The Tokyo Metropolitan Area External Drainage SystemThe “G-Cans Project” also known as the Tokyo Underground Tunnel, is an underground hydraulic infrastructure for the control of floodsand is the largest in the world. It was built between 1992 and 2006, at a cost of approximately 2.5 billion euros, and consists of a complex of 6.4 km of tunnellocated up to 50 meters below the surface, which connect 5 giant silos to a single large reservoir known as “the temple”. This flood water drainage system is capable of pumping out of the structure 200 m³/s of water. Since the city of Tokyo is located in an area below sea level and is therefore more exposed to the risk of flooding, this underground structure, made up of tunnels, serves as a flood control system, making it a true masterpiece of modern engineering.
How do G-Cans work and what are these flood tunnels?
Flood water is collected through drainage channels around the city which then flow into 5 silos located along the underground tunnel. Each of the silos is 65 meters deep and 32 meters wideand is large enough to fit a Statue of Liberty or a space shuttle inside. When the silos fill up, according to the simple principle of communicating vesselsthe water makes its way to a tank located at the end of the tunnel, measuring 25 meters high, 177 meters long and 78 meters wide. Once the emergency on the surface is over, four engine turbinessimilar to those on an airliner, push the water from the reservoir back out into the nearby Edo River with such force that it pumps 200 cubic meters of water per secondwhich is like emptying an Olympic-sized swimming pool in just 12 seconds.
The Benefits of G-Cans
The G-Cans system is located in Saitama Prefecture, in an area rich in rivers and streams that flow from the north of the country towards the Greater Tokyo Area. Since its completion, it is estimated that it has reduced 80% of damage caused by floodsprotecting an area of approximately 100 square metres from the danger of water 13 million inhabitants. The Tokyo Central Disaster Management Council has calculated that if a rainfall of 550 millimeters were to hit the city for three days, it would cause the Arakawa River to overflow and inundate 97 subway stations. A scenario that statistically could happen once every 200 years, but which G-Cans would still help to counteract. This is the largest existing flood control structure in the worlda prevention system that is in some ways even exaggerated, so much so that it has been the subject of various discussions regarding its cost-benefits.
Guided Tours in Tokyo’s Underground Tunnel
On average It fills up 7 times a yearand on the remaining days, when it is dry, it even welcomes visitors, with guided tours conducted, unfortunately, only in Japanese. The highlight of the tour is undoubtedly the reservoir, known as “the Temple” because of its grandeur and the 59 supporting pillars that recall those of a Greek temple. Down here the atmosphere is truly mystical, fueled by the mist that is created by condensation due to the large difference in temperature between the outside and the underground. This reservoir has been and is the location for many films, TV series and music videos, including a chapter of the Hunger Games series. And in fact, where else can you find a place like this?
