Tokyo is upgrading its flood defense systems to counter the increasing threat of extreme storms fueled by climate change. The city’s Metropolitan Outer Area Underground Discharge Channel, a massive network of tunnels and reservoirs, has played a crucial role in preventing major flooding since it became operational in 2006.
In August 2024, Typhoon Shanshan brought torrential rains to Tokyo, even though the storm’s center was nearly 400 miles away.
The storm filled a vast underground chamber known as the “cathedral,” part of the discharge channel.
The system has saved Tokyo from billions of dollars in potential flood damage by diverting water from densely populated areas.
The cathedral chamber, located six stories underground, contains 59 pillars that each weigh more than 1.1 million pounds and stand nearly 60 feet tall.
These pillars support a reservoir capable of holding water equivalent to nearly 100 Olympic-sized swimming pools. The chamber’s drop shaft is large enough to fit the Statue of Liberty inside.
Despite the system’s effectiveness, Tokyo’s defenses are under increasing pressure as storms grow more frequent and severe.
Sudden downpours that release large amounts of rain in a short time are testing the city’s infrastructure.
To address this, Tokyo has launched a seven-year, $250 million initiative to expand and upgrade its flood defenses. This includes reinforcing levees, improving drainage, and building new reservoirs to increase capacity.
Tokyo’s current sewer system is designed to handle up to 3 inches of rain per hour, but recent storms have brought as much as 4 inches, pushing the system beyond its limits.
New projects aim to raise this threshold to better manage extreme weather events.
The Metropolitan Outer Area Underground Discharge Channel, which cost $1.63 billion to build over 13 years, has become a global model for urban flood control.
Officials say further upgrades to the system will be completed by 2027 to ensure it can meet the growing challenges posed by climate change.
