The incredible "underground cathedral" that protects Tokyo from floods

Cecilia Tortajada recalls descending a long staircase and entered one of the wonders of Japanese engineering: a huge water reservoir that is Tokyo's biggest defense against floods.

When she finally reached the floor of the tank, she saw herself among the dozens of 500-ton columns that supported the roof. Inside the cavernous cistern (which looked like a sanctuary), she felt tiny.

"You realize that you only represent a small part of this gigantic system," recalls Tortajada, water management expert at the Water Policy Institute at the Lee Kuan Yew School in Singapore.

"You realize how well prepared Tokyo is."

Japan is a pilgrimage destination for disaster experts and risk management as it is. And it is one of its main temples.

The curious cathedral – hidden 22 meters underground – is part of the outer underground drainage channel leading to the metropolitan area (MAOUDC), a 6.3 km long tunnel and cylindrical chambers that protect North Tokyo from flooding. .

In recent decades, the Japanese capital has perfected the art of managing torrential rains and swollen rivers. Its complex flood protection system is a world wonder.

But as the weather and rain patterns change, the future looks uncertain.

Typhoons and other disasters

The Battle of Tokyo against floods has a long history.

The city is located on a plain crossed by five river systems and dozens of individual rivers that saturate each station.

Intense urbanization, rapid industrialization and irresponsible extraction of water that have caused the collapse of some areas have exacerbated the vulnerability of the city.

"I do not know who decided to install Tokyo there," jokes Tortajada, who has been working in water management for more than two decades.

Although Japan has been struggling with floods for centuries, the current Tokyo system really began to take shape in the post-war years.

Typhoon Kathleen hit Japan in 1947, destroying about 31,000 homes and killing 1,100 people.

A decade later, Typhoon Kanogawa (also known as Ida) devastated the city when about 400 millimeters of rain fell in a week. Streets, houses and businesses were flooded.

In the midst of chaotic consequences, the Japanese government has stepped up its monetary commitment.

"Even in the 1950s and 1960s, when the Japanese were recovering from the war, the government was investing 6-7% of the national budget in disasters and risk reduction," says Miki Inaoka, disaster expert for the country. Agency. International Cooperation of Japan (JICA).

A case study

Tokyo planners are tasked with staying alert to different types of floods.

In case of heavy rains, rivers can overflow and submerge neighborhoods or submerge the drainage system in some areas. Tsunamis can threaten coastal areas. And what would happen if an earthquake destroyed a dam or a dam?

After decades of planning for these scenarios, the Japanese capital now has dozens of dams, reservoirs and dams.

If you make an imaginary cut on the ground of the city – like a birthday cake – you will find an underground labyrinth of tunnels along subway lines and gas pipes running through the city.

The MAOUDC – at a cost of 2 billion US dollars – and its "cathedral to flood" constitute one of the most impressive works of art in the capital.

It was completed in 2006 after 13 years of work and is the largest water diversion facility in the world.

"Japan is a country that believes in learning," said Tortajada, who visited the MAOUDC in 2017.

"This makes it a very interesting case study."

Science fiction

The cbad draws water from the small and medium rivers of North Tokyo and transfers it to the Edo River, which can handle volume more easily.

When one of these rivers overflows, the waters fall into one of five huge cylindrical cisterns 70 meters high that extend along the cbad.

Each of these tanks is large enough to accommodate a space shuttle or the Statue of Liberty and is interconnected by a network of underground tunnels of 6.3 kilometers.

At the approach of the Edo River water, the "floodwater cathedral" visited by Tortajada reduces its flow, so that the pumps can push it towards the river.

A mental exercise can explain the power of the discharge channel.

Imagine a standard 25 meter pool, filled to the brim, connected to the 13,000 horse power pumps that release the water from the cbad.

If the bombs were lit, it would only take two to three seconds to empty the pool, since they can push 200 tons of water per second.

"It's like a kind of science fiction installation," says Inaoka of JICA, whose job is to collaborate with experts from developing countries to share Japan's experience.

However, Inaoka also recognizes that changes in rainfall patterns will challenge Tokyo's infrastructure. Climate change makes planning very difficult, he said.

Climate change

Based on historical rainfall data, urban planning authorities designed the Tokyo defenses to withstand 50 mm of rain per hour, especially in areas of concentration of people and goods.

But what was considered normal fifty years ago is no longer.

As in other parts of the world, the number of days of heavy rainfall has increased over the past 30 years, according to the Japan Meteorological Agency.

This is a sign that models are changing.

Some estimates suggest that rainfall in Japan could increase by 10% during the 21st century. In summer, this percentage could reach 19%.

The Tokyo Metropolitan Government Construction Bureau claims to be aware of these changes.

In at least three regions, projects are now designed to withstand rainfall of between 65 and 75 millimeters per hour.

But experts like Nobukuyi Tsuchiya, a former civil engineer from Edogawa district in Tokyo, say the authorities are spending too much time discussing what to do.

"Unfortunately, flood control measures against climate change have not yet been put in place in Japan," said Tsuchiya, director of the Riverfront Research Center.

In his 2014 book "Shuto Suibotsu" ("Submerged Capital"), Tsuchiya warned that Tokyo was not ready to cope with torrential rains likely to cause global warming.

According to the Japanese expert, in the low areas of Tokyo, about 2.5 million people could be affected by floods in the event of record tides, which should therefore be a planning priority.

To pay attention

In early 2018, heavy rains fell in western Japan killing hundreds of people and causing economic damage of a thousand dollars when the rivers overflowed.

Tsuchiya said that if it happened in Tokyo, the city would be devastated. And this risk is not unique to Tokyo.

Major cities such as New York, Shanghai and Bangkok will be increasingly vulnerable to floods and storms as a result of climate change and, like the capital of Japan, most evaluate this scenario and work slowly to build defenses.

London's climate change adaptation plan, for example, makes flooding the first threat, since a fifth of the city lies on the Thames Plain.

The area is well defended by embankments and the rugged Thames barrier to the east of the city, but planners feel there may be a time when these guarantees fail.

On the other side of the Atlantic, Miami is already fighting against rising sea levels.

In Singapore, Cecilia Tortajada and other experts are working to protect the city's state against the rising waters in the coming years.

The Local Building and Building Authority (BCA) recently commissioned a study to advance the national framework for coastal protection and new measures are introduced each year.

Meanwhile, everyone is watching Tokyo closely, trying to badess how well it protects its shield against typhoons and summer rains.

"If a country as prepared as Japan and a city like Tokyo are affected, we should all pay attention," said Tortajada.