RD Archives: Amazing waves and how they work

Caught in a timeless dance between ocean and air, waves are fuelled by the moon and wind, shaped by the sea floor. They are capable of inducing soothing meditation or a cataclysm akin to a small nuclear explosion. Waves can banish a beach overnight, then build a new one the next spring. Yet, perhaps the most romantic aspect of waves is the fascination they present for humans.

World-weary eyes should: "Feast them upon the wideness of the Sea…" urged the poet John Keats. The view is rich and endlessly varied, from the wind-blown scatter pattern known technically as "sea" to the immense tsunamis, the unquestioned wave kings. Tsunamis are often mistakenly dubbed "tidal waves", though in fact they have nothing to do with tides. Caused by undersea earthquakes and landslides, tsunamis are capable of travelling vast distances at 500 mph and can wipe entire towns off the earth.

Following the 1964 Good Friday earthquake in Anchorage, northern Canada, a tsunami virtually obliterated the neighbouring fishing town of Valdez (subsequently relocated and rebuilt). Hours later, more than a thousand miles away in Crescent City on the California coast, the Alaska-spawned waves killed ten people.

And yet waves do not destroy so much as produce life. They deliver fresh portions of rich ocean broth for filter-feeding mussels and barnacles. A wave may knock a loosened limpet into the centre of a hungry sea anemone, or unearth a luckless crab for a vigilant shore bird. Waves help propagate species—from distributing algae spores to carrying southern California grunion up a moonlit beach to bury their eggs.

What Keats praised as the "eternal whisperings" of waves is basically a version of what acoustic engineers dub "white noise", a mix of frequencies. "It's sufficiently irregular to be fascinating," says Walter Munk, a Californian oceanographer.

Researchers like Munk were among the first to develop ways of predicting waves, based on complex readings of shoreline structure, tides and weather.

Highly classified wave studies were crucial during the Second World War. In the Pacific, Europe and North Africa, Allied strategy sometimes depended on amphibious assaults. But heavy surf could create death by drowning even before the fighting started, or produce a squad of thoroughly seasick soldiers tumbling into enemy fire.

During the November 1942 landing in North Africa, scientists were able to predict a relatively calm spell on beaches that usually presented forbidding waves. And for D-Day, Supreme Commander Dwight Eisenhower was warned that the wave pattern looked rough. But delay seemed even more dangerous, so the invasion went ahead, encountering the expected tough conditions.

For amateurs, predicting specific waves is maddeningly difficult no matter how long you watch. There's rarely an entirely dependable sequence—no magical "seventh wave" or "ninth wave" that's always the biggest. The result of this unruliness can be lethal. Along the rocky coast of northern California, Oregon and Washington—meccas for devoted wave watchers—three or four deaths a year are invariably attributed to "killer" or "sneaker" waves. The killer may appear out of relatively calm waters, typically on nice days, sweeping the unwary victim out to sea or smashing him against the rocks.

Such killers amid apparent calm derive from the fact that few waves are simple. Almost any wave striking the beach is a combination of numerous others, some far out at sea, their crests and troughs meeting in a process of addition and subtraction. According to Steven Webster, education director of the Monterey Bay Aquarium in California, "The combinations are so complicated that the big one may turn out to be the 251st wave"—which, very rarely, may also turn out to be a killer.

The biggest "sleeper" waves of all are found far out at sea: full-sized ship killers. These leviathans, implicated in dozens of mysterious mid-ocean ship disappearances, are again the coincidence of various crests and troughs all adding up to one monstrous edifice.

The towering wall of water can sink a ship 500 feet long. "It's the trough that kills you," says Willard Bascom, oceanographer. "The bow drops away, and then the wave comes straight across and cleans the wheel-house off the deck." The tallest wave so far reliably measured at sea registered 112 feet. Experts suspect killer waves that leave no witnesses may be nearly twice as high.

Because of their random complexity, waves are still only partially understood. Engineers building jetties or piers can run into unpredictable situations: disappearing beaches or sudden new sand bars. Some biologists now suspect that migrating whales may ride beneath deep-water swells to help speed their passage. Oceanographers have lately noted that waves off the south coast at Land's End have grown a third taller over the past three decades, perhaps due to influences in the atmosphere.

Wave prediction still has some distance to go, but change is coming. Munk recalls a breakthrough experiment during the 1950s that used ships and land-based stations to track a single wave disturbance for two weeks, from its genesis off the coast of New Zealand to Alaska. "It took for ever to do that experiment," he says. Today, satellite-based radar can do the same job effortlessly, even through layers of clouds.

"In another couple of years," Munk says, "you'll be able to get a reliable wave forecast for anywhere in the world."

Except, of course, you probably won't want one for that isolated beach where you go merely to contemplate the universe, to gaze out into the distance over the swells. For the final sweet consolation of waves is simple: each one is slightly different—and there will always be another.