Winter 2016

G-Cans

Rain Rain Go Away: Underground Flood Management in Tokyo

Rivers have always sustained and supported Tokyo, providing water for agriculture and moving goods along the city’s waterways. However, when it rains these rivers can become Tokyo’s greatest threat. Over the years various systems have been developed to control the flow of water through Tokyo, nevertheless a series of strong storms overwhelmed the city’s flood defenses in 1980s and 1990s. These storms caused massive flooding and damage to large portions of the city and left city planners scrambling to find new ways to protect the city from increasingly heavy rainfall. Normally, the simplest solution would be to build a reservoir; the question was how to build a reservoir in a city twice as dense as New York. The answer: build it underground.

One system devised to bolster Tokyo’s flood defenses is The Metropolitan Area Outer Underground Discharge Channel, which was completed in 2009. Also known as G-Cans, the discharge channel is the world’s largest underground flood water diversion system. It took 17 years and billions of dollars to compete the project, but its benefits to the city are immeasurable.

G-Cans is connected to five of Tokyo’s rivers via vertical drain shafts, each large enough to hold the Statue of Liberty with room to spare. These drains are connected by a four mile long underground tunnel, through which excess water from heavy rain travels to be collected in the channel’s massive subterranean reservoir, dubbed The Underground Temple. Once the storm subsides, the water in the reservoir gets pumped into the Edo River at a combined rate of 53,000 gallons per second, which is the equivalent of completely draining one Olympic-sized swimming pool every 12.5 seconds. When it is not filled with water The Underground Temple is a popular tourist attraction and has even been used as a movie and television commercial set.

In recent decades the intensity and frequency of rain storms in Tokyo has increased dramatically. On average Tokyo receives around 60 inches of annual rainfall. That is about the same as Miami, Florida, which receives over fifty percent more rain than the U.S. national average. By comparison, New York City receives approximately 45 inches of rain per year. In recent years localized downpours called “Guerilla rainstorms” have also become a problem in Tokyo, further taxing local drainage systems and providing impetus for the expansion of Tokyo’s water diversion network.

The Furukawa Underground Regulating Reservoir, which should be completed later this year, will be the next addition to Tokyo’s network of underground water storage facilities. The Furukawa River winds through Tokyo’s Shibuya and Minato wards, which are some of Tokyo’s most densely packed neighborhoods. Due to space constraints the reservoir is being built 50 feet directly underneath the course of the river, which often makes acute turns that complicate its construction. When complete, the two mile long, 26 foot diameter drainage tunnel will be able to hold 35,663,227 gallons of storm water, which is equal to the volume of 54 Olympic-sized swimming pools.

Systems like the Metropolitan Area Outer Underground Discharge Channel and Furukawa Underground Regulating Reservoir have dramatically reduced the flooding and damage caused by heavy rain storms. It is also largely thanks to these kinds of underground and out of sight systems protecting Tokyo that it is uncommon to see pools of standing water on the city’s streets, even after the heaviest of rains. As Tokyo continues to adapt to changing weather patterns, it is likely that these systems will continue to play a key role in protecting the city for many years to come.



Banner Photo: The Metropolitan Area Outer Underground Discharge Channel Reservoir.

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Japan Info is a publication of the Consulate General of Japan in New York. However, the opinions and materials contained herein do not necessarily represent the views or policies of the Government of Japan.

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