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 out skirts 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.
Boring the G-Cans tunnel
Tokyo has a history of major flooding dating back long before the city was even founded, but in the 20th century – with its population growing -monsoon rains and typhoons claimed the lives of thousands and destroyed millions of homes. So in 1992, work began on the G-Cans project to mitigate the effects of any such disaster. Due to the depth of G-Cans and the area’s soft soil, engineers employed a tunnel-boring technique known as the shield method. The tunnel was bored using huge metal cylinder (the shield), followed by a series of hydraulic jacks that push the machine forward and a system that erects concrete support segments. Because of the scale of the project, size of the tunnel and wet conditions, a slurry shield borer was also used, which uses chemical additives to soak up the moisture in the soil and stabilize the tunnel.
Located next to Tokyo Bay in the Kanto region of central Honshu -and with over 13 million inhabitants – Tokyo spans the alluvial floodplain of three major rivers in the area. The Edogawa and Arakawa Rivers meander around Tokyo’s outskirts, while the Sumida River flows right through the centre. When the weather is relatively dry in springtime and late autumn/winter, living in one of the many regions of Tokyo that lie below the flood level isn’t a problem. But the rainy season (or tsuyu) hits Japan in June and July, while typhoon season peaks in late-August through to October. A major part of Japan’s annual average 1,800 millimeters (70.9 inches) of precipitation occurs during these months and devastating flash floods, as well as tidal surges brought on by terrible typhoon winds, can sometimes wreak havoc on communities -even with the additional flood protection afforded by G-Cans.