Japan, being a mountainous country, has lots of rivers. Even the relatively flat plain on which Tokyo is built is crossed by a number of rivers, carrying water from the mountains to the Pacific Ocean. These rivers, often rising in mountains just tens of kilometers from the city, are prone to flash flooding whenever there is heavy rain.
The Sumida River, the largest of the rivers running through central Tokyo, is also tidal for 23 kilometers inland from its mouth, making it particularly vulnerable to such flooding. For this reason, much of the Sumida’s flow was diverted to the Arakawa River a century ago, but the risk of flooding remains. A more recent flood control measure, the G-Can Project, is also a fascinating tourist destination.
Completed in 2006, the project involves diverting water from the Sumida River upstream in Saitama whenever the river’s waters reach a certain level. The water flows 6.3 kilometers through an underground aqueduct 50 meters below ground (reportedly the deepest such aqueduct in the world) and collects in several large cisterns, one after another, ultimately arriving in the so-called surge tank, known as the Metropolitan Area Outer Underground Discharge Channel, near Kasukabe, Saitama. From there it is slowly pumped into the Edogawa River, which is broader with wider banks and can therefore handle the extra water more readily.
A tour of the G-Can Project begins on the second floor of the Ryukyukan Showa Drainage Pump Station with a video presentation of how the flood control system works, followed by an explanation of the water flow using a diorama. After a safety briefing, tourists walk to the entrance to the surge tank, where they descend about 100 steps to the bottom of the massive tank. This is the real highlight.
Tours can only take place when the tank is empty, but even then the air is often misty, giving the tank an eerie, lost world, feeling and making it difficult to see the outer dimensions of the tank, which measures 177 meters by 78 meters and is 25 meters deep. The tank itself weighs 500 tons and contains 59 pillars that anchor the tank in the ground and keep it from being pushed upward by ground water and other pressures.
There may be patches of water on the floor as well, giving one the impression that the tank was only just recently full. The water actually enters the tank from a large pit at one end (remember, the aqueduct is 50 meters below ground), and is pumped out to the Edogawa from the opposite end. Up to 67,000 cubic meters of water can be stored in the tank if necessary, before being disgorged into the river by 14,000 horsepower turbines at the rate of 200 tons per second.
Tours are conducted 3 times daily, from 10 a.m., 1 p.m. and 3 p.m., Tuesday through Friday. The tour takes about 90 minutes. Non-slip footwear is required.
To join a tour, reservations are required, and must be made on-line or by phone at least one week in advance. The tours are conducted in Japanese only and English speakers are asked to bring their own “interpreter” (this can simply be a member of your party who understands enough Japanese to be able to explain the safety instructions to non-Japanese speakers before descending into the tank — otherwise there is no real need for interpretation). There is also an English language video that can be viewed before or after the official tour.
The Ryukyukan Showa Drainage Pump Station of Kasukabe, Saitama, is a bit remote, as one might expect of a flood control site. It is most easily reached by car (30 minutes on Route 16 from the Iwatsuki Exit of the Tohoku Expressway or 40 minutes on Route 16 from the Kashiwa Exit of the Joban Expressway). By public transportation, take the Tobu Noda Line to Minami-Sakurai Station, then take the “Haru” bus from the north exit, grab a cab, or be prepared to walk 40 minutes (3 kilometers). Fortunately, there’s a decent map and instructions on the website.
Shutoken Gaikaku Hosuiro (in Japanese):
Metropolitan Area Outer Underground Discharge Channel (in English):