Waving the world

Pressing north through the mountains, the first full-blooded gale of autumn raised whirling waterspouts on Doo Lough and scythed the hillside bracken to a rusty stubble. Swinging westwards, it heaped and hustled the ocean swells into terraces of spilling breakers and blurred the islands in a gloom of spray.

Along at the little pier at Roonagh, where the tide swings round into Clew Bay, I sat for a while as the waves peaked above the rocks and lurched and surged into the shore. Is the seventh wave really the big one? I could find no special rhythm to the train of crests: out at the reefs and islands, open to the ocean's full fetch, such a magical pulse may be easier to see.

Across the ocean, the big ones seem to travel in threes. "Again and again," wrote Henry Beston in The Outermost House, his 1920s classic written in the dunes of Cape Cod, "have I watched three giants roll in one after the other out of the Atlantic and follow each other in to fulfilment and destruction."

More recently, in his powerful bestseller The Perfect Storm, Sebastian Junger wrote of "the three sisters" - accumulations of wave energy so huge they can be tracked by radar. Sometimes, he says, these great waves cross the Atlantic and start to shoal at the 100-fathom curve of the continental shelf, far off French coast. Descriptions of awesome trains of swells, travelling thousands of kilometres across open ocean, fit oddly with the fact that waves arrive at Cape Cod with the same, incessant fall as at Thallabawn. Most of our swells are born in storms to the east of Newfoundland; others spill out from Caribbean hurricanes.

But Henry Beston, looking the other way across the Atlantic, saw the birthplace of his waves as somewhere between Cape Cod and Spain. Rachel Carson, too, in her book The Restless Sea, looked 1,000 miles east for the origin of waves at the New England shore. That everyone is right must have to do with the spinning of storms, swinging the wind around the compass and radiating swells like massive ripples.

As they travel they lose height and become more widely spaced, but they may still roll on for great distances at about 15 m.p.h. (a speed gracefully known as "wave celerity"). They meet other swells from other distant storms, and local waves generated by different winds again. Mix in tidal currents and seabed contours and the result is the seeming chaos of the real world of waves. How high they get depends, in general, how hard the wind blows, for how long and over what length of open ocean. There is a limit to the amount of wind a wave can absorb: the rule is that when a wave's height is more than one-seventh the distance between one crest and the next, it will begin to topple into foam: a "whitecap".

Two revelations are basic to waves. One is their nature as liquid power: the energy of wind transferred to water. The other realisation, somewhat chastening to many people, is that the water in waves is going nowhere. As the wave passes through deep water, it spins the water particles like a wheel but leaves them back more or less where they were: all that travels onwards is the energy, the momentum, the shape of the wave.

As it approaches the shore, things start to change. The wheel of energy in the wave drags on the bottom and becomes an ellipse, then finally a back-and-forth movement at the seabed that stirs up the sediment. And if, as is likely, the wave is approaching the coast at an angle, it is swung into line with the seabed contours like a door on a hinge.

As the drag slows it down, the following wave crowds in, shortening the wavelength. The lead wave steepens and increases its height, then collapses into foam, a turbulence that helps to spend its energy; even the sound it makes is energy returning to the air.

HOW waves break depends a great deal on the nature of the bottom. Thallabawn strand is a gently sloping beach and the waves are "spilling" breakers, unravelling into smaller waves within a broad surf zone. What the surfer needs are the "plunging" breakers of a steeper shore, with a wave crest that curls forward into the trough.

A third kind of wave is the "surging" breaker that charges up a rock face or steep shingle beach, makes little or no surf and bounces half its energy back to sea. This is the one that, hitting a vertical sea wall, builds a pattern of waves that scour and hammer at its foundations. Coming into an era of rising sea levels and fiercer storms, the engineers of coastal local authorities are launched on their own steep learning curve about the science of waves, the harmonics of tides, the numerical models that help to map the behaviour of seas, in particular in bays and estuaries. In building new structures, they have to plan for the "design wave" - the ultimate, theoretical breaker to be expected every 50 years or so.

In current textbooks, a 50-year wave of 20 metres far out in the Atlantic approaches the west coast at 16 metres and Cork Harbour at 14 metres. But as Ecopro, the new government manual on coastal protection, warns, wave heights seem to be increasing over the whole of the Atlantic by 1 or 2 per cent a year.

The conservative 20 metres for the biggest possible storm wave has long been contested in the first-hand experience of mariners. On the day I was born, in February, 1933, the crew of the American steamship Ramapo, in a Pacific storm, saw a great sea rise astern in the moonlight that could be calculated by geometry at 112 feet (34 metres). This humbling event may have made me a beachcomber, with one eye on the horizon.