The world’s biggest drain

What does the planet’s largest subterranean flood diversion facility actually do?

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The world’s biggest drain

Shutoken Gaikaku Housui Ro, otherwise known as G-Cans or the Metropolitan Area Outer Underground Discharge Tunnel, is essentially the world’s largest drain. It can be found underground between the Showa region of Tokyo, Kasukabe in Saitama prefecture and the outskirts of Tokyo City. Loosely speaking it performs the same function as a standard drain – that hole in the road with a metal grille over the top that we encounter every day, which ultimately diverts excess surface water to avoid flooding. But it’s on a totally different scale and much more sophisticated than that.

Five enormous silos, each 65 metres (213 feet) deep and 32 metres (105 feet) wide are spaced at regular intervals, within a certain distance from Tokyo’s main rivers, including the Oochi Kotone, Kuramatsu, Arakawa and Nakagawa. They’re connected by 6.5 kilometres (four miles) of tunnels 11 metres (35 feet) high and 50 metres (164 feet) under the ground that act as a flow regulator for floodwater. The real show-stopper though has to be the water storage tank into which this tunnel network empties.

The storage tank – aka the ‘Underground Temple’ – is a marvel of engineering. 177 metres (580 feet) long and 25 metres (83 feet) tall, it’s supported by 59 pillars and connected to 78 pumps. These in turn connect to ten-megawatt (14,000-horsepower) turbines that are monitored by a control room also located in the tunnel. These turbines enable G-Cans to pump tons of water out on a safer course farther upstream.

G-Cans was built to protect Tokyo City from flooding, which it is particularly prone to during typhoon season. This facility channels surface floodwater that can’t be handled by the normal drainage system into the silos and then out to the Edogawa River on the outskirts of the city.

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