Natori, Japan

Tsunami defences - can you keep the waves at bay?

Some of the world's most extensive sea defences did little to protect Japan's coast from the devastating tsunami - is it time to develop buildings to withstand such an impact?

In September last year, the picturesque city of Kamaishi on Japan's north eastern coast celebrated the completion of a vast breakwater built across the mouth of the bay to protect its residents from tsunamis.

The world's deepest seawall, it rises 63m from the ocean floor and stretches 1,960 metres across the bay, the culmination of 31 years of engineering endeavour. But the Kamaishi breakwater had no appreciable effect on the tsunami that struck Japan on 11 March, triggered by a 9-magnitude earthquake just 129km off the north east coast.

Waves more than four metres high surged over it and into the city, sweeping up cars and boats, crushing buildings, ripping up telegraph poles and drowning hundreds, possibly thousands of people. The sheer power of the water is evident on video recordings and must have been especially terrifying for residents who had presumed they were safe behind their world-beating barrier.

The Japanese government has invested heavily in seawalls and breakwaters, which stretch along some 40 per cent of the country's 30,000km coastline. The policy reflects a general belief by authorities in tsunami-prone regions around the world that concrete barriers offer the best hope of protection from the sea. Indeed, that may be true for smaller storms, but the disaster in Japan illustrates that seawalls offer precious little protection against the most ferocious tsunamis.

Many coastal engineers are now calling for a shift in policy. Rather than trying to keep the sea out by building ever higher walls, more lives might be saved by designing buildings that can withstand the waves - an area of research that up to now has received little funding.

Where walls work

'If you look at the pictures, you can see that most of the buildings were okay after the earthquake, but were then washed out by the tsunami,' says Ioan Nistor, associate professor at the University of Ottawa and expert in coastal and hydraulic engineering.

Dan Cox, director of the Hinsdale Wave Research Lab at Oregon State University, says it is too early to make any conclusive statements about the effectiveness of Japan's seawalls in the recent tsunami, but that while they did not head off the disaster they may have lessened the overall impact by giving people more time to climb to a safe height, he says. 'Many residents did not have enough time to seek higher ground, so they went to the roof of concrete reinforced buildings, such as local schools and civic centres.'

Experiments conducted in 2009 by Cox and others, in which engineers fired artificial tsunami-like waves at an aluminium sheet fronted by seawalls of various heights, showed that even small seawalls reflect the wave sufficiently to reduce the force on whatever stands behind. Moreover, a assessment of the damage inflicted by the 2004 Indian Ocean tsunami, carried out by the Coasts, Oceans, Ports and Rivers Institute of the American Society of Civil Engineers (ASCE), concluded that if more seawalls had been in place, fewer buildings and lives would have been lost.

A team of civil engineers from the Indian Institute of Technology found that on the south west coast of India damage from the tsunami was greatest in areas not protected by a seawall.

Where walls don't work

However, the devastation caused in Japan illustrates the need for other approaches. Rescue teams have reported entire towns and villages swept away, with sea defences redundant in the face of waves that, according to the Japan Meteorological Agency, exceeded 7m in places.

The near-meltdown at the Fukushima Daiichi nuclear plant was caused by water sweeping over walls designed to protect it, disabling the diesel back-up generators that are meant to keep the reactors' cooling systems running when the mains power fails.

The most effective way of saving people in a tsunami is by evacuating them, if there's time. Unlike in the 2004 disaster, Japanese populations are well drilled in evacuation procedures. Escape routes are well marked and emergency sirens regularly tested. They are also protected by technology, with automatic sensors designed to close floodgates when an earthquake strikes and a network of tsunami detection buoys in the Pacific Ocean giving estimates of the arrival time and height of tsunami waves.

In 2005, the Japanese government's Tsunami Protection Committee recommended that even more should be done to enhance forecasting and early warning systems, improve evacuation procedures and promote response drills in schools and elsewhere.

Yet an improved evacuation plan may have made little difference during this tsunami, since the epicentre of the earthquake that triggered it was so close to the shoreline, leaving little response time. Lack of escape time is a problem with many tsunamis, says Nistor.

'It's always going to take you a couple of minutes to get out of the building, then there will be congestion on the roads and so on. It's especially difficult for older people.' In such situations, the best solution is what researchers call 'vertical evacuation' – climbing to the higher floors of buildings that are specially designed to withstand the flood.

Unfortunately, as the images of the Japanese coastline show, such structures are thin on the ground, perhaps because, as Nistor points out, there are no compulsory guidelines for buildings in tsunami-prone areas – not even in ready, willing and able nations like Japan. To address this, the ASCE early this year convened a subcommittee of experts to evaluate the forces exerted on structures during a tsunami and to develop design guidelines.

This has now become an active research area. Nistor and others at the University of Ottawa have been reproducing tsunami wave bores in the lab to examine what happens to structures when hit by water, travelling at tens of kilometres an hour, testing their effects on walls and model homes.

Translating such findings to real-world scenarios involves complex calculations: the size and nature of tsunami forces depend on the wave's depth, velocity, direction, height and period, as well as the topography of the seabed and the land over which it travels. Precision is important; since the hydrodynamic forces involved are a function of the square of the velocity of the wave, a small increase in velocity results in a dramatic increase in force.

Turning the tide

From what they have found so far, it's clear that building houses with rigid walls on the ground level may not be a good idea, since the full force of the wave will be deflected to the columns that support the walls. On the other hand, says Nistor, 'if the lower walls are collapsible, or if you have large window panels, the tsunami will pass through the base of the structure without putting a load on it'.

What's also clear is that while wooden-framed buildings are more than likely to withstand earthquakes, they have little chance against tsunamis. 'They become detached from their foundations and get washed away, even in water depths of a metre,' says Harry Yeh, who specialises in coastal and ocean engineering at Oregon State University. Reinforced concrete structures are more likely to remain standing, even if completely submerged.

Improved building design is not the only technological innovation under consideration. Brahman Developments, a Puerto Rican company, has drawn up plans for a floating barrel-shaped concrete capsule that would provide protection for up to 80 people. Known as the STATIM Shelter System, they could be placed at strategic points across a city and equipped with communications systems and food and water to last a week – the modern equivalent to wartime bomb shelters.

Some towns in Japan already have tsunami escape towers, such as the five-storey tube-shaped structure in Nishiki that doubles as a public museum. In the US, the Pacific town of Cannon Beach in Oregon is planning something similar, with a $4m city hall that acts as a tsunami shelter in emergencies.

The case for focusing on improved building design as a way of reducing the risks is given added strength by suggestions that in many areas high seawalls are culturally inappropriate, since they are unsightly and cut people off from the sea. In fishing communities, this can be critical.

Yeh points out that in some places on Japan's eastern coast, people have been killed by tsunamis after building their houses on the seaward side of a seawall, preferring a view of the sea to the reassurance of protection from an event that may not have occurred in their lifetime. 'When you're trying to protect people from tsunamis you need to think about their culture, the way they live.' *

Sign up to the E&T News e-mail to get great stories like this delivered to your inbox every day.

Recent articles