
Groundwater controls
Restrictions on pumping have largely halted subsidence.
Expected effectPrevents further lowering of the land.
05 · Key indicators
What can be measured
No reliable quantitative indicator is included for this issue. TerraLens shows figures only where a credible source supports them — consult the sources below for current data.
06 · Sources
Where this comes from
- [1]Ministry of Land, Infrastructure, Transport and TourismGovernment of Japanmlit.go.jp(opens in a new tab)
- [2]Japan Meteorological AgencyGovernment of Japanjma.go.jp(opens in a new tab)
Links open the publishing organisation's site. Some point to the publisher's main page, where the named report or dataset can be found.