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Monday, June 26, 2017, 14:30
Tackling Tokyo’s flood threat
By Karl Wilson in Sydney
Monday, June 26, 2017, 14:30 By Karl Wilson in Sydney

In Japan, world’s largest man-made drainage system offers model for dealing with heavy rainfall in urban areas

People walk with their umbrellas during rainy weather in Tokyo on Oct 28 last year. In recent decades, the intensity and frequency of rainstorms in the Japanese capital has increased dramatically, which could put pressure on the city’s drainage system. (AFP)

Ever wonder why Tokyo does not flood?

Next time when you are walking around the Japanese capital, stop for a minute to consider this: Just 50 meters below the ground is a labyrinth of tunnels, cathedral-like water tanks and massive pumps that can remove 200 tons of floodwater every second.

It has been described as the biggest underground man-made drainage system in the world. 

With around 30 percent of Tokyo’s population living below sea level, mostly along Tokyo Bay or the many rivers that feed into it, the metropolis has had a long history of disastrous flooding. 

As the capital city grew and Japan prospered, protecting the world’s largest metropolitan economy and its inhabitants — over 13 million in Tokyo today and more than 36 million in the greater metropolitan area — became a national priority.

The simplest solution would have been to build a reservoir. But the question facing urban planners was: How do you build a reservoir in a city twice as dense as New York? The answer: Build it underground.

The results were the Metropolitan Area Outer Underground Discharge Channel, also known as G-Cans, and more recently, the Furukawa Underground Regulating Reservoir.

Completed in 2006, the G-Cans gathers the overflowing floodwater from rivers in Tokyo, such as Oochi Kotone, Kuramatsu, Nagakawa and Arakawa, and drains it into the Edo River.

It is the world’s largest underground floodwater-diversion system, said Koichiro Omoto, a water specialist with the Asian Development Bank Institute in Tokyo.

Costing around US$3 billion, the G-Cans connects five of Tokyo’s rivers via vertical drain shafts, each big enough to hold the Statue of Liberty with room to spare. 

“The shafts are connected by a 6.3 kilometer underground tunnel, through which excess water from heavy rain travels and is collected in a subterranean reservoir,” Omoto told China Daily Asia Weekly.

Once the storm subsides, the water in the reservoir gets pumped into the Edo River at a point where it has the capacity to carry the excess water to the ocean without further flooding. 

The water is released at a combined rate of 200 cubic meters per second — the equivalent of completely draining one Olympic-sized swimming pool every 12.5 seconds. The pumps are driven by four turbines powered by Boeing 737-sized jet engines.

And when it is not filled with water, the reservoir becomes a popular tourist attraction.

Omoto, citing government figures, said the project has likely saved around 48 billion yen (US$430 million) in property damage alone since it became fully operational.

In recent decades, the intensity and frequency of rainstorms in Tokyo has increased dramatically, according to the government’s information agency.

Tokyo’s annual rainfall averages around 1,500 millimeters which is not unusual. Singapore and Manila, by comparison, have over 2,000 millimeters of rainfall a year.

In recent years, localized downpours — dubbed ‘guerrilla rainstorms’ — have also become a problem, taxing the existing drainage systems and forcing authorities to expand the city’s water diversion network.

The Furukawa Underground Regulating Reservoir is the latest addition to Tokyo’s network of underground water-storage facilities.

The Furukawa River winds through Tokyo’s Shibuya and Minato wards, which are some of the city’s most densely packed neighborhoods, according to the government.

Due to space constraints, the reservoir has been built 15.24 meters directly underneath the course of the river, which often makes acute turns, complicating the project’s construction. 

The 3.3 km long, 7.5 meter diameter drainage tunnel can hold 135 million liters of storm water, which is equal to the volume of 54 Olympic-sized swimming pools.

Systems like the Metropolitan Area Outer Underground Discharge Channel and the Furukawa Underground Regulating Reservoir have dramatically reduced the flooding and damage caused by heavy rainstorms. 

“It is also largely thanks to these kinds of underground and out-of-sight systems protecting Tokyo that it is uncommon to see pools of standing water on the city’s streets, even after the heaviest of rains,” the government said. 

“As Tokyo continues to adapt to changing weather patterns, it is likely that these systems will continue to play a key role in protecting the city for many years to come.” 

But there are some who do not share that optimism. With the effects of global warming becoming increasingly obvious, there is no degree of certainty as to the severity of storms and rainfall.

Nobuyuki Tsuchiya, a civil engineering expert and author of the 2014 book Shuto Suibotsu (The Capital Submerged), has his doubts.

In an interview with The Japan Times, he said weather conditions previously considered very rare are becoming increasingly prevalent due to the growing impact of global warming. He noted that weather and rainfall patterns are changing.

Akira Kawamura, a professor of engineering at Tokyo Metropolitan University, agreed. In the same interview, he said: “Should the banks of the Arakawa River (which flows from Saitama prefecture and through Tokyo to Tokyo Bay) break, the damage would be so extensive that it could annihilate the entire capital, with the below-sea-level areas in eastern Tokyo, including Koto and Edogawa wards, likely to be hit the hardest.”

If the Arakawa River banks were to give way, the damage to the city could amount to 33 trillion yen, according to official government estimates. 

A 2010 Japanese government report by a panel of outside disaster-prevention experts calculated several possible death tolls in the event that the Tone (which also runs through part of Tokyo) and Arakawa rivers flood.

The deadliest scenario was if the Tone River broke its banks near the cities of Koga and Bando in western Ibaraki, in which case the death toll could rise to as many as 6,300, the report said.

The World Bank noted that extreme weather conditions are among the most devastating effects of global warming. These include powerful typhoons, monsoon rains, hurricanes and tropical cyclones that wreak havoc on cities and other low-lying areas. 

It said floods are considered the world’s “most frequent destructive natural disaster and the cost of rehabilitation is taking a toll on most of the world’s economies”.

In a report earlier this year, the World Bank said global natural disasters cost about US$520 billion annually in lost consumption, 60 percent larger than the commonly reported asset losses.

The estimate is based on the impact of disasters such as floods, windstorms, earthquakes and tsunamis on people’s well-being, measured by the decline in their consumption.

Asia Pacific is the world’s most exposed region to natural disasters, accounting for about 40 percent of the global tally.

The region has high population densities and suffers from a relatively high frequency of typhoons, earthquakes and tsunamis, such as the one that led to the 2011 meltdown of the Fukushima nuclear reactors in Japan, the report said.

With the growing frequency and intensity of abnormal weather conditions across the region, Tokyo is held up as a model of what can be done to protect urban areas from flooding. However, such preventive measures come with no guarantees.

karlwilson@chinadailyapac.com

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