At the core of the plan, approved by the Hanoi People's Council in late March, is a multi-functional underground network featuring a large-scale water storage system with a total capacity of about 125 million cubic meters, slated for development between 2036 and 2045.

City planners estimate the project could help reduce annual flood-related damages by trillions of Vietnam dong (VND1 trillion = $37.97 million).

To put the scale into perspective, the proposed capacity is roughly equivalent to 50,000 Olympic-sized swimming pools, far exceeding most existing urban flood control systems globally.

Notably, it is projected to be around 180 times larger than Tokyo’s Metropolitan Area Outer Underground Discharge Channel, currently the world’s largest underground flood diversion facility.

The scale of Hanoi’s proposal reflects mounting pressure from increasingly extreme weather patterns. The city frequently experiences heavy rainfall events of 300-500 mm within a single day. Rapid urbanization, extensive concrete surfaces, and low-lying terrain have significantly reduced natural drainage capacity, amplifying flood risks.

The proposed system would operate on principles similar to Tokyo’s model. Rainwater would be collected through an extensive network of underground tunnels and directed into central storage reservoirs. When river conditions permit, the stored water would be pumped out into major waterways such as the Red River and Duong River, alleviating pressure on inner-city drainage systems.

Beyond flood control, the underground infrastructure strategy is expected to support broader urban objectives, including reducing traffic congestion through integrated underground transport corridors and mitigating pollution via improved water management.

In 2025, Hanoi posted an impressive growth rate of 8.16%, with GRDP of $63 billion, ranking second nationwide. The current population is over 8.8 million.