Kasukabe Underground Flood Protection Tank

Kasukabe Underground Flood Protection Tank (aka G-Cans) 首都圏外郭放水路, 春日部市

by Johannes Schonherr

The official English name for whole of the giant installation is the Metropolitan Area Outer Underground Discharge Channel. It has been designed as the central flood protection facility for eastern Saitama Prefecture and parts of north east Tokyo, namely, the Adachi, Katsushika and Edogawa wards.

Kasukabe Underground Flood Protection Tank, Saitama Prefecture.
Kasukabe Underground Flood Protection Surge Tank, Saitama

Open to the public is only the cathedral-like underground surge tank next to the bed of the Edogawa River, right at the border to Chiba Prefecture, commonly known as the G-Cans.

It is an impressive sight indeed. That booking a visit can be a bit tricky only adds to its lure.


But before I go into the details of a visit, I should provide a little background. Eastern Saitama Prefecture is located in the so-called Nakagawa Basin. It is low-laying land, below the level of the wide Edogawa River, and several small rivers, the Nakagawa being one of them, run through it.

The area has traditionally been very flood-prone especially during the snow melt period in the mountains were the rivers originate and even more so as a result of the frequent typhoons hitting the area. Farmers often lost their harvests to the floods but they were of little concern to the authorities.

From the 1960's on, however, the Tokyo Metropolitan Area began to extend into the Nakagawa Basin. By the 1980's, the farms had been almost completely replaced with suburban housing and industrial facilities. New train lines spread out and the real estate bubble of the late 1980's created a feverish housing market in the area which spent little thought on the flood dangers the prospective buyers faced.

Water containment silo seen from the surge tank, Saitama.
Water containment silo seen from the surge tank
Pump station control room, Saitama.
Pump station control room, Kasukabe Underground Flood Protection Tank (aka G-Cans), Saitama Prefecture

Flood Control

As a result, the area became repeatedly the victim of serious flooding, causing huge financial losses for a vital part of the economy of the Tokyo Metropolitan Area.

Something had to be done. The man-made problem of settling a huge population in a flood-prone environment had to be solved by a technological solution.

Thus, in 1992, the construction of the "Outer Underground Discharge Channel" was launched. The project was completed in June 2006 and, voilà, no floods in the area anymore. Until today at least (October 2014).

To describe the way the flood protection system works very briefly, five collection silos collect the surplus water via a 6.3km long central tunnel from the minor rivers. Each of the containment silos is about 70m high and 30m wide and would thus be able to easily contain the Space Shuttle or the Statue of Liberty - as the booklet of the Discharge Channel claims. The water is then shot through the 10m wide underground water tunnel running 50m below Highway 16 (a major thoroughfare) towards the surge tank facility in Kasukabe.

Water contaiment silo from above, Saitama.
Water contaiment silo from above, Kasukabe Underground Flood Protection Tank (aka G-Cans), Saitama Prefecture
Kasukabe pump station and visitor center, Saitama.
Kasukabe pump station and visitor center, Kasukabe Underground Flood Protection Tank (aka G-Cans), Saitama Prefecture

Release in Tokyo Bay

Once the huge rainfall is over and the situation reverts to normal, the stored water is pumped up into the Edogawa River and through it, eventually safely released into Tokyo Bay.

The flood protection system as such is certainly a great engineering feat and indeed, engineers from all over the world have been intensely studying the system since it was inaugurated.

But why would anybody bother putting it on their Tokyo visiting agenda? The answer is that the underground surge tank is an incredible sight. An industrial cathedral of enormous proportions. The engineers designing it had probably not art on their mind when they drew their sketches but nonetheless, they created one of the most elegant and beautiful underground spaces in the whole Tokyo Metropolitan Area.

The surge tank has been featured extensively by both domestic and international media and thus attained a popularity which makes booking a spot on one of the guided tours a bit challenging.

