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Dublin: -1 °C Sunday 18 March, 2018

Storm waves in the west moved boulders onto the top of this cliff

Previously it was assumed that only a tsunami could be that powerful.

Image: Earth Science Reviews

NEW RESEARCH SHOWS that storm waves in 2013 and 2014 along the west coast of Ireland were strong enough to move boulders – and deposit new ones on the top of a cliff.

In a new paper in Earth Science Reviews, researchers from Williams College in the US show that the storms shifted smaller boulders as high as 26 metres above high water and 222 metres inland.

Many of the boulders moved were heavier than 100 tons, and the largest moved was 620 tons – the equivalent of six blue whales or four single-storey houses.

They say that it was previously assumed that only tsunamis could move boulders of the size seen displaced in Ireland. But their work provides direct evidence that storm waves can do this kind of work.

The researchers say the work is important as according to the UN, about 40% of the world’s population live in coastal areas (within 100 metres of the sea), so millions of people are at risk from storms.

Understanding how those waves behave, and how powerful they can be, is key for preparation. It is therefore important to know the upper limits of storm wave energy, even in areas where these kinds of extreme wave energies are not expected.

“The effect of the storms of winter 2013-14 was dramatic,” said Dr Rónadh Cox, Professor and Chair of Geosciences at Williams College and lead author of the study.

“We had been studying these sites for a number of years, and realised that this was an opportunity to measure the coastal response to very large storm events.”

1-s2.0-S0012825217302350-gr4_lrg Feld photographs of the two largest blocks to have moved on Inishmore. A: Boulder 267, on the lower platform, weighs ≈ 475 t. The yellow box outlines two full-size adults on the upper platform. B: Boulder 293 weighs ≈ 620 t. The white patch marks the previous location of a 60-ton slab that was dislodged during storms.

So in the summer after the storms, Professor Cox and a team of seven undergraduate students from Williams College surveyed 100 sites in western Ireland, documenting with photos the displacement of 1,153 boulders.

They measured the dimensions and calculated the mass of each boulder.

As they knew where 374 of the boulders had come from, for these they also documented the distance travelled.

The largest boulder, at 237-239 m3 was an estimated 620 tons; the second biggest, at 180-185 m3, was about 475 tons. These giant rocks were close to sea level (although above the high tide mark). At higher elevations, and at greater distances inland, smaller boulders moved upwards and inland.

The boulders are usually created quite near to where they are deposited.

Thus boulder creation generally happens quite close to the site of deposition, so although clasts are deposited in many cases quite far inland and well above the high water mark, net transport distances are often not that large.

Previous studies seemed to indicate that storm waves couldn’t move or deposit boulders that large or too high above sea level. But this research shows that this isn’t always the case.

Their analysis showed that the waves had most power at lower elevations and closer to the shore. They say that while this may not be surprising, “the sheer energy of the waves and their ability to move such large boulders was – and this evidence proves that not only tsunami but also storm waves can move such large objects”.

Cox added: “Now that we know what storm waves are capable of, we have much more information for policy makers who are responsible for preparing coastal communities for the impact of high-energy storms.”

Read: ‘In Boston they’d throw you in the clink’: Here’s why you should never drive in a blizzard>

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