Five giant cisterns, 500-ton columns, and more than 6.5 kilometers of cavernous tunnels all lie below the world’s largest metropolis. Tokyo, one of the densest cities on the planet, is home to one of the most ambitious and enormous flood-control systems ever created. And it is almost entirely invisible to Tokyo residents.

Two massive construction projects protect Japan’s capital from the ever-present threat of floods. The first project, known as the Metropolitan Area Outer Underground Discharge Tunnel, or more simply as the “G-Cans Project,” was completed in 2009. A network of 3.3 kilometer tunnels, called “The Furukawa Reservoir,” will be in place later this year.

Tokyo’s full flood control system is an engineering marvel and a global trailblazer in dealing with water in rapidly expanding urban areas.  It is a leadership position the east Asian nation earned through hard experience.

Japan’s history with floods

Tokyo, and the rest of Japan, has a long history of devastating and deadly floods.

Image: Utagawa Kuniyosha (1797-1862)

A storm in 1910 destroyed 4.2% of the nation’s GDP. And storms throughout the 20th century left the rapidly urbanizing nation’s cities and towns underwater.

And just last year, floods ravaged cities in Japan’s northern regions.

Building flood protections

Starting in the 1920s, Japan began building water channels and an extensive set of levies. But storm after storm and flood after flood proved that the system was not enough. After a particularly deadly stream of six floods in the 1980s left tens of thousands of homes in the capital city damaged, national leaders decided Tokyo needed special attention.

The G-Cans Project

Commissioned in 1992, construction began on the “world’s largest drain” in 1993. The concept was relatively simple. The plan connected existing rivers and waterways to overflow pipes and drains. This allowed the above ground drainage system in central Tokyo to continue in operation, while its overall capacity was massively expanded underground.


The underground system was composed of 5 giant cisterns that collected excess water from rivers and waterways. Each cylindrical shaft measures about 70 meters in height and 30 meters in diameter, spacious enough to park a space shuttle.

These cisterns are connected by 6.5 kilometers of tunnels 10 meters wide buried 50 meters deep. The tunnels allowed the cisterns to provide capacity support to each other.

Image: Hiroshi Ayukawa

Six construction firms worked on these tunnels, in the process developing new digging and tunnel wall building methods that are now the international standard. At the cost of about $3 billion US dollars, the on the G-Cans Project was completed in 2006.

The pipes take the water from the cisterns to an enormous pressure-controlled tank. This tank measures 177 meters long, 78 meters wide, and 18 meters tall. 59 pillars of 500 tons each support the ceiling and break up the power of incoming water.

This enormous structure is dubbed “the temple” by visitors (there are tours during low-water periods of the year). The inside of the tank has even been used in music videos and movies.

In terms of flood safety, the pressure tank is a staging area before a set of four turbines pump the water into an area of the Edo river that can take the overflow safely out to sea. The pressurized nature of the tank allows the system to regulate in case one of the turbines breaks down.

The turbines are powered by jets similar to those used on a Boeing 737 airplane. The system is capable of discharging 200 cubic meters of water per second.

The massive drainage system is run through a state-of-the-art control center.

Measuring success

It can be hard to measure the impact of a preventative measure, because, by its nature, it prevents a damaging event from happening. By some estimates, the G-Can flood control system has cut flood damage in half. The construction’s byproducts have even been put to good use. New levies have been built up around flood prone rivers using the earth excavated to build the enormous underground system.  

In some ways, the success of the project has been its own worst enemy.

Impact on Urbanization

Tokyo and the surrounding area are much safer. The risk to homes, businesses and public services has reduced significantly. Making the area safer has encouraged more growth and even more urban sprawl. This is creating new problems.

Super levees designed to create public walking areas and shopping centers have played a part in new developments moving into areas that are historically flood prone. It seems the system designed to protect Tokyo from floods has been so successful it is encouraging growth into danger areas still relatively unprotected.

Still a threat

Continued urban expansion, the recent rise in storms across east Asia, and the prospect of further changes from climate change has city planners worried. Particularly when it looks like the system is being overwhelmed in some areas.

Video shot in 2011 by a Tokyo resident of the above ground drain systems (supported underneath by the G-Cans Project) during a typhoon shows what the city is still facing.

To avoid watching the full 8-plus minute, here are some of the highlights. The start of the video shows the water system before the typhoon has fully hit the region.

Image: YouTube: Will George

This screenshot from about 6:57 in the video shows the dangerously high level of the above ground flood system.

Image: YouTube: Will George

The uploader of the video noted that the full force of Typhoon Roke did not hit till later in the day when it was too dark to film. Looking at the water level, that is a scary concept.

The Furukawa Reservoir

In 2008, Japanese urban planners recognized that increased urban sprawl required a flood control solution beyond the G-Cans. The Furukawa Reservoir would utilize old and new concepts to protect the fast growing metropolis.

Similar to the above ground discharge waterways, the system would focus on moving water from flood areas into rivers that could carry the water to the ocean. Instead of giant cisterns, the system would use 3.3 kilometers of 7.5 meter diameter tunnels. But like the G-Cans system, the Furukawa project would be entirely underground.

“What makes this system particularly noteworthy is just its scale, because it’s underneath one of the largest cities in the world,” said Patrick Lynett, a civil engineering professor at the University of Southern California in Los Angeles. “It allows them to have this flood control out of sight.”

“Japan has no choice. With the lack of space they have, they have to come up with some ingenious way of doing this,” Marcelo H. Garcia, director of the Ven Te Chow Hydrosystems Laboratory at the University of Illinois at Urbana-Champaign, told the Japan Times.

The project should be completed later this year. It is a feat of construction only matched by its G-Cans peer. Starting with its connection to above ground water collection systems.

Then dropping down 52 meters to the underground tunnels.

When finished, water will be able to be pumped away from metropolitan areas, protecting lives and property.

The full capacity of the Furukawa Reservoir is expected to surpass 135,000 cubic meters of water, about the equivalent of 54 Olympic-size swimming pools.

A less waterlogged future

Completion of the Furukawa Reservoir is the second of three major underground flood control projects in Japan’s capital region. In the next few years, a third system featuring another 3.2 kilometers of massive tunnels in the northwest of Tokyo will be complete.

While the cost of these systems may not be reasonable for most countries, the concepts and lessons learned will be essential in a more and more urban world facing increasing threats from climate change.  

For more on the G-Cans system, check out this video from NDTV.


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