- 2018年01月08日10:10 来源：互联网
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Modelling tsunamis——The dangers of insularity
Islands may amplify the biggest waves, not break them
SURFERS shun beaches shielded by islands off the coast.
That, as generations of swarthy, golden-haired hulks will tell you, is because such islands create a natural breakwater.
This dampens waves and makes for a boringly calm surf best left to sunbathers.
The surfers’ reasoning is sound for the short-wave, wind-generated swells that they ride.
But Themistoklis Stefanakis, of the Ecole Normale Superieure in Cachan, France, warns it may not be true for the longer wavelengths of tsunamis.
As he and his colleagues show in a paper posted on arXiv, an online repository, littoral islands may actually exacerbate, rather than diminish, the effects of these waves.
Anecdotal evidence for this counterintuitive assertion comes from episodes where the same tsunami has battered different types of coastal topography.
In 2010, for instance, when one hit the Mentawai islands in Indonesia, areas of coastline directly behind islets bore the brunt of the damage, according to Costas Synolakis, a tsunami expert at the University of Southern California who is one of the study’s co-authors.
Dr Synolakis, Mr Stefanakis and their colleagues decided to try to put numbers on their hunch.
博士Synolakis Mr Stefanakis及他们的同事，决定验证这个设想。
Rather than valiantly staking out seafront locales, though, they tested the idea with a computer model.
This is harder than it sounds.
Simulating the way waves behave as they make landfall means taking account of, literally, oceans of data.
To simplify the problem, the researchers looked at what happens when a computerised wave encounters a cone-shaped island on a smoothly sloping seabed in front of a straight cyber-coastline with a beach that continues to rise smoothly as it proGREsses inland.
These approximations allow a computer to cope with the problem, yet are sufficiently similar to many real places for the conclusions drawn from them to, as it were, hold water.
The team made their virtual islet jut out of the ocean to an altitude of 100 metres, a typical height above sea level for such outcrops.
They then looked at 200 combinations of gradients for the sides of the island, the seabed and the beach; the distance between the island and the beach; and the wavelength of the encroaching tsunami.
In none of these did an island offer any succour to the coastline behind it. Instead, it acted as a lens, focusing the wave’s destructive power and amplifying its size by between 5% and 70%.
The upshot is that, far from shielding a coastline, offshore islands can make things worse—information that should be incorporated into tsunami evacuation plans.
For if a big wave is coming, running from it is not enough. You also have to know how far to run before it is safe to stop.