Tokyo’s massive flood protection facility might not be “enough” due to climate change

Tokyo’s massive flood protection facility might not be “enough” due to climate change

By Elisa Jiménez Alonso

In Tokyo, an enormous underground flood protection system pumps excess water out of the metropolitan area into the sea and has reduced flood occurrences massively. But, climate change might take it to its limits.

When you look at a photograph of the Metropolitan Area Outer Underground Discharge Channel, or G-Cans, you might be reminded of J.R.R. Tolkien’s Mines of Moria and the terrifying Balrog that lived there. However, it is the world’s largest underground flood water diversion infrastructure, built on the outskirts of Tokyo.

The numbers associated with this cavernous super structure are truly impressive: 50 metres beneath the surface, five containment silos, each 65 metres high and with a diameter of 32 metres, are connected by a 6.3-kilometre network of tunnels. The silos are so big, they could fit the Statue of Liberty inside. The structure also has a large cistern, the “Underground Temple”, 18 metres high, 78 metres wide, and 177 metres long with 59 massive pillars and connected to the drainage facility of the system which consists of 4 pumps that can pump a total of 200 cubic metres of water per second.

Concept drawing of G-Cans. Source: Edo River Office, Kanto Regional Development Bureau, Ministry of Land, Infrastructure, Transportation, and Tourism.

The underground system was built in 2006 and cost roughly $2 billion. Now, climate change threatens to erode the capacity thresholds of G-Cans. According to the Japanese Meteorological Agency, Japan, already one of the wettest areas of the world, will see even more rainfall. Additionally, sea level rise is threatening Tokyo, which is further exacerbated by subsidence. In 2015, heavy rainfall caused by a typhoon filled Tokyo’s flood protection system with almost 19 million cubic metres of water – which could roughly fill 7600 Olympic size pools – and took four days to be pumped out.

For the time being, the facility remains crucial to Tokyo’s flood defences. However, in the face of climate change and possibly looking at a future where this structure alone will not be able to protect Tokyo’s 38 million inhabitants from floods, Nobuyuki Tsuchiya, former chief civil engineer of Tokyo’s Edogawa ward, said to the New Statesman that current flood protection measures “are not enough.” Kuniharu Abe, who heads the Metropolitan Area Outer Underground Discharge Channel, further adds he is “not sure Japan can build something like this again.”

It begs the question if attempting another infrastructure project of such enormous proportions (literally and financially) even is the correct way forward. Concrete defences often offer a very obvious and visible form of flood protection and can attract more people to flood-prone areas. This is what happened to Saitama, where the G-Cans facility has reduced floods significantly. Many businesses settled in the area and might face a future when frequent flooding returns.

It is important to emphasise that the Metropolitan Area Outer Underground Discharge Channel is by no means a story of failure, it has already avoided many floods and will, at the very least, continue to alleviate them in the future. But it tells a story about the fact that adaptation to increasing flood risk, or any climate risk for that matter, is never one-dimensional. No single project will remove the risk. It is important to consider many aspects from infrastructural solutions, zoning and land use, to public risk awareness and preparedness.


Cover photo by Kunitaka NIIDATE/Wikimedia Commons: Metropolitan Area Outer Underground Discharge Channel Kasukabe, Saitama, Japan

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