Edogawa River near the tank facility, Saitama.
Edogawa River near the tank facility

Booking A Tour

You must book your tour exactly 4 weeks in advance of your planned visiting date. Tours take place only from Tuesday through Friday, there are 3 tours per day.

The booking site is only in Japanese and you got to be quick with your booking. Check into the site right after midnight. There are only 25 visitors per tour. Those spots book out very quickly.

The surge tank web site also informs you that you must bring serious footwear (no access permitted wearing sandals) and, if you are a foreigner, you must either speak Japanese reasonably well or bring a translator.

The tours are in Japanese and if suddenly flood water surges in, you must be able to understand the emergency directions of the tour guides.

Once you have successfully booked your spot on one of the tours, be sure to arrive at the 2nd floor of the central pump station building about half an hour before the scheduled starting time. You will be registered and, if you are not a native Japanese speaker, an 8-minute English-language video will be shown to you that explains all the workings and history of the whole Metropolitan Area Outer Underground Discharge Channel.

The tour itself will start with someone explaining exactly the same background information, but in Japanese, for the whole group. Using various videos and scale models, she will detail the workings of the channel system 3 times over before the tour group eventually walks the steps down to the first floor.

You will be asked to do some stretching exercises before the tour leaves for the actual surge tank. It's a 5 minute walk along a soccer field before you arrive at the stairway down into the tank. That soccer field sits right on the top of the underground surge tank.

The tank is about 22m meters deep. Various galleries on the way down offer great views, but you are not supposed to take photos from there. Eventually, you arrive at the bottom of the tank.

When I went, the floor was still wet and plenty of mud covered the floor space between the columns. The last big rain had just been a few days before. The visitor area had, however, already been cleaned.

After listening to another talk about the tank and its details (the tank is 177m long, 78 meters wide, 18m high, with 59 pillars and each of those pillars is 7m long, 2m wide and weighs about 500 tons), you can eventually walk around on your own within the limited visitor area and try to find the best photo opportunities.

The giant surge tank is definitely very impressive. But there is no way to see any of the other underground facilities of the huge flood protection system. It would certainly be great to walk through one of the huge tunnels belonging to the system, and look at the giant pump that sucks the stored water from the tank and discharges it into the Edogawa River, but you can experience them only through the videos provided.

The tour is over once you arrive back on the surface after climbing up the steps again.

Take a walk over to the Edogawa River. Interestingly, the river is here not pressed into a concrete bed like so many other Japanese rivers are, it is free to expand its size as needed. The land bordering it is farmland with hardly any buildings. Just like in the times before the suburban sprawl set in.

Kasukabe Underground Flood Protection Tank, Saitama.
Kasukabe Underground Flood Protection Tank

Web Information

Visitor website of the Metropolitan Area Outer Underground Discharge Channel: http://www.ktr.mlit.go.jp/edogawa/gaikaku/index.html

The site has an English-language section with information about the surge tank and the booking requirements.

You can't book tours via that English-language section, however. For that, you have to go through the Japanese-language part of the site.


You must book your visit four weeks in advance via the facility website. From which day on which tour can be booked is announced on the site in advance. Be fast, tours quickly fill up.


From Tuesday through Friday at 10:00, 13:30 and 15:00. Each tour takes one hour, maximum 25 people per tour. Requirements: Bring serious footwear. If you don't speak Japanese, bring a translator (who must of course be registered as part of the tour). The tours are free of charge.

Please read the instructions given on the English-language visitor site carefully.


By train: The closest train station is Minami Sakurai (南桜井駅) on the Tobu Noda Line from Omiya Station in Saitama. To get to Omiya Station take a shinkansen from Tokyo Station (25 minutes) or the Takasaki Line from Ueno Station (25 minutes).

From the station it is about 3km to the Flood Protection Tank. Take a taxi from the North exit of the station. No bus service.

By car: Parking is available. Please note that driving in the northern suburbs of Tokyo tends to be very slow due to congestion and poorly coordinated traffic lights.

Kasukabe Underground Flood Protection Tank, Saitama.
Kasukabe Underground Flood Protection Tank

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