Tokyo's epic engineering defences against disaster

Japan is vulnerable to a range of devastating natural disasters, but has produced some ingenious engineering feats to protect itself. File photo / Getty Images

During a tour of the city on the fourth anniversary of the Great East Japan Earthquake, the Herald looked at Tokyo's defences against disasters, from the depths of its reservoir to the peaks of its modern skyscrapers to its barriers against the ocean.

We are in the bowels of Tokyo. A dank, eerie tunnel which curls deep under the city's CBD and is fed by three rivers.

We descend 11 storeys in a little elevator made for five people, squeeze through a submarine-like passageway and emerge in the cavernous subterranean reservoir. It appears part bat-cave, part-gigantic sewer.

The green-grey tunnel is 12.5 metres across, about half the width of the Puhoi tunnel north of Auckland but ten times as long.

Our guide shines a torch on the floor, where a trickle of water pools at his feet.

"These are the dry months," he says. "So don't worry - no water is coming."

In Tokyo's wildest weather, known as "guerilla rainstorms", the Kanda River regulating reservoir is a saviour. About twice a year during heavy rain, when rivers in the central suburbs become swollen, it swallows up to 405 sq km of floodwater (the equivalent of 1800 25-metre swimming pools).

The reservoir is one part of Tokyo's fortress.

In the city of 13 million people, science and technology battle against the forces of nature. One of the world's most vulnerable capitals for natural disasters, Tokyo is also home to great engineering feats and innovation.

Since 400 years ago, when city authorities diverted the Tone River away from Tokyo and out to the Pacific Ocean, the city has found practical engineering solutions to a range of natural threats.

THE BATHROOM SINK

Before descending underground in central Tokyo, our guide shows us pictures of the city before the reservoir was built. When 280 mm of rain fell during a typhoon 22 years ago, floodwaters sluiced through the main streets and rose to first storey windowsills, drowning 84 hectares of land and flooding 3100 houses.

In response, the Construction Bureau widened and deepened three major rivers in 1994.

A 2km reservoir was dug out using a similar mega-drill to the "Alice" machine used to dig the Waitemata tunnel. It was extended another 2.5km in 2004.

Since construction, it has been used 36 times to prevent flooding. When another huge storm dumped 280mm of rain on the city in 2004, just four hectares and 46 houses were flooded.

The reservoirs effectively work like the hole at the top of a bathroom sink. When the rivers become too full, the water flows through a gap high on the river bank and then disperses below ground. The water is later pumped back into the rivers, live fish and all.

Back in the Construction Bureau's office, our guide sheepishly points out a few river fish which the staff have kept as pets.

I later ask Earthquake Recovery Minister Gerry Brownlee whether similar innovations could be used for Christchurch, where flooding has become more common since the Canterbury earthquakes lowered the ground level by around half a metre.

He is sceptical. Water just goes to the lowest point, Mr Brownlee says, so a catchment can sometimes "just funnel the problem somewhere else".

Another problem is the cost - at least 110 billion yen (NZ$1.2 billion) for the Tokyo system.

"WE CAN HANDLE IT"

A day later, we are above ground at a riverside government building, where director of High Tide Management Centre director Mitomo Matsuo talks about the threats posed by the ocean on Tokyo's doorstep.

The city's location inside the Bay of Tokyo, with its a narrow, south-facing harbour mouth, means it is not vulnerable to a tsunami. More dangerous are high tides propelled by onshore winds, which pile up water against the harbour front.

Mr Matsuo is the city's gatekeeper, looking after a network of 19 steel gates which can be lowered for storms and raised for boat traffic.

When his staff detect rising atmospheric pressure and growing winds, the 70-tonne gates are lowered, a process which takes about five minutes. They can keep out surges of up to 3.7 metres.

I ask him about the gates' limits. Could a king tide ever creep over the top?

"We can handle it," he says with a wry smile. What if a tsunami coincided with a high tide and onshore winds? "We can handle it." Is he worried about rising sea levels? "We would like to increase the seawalls' height one day," he says.

The Sky Tree can withstand a once-in-500 year storm with wind speeds of 80km/second.

21ST CENTURY PAGODA

On the north-eastern side of Tokyo is the towering Sky Tree, a 634-metre building which resembles a rocket ship combined with a broadcasting tower. It is nearly twice as high as Auckland's Sky Tower.

While Christchurch was debating whether to introduce seven-storey limits after its devastating quake in 2011, Tokyo audaciously opened the largest tower in the world just 14 months after its own mega-quake.

The Sky Tree is a symbol of Japan's confidence in its engineering and its refusal to be cowed by its threatening environment. Its designers Nikken Sekkei say it can withstand a once-in-500 year storm with wind speeds of 80km/second.

When considering its construction, its architects questioned why no pagoda, or traditional Japanese tower, had ever fallen in an earthquake over hundreds of years.

Imitating a pagoda's design, its engineers kept the the core column at the centre of the building structurally separate from the outside steel framing, allowing it to dramatically reduce the amount of shaking during an earthquake.

It is so safe, our guide says, that people were more likely to rush inside the building than flee it during a tremor.

WATCHING AND WARNING

Our tour ends on the other side of Tokyo in a grey, low-ceilinged office, where staff at the Japan Meteorological Office pore over seismic data around the clock.

The office is split into two halves. One side keeps an eye on active volcanoes, watching every blip of seismic activity and staring at a screen of 20 webcams of cones all over the country. The other half of the room keeps an eye out for earthquakes, with the help of 290 seismic stations scattered around Japan.

The monotony of computer monitors and is broken up by a K-Pop poster in the middle of the room. Alarms go off disconcertingly often. The tour is halted after the seismographs spike in northern Japan, and we are hustled to the back of the office while officials fast-walk to their monitors.

An earthquake has just hit Fukushima Prefecture, one of the regions struck hardest by the March 2011 disasters. The room goes silent for a moment at the mention of the region, now notorious for the nuclear crisis which followed the quake.

After 20 seconds, the staff breathe again. It is a small tremor, lower than Magnitude 1. Tens of thousands of these occur each year.

What was this room like when the Magnitude 9 tremor hit Tokyo on March 11, 2011?

"We did not panic," the Seismic and Volcanology department's deputy director Yugi Nishimae says. The staff do regular drills which teach them to remain calm and to get information out as soon as possible.

There was no running or screaming. Just an intense, quiet three minutes before a warning was distributed to anyone in the country with a mobile phone or television.

Tokyo's technological fortress is not yet perfect. Clouded by the sheer trauma of the faultline breaking, the meteorological agency's instruments warned people a Magnitude 7.9 quake would produce a tsunami of 3 to 6 metres on Japan's east coast. After the first alert had gone out and residents had begun scurrying for higher ground, tidal gauges closer to picked up a much more alarming tsunami.

The waves which rose up on Iwate, Miyagi and Fukushima's horizon were all greater than 10 metres, and in one place in Miyagi, higher than 40 metres.

It was not until three days later that the quake was confirmed as Magnitude 9, the biggest in the modern era.

"When we look back, 20,000 people died, and that is regrettable," Mr Nishimae says.

The agencies' underestimates were a reminder that this highly sophisticated nation cannot yet place its whole trust in technology.

At a fourth anniversary event last week, the governor Yuhei Sato of Fukushima, where nearly 1000 died, said: "Were we sufficiently prepared for threats posed by nature? Had we overestimated science and technology? In these questions lie important lessons for the future."