TOKYO – Underneath a stretch of highway north of Tokyo lies an example of Japan’s prowess in engineering solutions to potentially destructive weather events: a network of concrete tunnels and colossal pits built to prevent devastating floods in Saitama prefecture, near the bottom of a bowl-shaped river basin. In the event of an extreme storm or a typhoon, it can even help protect the capital.

Completed in 2006 at a cost of 230 billion yen (S$1.8 billion), the Metropolitan Area Outer Underground Discharge Channel exemplifies the country’s willingness to finance enormously expensive infrastructure projects to protect cities from catastrophe, making Japan among the best-prepared for disasters.

Yet, as climate change makes storms stronger and more common, such advanced hard-engineering projects on their own are not guaranteed to protect the region from future flooding.

“Rainfall will increase, and such structures cannot prevent everything,” said Dr Mikio Ishiwatari, senior adviser in disaster and water resources management at the Japan International Cooperation Agency, a government arm that provides development assistance overseas.

Flooding accounts for more than 70 per cent of the total number of natural and climate-related disasters in Japan. Tokyo has historically been on the front lines of the challenge, with more than 100 rivers coursing through the capital region, including four major waterways that converge on the city.

Rising sea levels, more frequent “guerilla storms” and the continual sinking of land due to excessive groundwater extraction further exacerbate the region’s vulnerability. In the last three decades, short but intense rainfalls with more than 50mm of rain an hour have become 1.4 times more frequent, according to the Japan Meteorological Agency. And, downpours with double that hourly precipitation rate have become 1.7 times more common. The global mean sea level, meanwhile, is projected to rise by up to 2m by 2100, making storm surges a major threat to large parts of the region that sit below sea level. 

“The disasters from floods and storms will rise, and rise sharply,” said Dr Vinod Thomas, senior associate fellow at the Institute of South-east Asian Studies in Singapore, and an expert in climate resilience. “While Japan is among the more prepared on adaptation, the climate equation is changing so rapidly that, as you find in country after country, old adaptation plans just are not good enough.”

Rapid urbanisation means that flooding can result in devastating human and economic loss. The Tokyo metropolitan area is one of the densest in the world, with some 37 million people packed into 13,500 sq km of land. Some 1.5 million people live in “zero-metre zones”, which sit below sea level and are expanding as land in the region continues to sink. 

Extreme flooding also threatens Tokyo’s concentration of financial institutions and government offices, and can paralyse the metro area by inundating roads, railways and underground subway networks, rendering recovery both costly and complicated.

Amid a scarcity of land space, flood control systems are built dozens of metres below ground, consisting of reservoirs and tunnels that work with dams and levees to prevent rivers from overflowing during heavy rainfall.

The discharge channel in Saitama, also known as the G-Cans Project, is charged with redirecting excess water from five small- and mid-sized rivers and a water channel throughout the prefecture to the larger Edo River outside of the Nakagawa and Ayase River basins. Levees built on the river banks carry the excess water to the five underground cylindrical shafts, some measuring up to 77m deep and 31m in diameter, large enough to fit the Statue of Liberty.

A massive 6.3km tunnel, buried 50m below ground and running parallel to a stretch of highway, connects the shafts and carries the water to a cavernous pressure-adjusting water storage tank, known as the “underground shrine” for its cathedral-like appearance. A four-pump system then gradually discharges the water out to the Edo River.

The G-Cans Project can withstand 356mm of rain within a 48-hour period, and drain flood waters at a maximum speed of 200 cubic m per second, according to officials. 

“The flood tunnel system can withstand once-in-a-century scale floods,” said Metropolitan Outer Floodway Management Office subsection chief Takeshi Ooyama during a tour of the site.

It went into action during Typhoon Hagibis in October 2019, when rain fell in the Nakagawa River Basin at a rate of 216mm over 48 hours. The system diverted some 12.18 million cubic m of flood water – equivalent to over 4,800 Olympic-size swimming pools – according to officials. Compared with the destruction of a similarly powerful typhoon from 1982, the system cut the number of flooded homes in the river basin by 90 per cent and prevented 26.4 billion yen in related damages.

The system has been activated seven times a year on average, and from 2002, when the system partially opened, to 2019, officials calculate that the G-Cans project prevented nearly 148 billion yen in flood destruction. When the weather does not call for rain, the underground shrine doubles as a tourist attraction and even as a backdrop for films and celebrity photoshoots.

Across the region, local governments and engineers are constantly preparing for the next big disaster. Closer to the capital, another labyrinthine network of underground tunnels and reservoirs works with levees and dams to divert water away from the capital during typhoons and rainy seasons.

“Until 30 or 40 years ago, flooding happened almost every year in Shinjuku or such a downtown area, but because of huge efforts by the Tokyo Metropolitan Government, the situation has quite improved with no severe flooding recently,” said Dr Ishiwatari. “Still, because of climate change, there’s still some risk, and the risk is increasing.”

Dramatic increases in the intensity and frequency of storms in recent years is testing the limits of the city’s existing system. Under the 2022 Tokyo Resilience Project, a 15-trillion-yen government plan to fortify the capital’s infrastructure against natural and man-made disasters, flooding was named one of the five most urgent threats alongside earthquakes, volcanic eruptions, power and communication failures and infectious diseases.

Roughly 6.6 trillion yen of the resilience plan funding will go toward boosting the metro area’s flood infrastructure. That includes expanding the sewer system to hold storm run-off, building communities on higher ground and doubling the capacity of underground reservoirs, which can currently hold 2.64 million cubic m of flood water.

The plan also calls for raising the height of sea walls, using artificial intelligence to predict sea levels, and protecting high-traffic underground spaces like subways and shopping malls, with barriers at the entrance to prevent flood water from entering.

Other Asian cities have also acknowledged the need to expand their protection measures amid heavier rainfall and rising seas. Singapore has put S$5 billion into a coastal and flood protection fund, and is using a mix of hard-engineering measures and sensor technology as part of its strategy. In China, which has invested billions of dollars since 2012 into water management projects, extreme flash flooding this past summer could force the government to reconsider its adoption of “sponge cities”.

But when it comes to climate change, it is not just about adaptation.

“Japan has a huge role both in mitigating – that is, preventing climate change – and adapting to it, and it’s very much still on the adaptation side of the equation,” said Dr Thomas. “It’s on the mitigation side where the needle has not moved.”

Japan, the fifth-largest global emitter in the world, has set a target to become carbon neutral by 2050, but lags behind other advanced economies in making progress towards that goal. Earlier in 2023, the country’s emissions rose for the first time in eight years, driven in part by pollution in its industrial and commercial sectors.

“No amount of adaptation will be enough,” said Dr Thomas. “It is like mopping the floor forever, but the tap is running.” BLOOMBERG