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Japan must relocate its towns, say flood experts

Building higher sea walls and relocating towns are the only ways Japan can safeguard its infrastructure against another tsunami of the magnitude that followed last week’s earthquake, experts said this week.

Sea wall ‘only structural option’

A well designed sea wall is the only structure that can prevent a tsunami reaching the shore rather than just slowing it down, Royal Haskoning coastal and rivers business development director Hamish Hall told NCE.

“In a tsunami you need height; you have to build a very high wall,” said Hall. “The energy of that wave is so significant that you’re not going to take out much of it [with a breakwater].”

But he said making the coastline truly resilient would be “a major undertaking”. “They’ll need to think about moving [buildings and infrastructure] away from those parts of the coast,” he said.

‘Dedicated areas’

Former ICE president Jean Venables said certain parts of the coast could be dedicated as areas into which tsunamis could be channelled, minimising damage elsewhere.

“You would have to have several of these along the coast and then put a lot of effort into defending key sites such as nuclear plants,” she said. The towns destroyed by the recent tsunami which was up to 15m high could be rebuilt on higher ground, and former locations used for this purpose, she added.

A lack of definitive design standards for tsunami sea walls means it is unclear what level of tsunami event Japan could comfortably withstand. Rikuzen-Takata city in Iwate prefecture had a 6.5m high sea wall, but was one of the one of the hardest hit towns.

“There are still surprisingly few coastal design codes that mention tsunami hazard in a design context”

Tristan Lloyd, UCL

In Miyagi prefecture, Minami-Sanriku town’s 8m high sea wall was designed to withstand a tsunami 6.5m high, but it was also overtopped.

“There are still surprisingly few coastal design codes that mention tsunami hazard in a design context,” said University College London (UCL) PhD student and tsunami specialist Tristan Lloyd. “Most of the guidance in existence today is still based on formulae originally derived for fluvial flood engineering.”

Lloyd said quantifying return periods for a tsunami is notoriously difficult and they can only be very roughly defined. Tsunami risk is often assessed on a case-by-case basis by determining the largest tsunami that would be produced by the largest expected earthquake.

Even then, Lloyd said there is “no definitive relationship that says a certain magnitude earthquake will produce a tsunami of a certain size.”

Tsunamis difficult to predict

Tsunamis vary according to the specifics of the earthquake that produces them, and the qualities of the coastline that they approach − because a tsunami’s wave height and flow velocity increase rapidly once it enters a bay.

The Overseas Coastal Area Development Institute of Japan’s guidance document Technical Standards and Commentaries for Port and Harbour Facilities in Japan says tsunami risks can only be assessed for design purposes on a specific local basis, using past tsunami records and calculations for the specific coastline in question.

Structures must survive an earthquake first

Designing flood walls and breakwaters for tsunamis is further complicated by the fact that the structure has to survive an earthquake before the tsunami even comes, and has to be able to withstand the backwards motion of large amounts of water and heavy debris moving back out towards the sea after the event. “It’s not like a reservoir collapse where you’ve only got the flow in one direction,” Venables said.

She said research into anti-tsunami engineering has increased since the 2004 Indonesian tsunami, but said: “It’s probably too soon to have had an effect in terms of construction.”

UCL Earthquake and People Interaction Centre director Tiziana Rossetto said the shape and behaviour of tsunamis is still little understood. The rarity of extreme events coupled with the fact that measuring equipment in the worst-affected places is usually destroyed by the tsunami means there is no comprehensive historical data.

“We’re not 100% sure what a tsunami looks like”

Tiziano Rossetto, UCL

“We’re not 100% sure what a tsunami looks like,” she said. “There aren’t any observational measurements of wave forces.”

A newly invented tsunami generator is central to ongoing UCL research at HR Wallingford that Rossetto expects to be of interest to countries like Japan. The machine uses valves to create a very long, tsunami-like wave in a water column.

No previous wave generators exist that have been able to achieve the wavelengths typical of a large tsunami, said Rossetto.Researchers are using the equipment to discern a wave’s shape and to observe its effects on a model coastline.

Relief efforts underway

The Japanese government has announced plans to build 33,175 temporary homes for displaced residents, at a cost of 2,387,000 yen (£18,000) each.

The project includes 14,000 temporary homes in Fukushima prefecture, 10,000 in Miyagi and 8,800 in Iwate, with a typical family shelter measuring 30m² in size. The project began this week with the construction of the first 200 homes in Iwate prefecture.

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