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Rogue Waves

A rogue wave is fortunately a once-in-a-lifetime event for offshore sailors.

Captain Warwick peered out of a bridge port and watched what looked like a small mountain looming out of the night. Piloting one of the largest ships in the world did not make him feel more comfortable about what he was seeing. The wave seemed to take forever to reach the ship but struck actually less than a minute after being sighted. The monster wave broke over the bow with incredible force, followed immediately by a shudder throughout the entire ship. Warwick reported that there seemed to be two waves, one following closely behind the other. As the massive ship fell into the hole left by the first wave, the second inundated the foredeck, ripping away the forward whistle mast.

According to Captain Warwick, it was difficult to estimate the height of this wave. Its crest was more or less level with his line of sight on the bridge—about
29 meters (87 feet) above the surface of the water. The other officers on the bridge agreed with Warwick that it was a true wave, not a swell. The presence of extreme waves was also recorded by Canadian weather buoys moored in the area, and the maximum measured height from one of these buoys was 30 meters (98 feet).

The ship was the Queen Elizabeth II and the event was recorded en route from Cherbourg, France, to New York. During this trip the ship changed course several times in order to go around Hurricane Luis. The Grand Lounge windows, which are located 22 meters (66 feet) above the surface, were struck with such force that they caved in.

One wave larger than all the rest can easily catch sailors off guard.

These events are generally referred to as "rogue waves," and they sometimes appear under ostensibly calm conditions. Many people refer to such events as "tidal waves," a misnomer that has stayed with us for generations—such waves have nothing to do with the tides. Mostly due to the movement of the sun and the moon, tides can rise and fall in open water with no effect at all on shipping or the state of the sea. Indeed, even tsunamis generated by underwater seismic activity only create the towering walls of destruction near shore where the water becomes shallow. On the open sea, a tsunami might pass under a ship unnoticed.

Wind waves, or the rippling of the ocean surface by the friction and driving force of the wind, is the most ever-present oceanic feature. Winds blowing across the sea surface form a series of irregular hills and valleys. Light winds create small wavelets, but when strong winds blow, huge storm seas can develop.

Waves travel quickly across the surface of the sea, deceiving our senses into thinking the water is moving. In fact, the water itself does not move very rapidly—it is mainly traveling up and down in a circular, or rotary, motion. This is actually a fortunate illusion of nature, since water moving at the speed of most storm waves would make ocean navigation in ships nearly impossible.

How big the waves get depends on three things—the speed of the wind, the duration, or how long the wind blows, and the fetch, or the distance of open water over which the wind is blowing. Light winds, or those that only blow hard for a short time, cannot generate large waves. Ships seek out the safety of a harbor in a storm because they can block a portion of the wind with a body of land, thereby reducing the fetch of open water. Storms of equal size generate much larger waves in the open Pacific than in the Adriatic Sea where fetch is much more limited.

It takes a while after the wind begins to blow before large waves agitate the sea surface. As the wind continues to blow, the waves get higher from trough to crest, and both the wave length and period become longer. Wave length is measured from crest to crest, while the period of a wave is the time it takes for one full wave from crest to crest to pass the same point. Wave height is actually less important than the steepness of the seas and this is a critical point for sailors since steepness is a product of the height and the length, being the angle between crest and trough. Sailboats will glide over very high, long ocean rollers and barely notice them but when the wave length is shortened, the waves become steep.

It is never just the height or the length of the waves that matter, but the relationship between the two.

It is this wave gradient that creates dangerous conditions for small boats. Waves are generally steeper at the beginning of a storm and are at their worst near its center. As the wind continues or strengthens, the water first forms whitecaps and eventually the waves start to break. Waves as high as 30 meters (nearly 100 feet, or as high as a nine-story building) have been recorded during severe storms. This is referred to as a fully developed sea, and in one way this is good because it tends to limit the waves future growth. Good, that is, if you’re not out among them.

These waves radiate out great distances from the storm that generated them, eventually lengthening and becoming reduced in height. We call these swells, and mariners have known for centuries that an increasing swell means bad weather is coming. Beach lovers are often warned of high swells and surf on beautiful sunny days because of storms that may be far over the horizon.

The longer the wave, the faster it travels. As waves leave a storm area, they tend to sort themselves out with the long ones ahead of the short ones, and the energy is simultaneously spread out over an increasingly larger area. As the waves close in on the coast, they begin to feel the bottom and their direction of travel might change due to the contour of the land. Eventually, the waves run ashore, increasing in height up to 1.5 times their height in deep water, finally breaking up as surf.

A sight that none of us ever want to see.

A dramatic change occurs when the energy in a wave meets the land. In the area of the surf, the waves become steep and the water begins to move forward at the speed of the waves. Waves can also be amplified when traveling into an opposing current, forming dangerous riptides and undertows. These two effects together make the surf area a potentially dangerous place for both people and boats. Many cases of rogue waves actually have their origins in high swells from a distant storm moving over an area of reduced water depth, around a headland, or into an opposite current. Areas with these land features, like the Hourglass Shoals, between Hispaniola and Puerto Rico in the notoriously rough Mona Passage, have developed nasty reputations among sailors.

The most common explanation for rogue waves involves the chance meeting of two smaller waves from different wave trains. It is not uncommon to have a swell from one, or even several directions, at the same time as seas are being driven by a local wind. If a large local wave happens to coincide and be reinforced by an unusually large swell from a more distant area, a wave much bigger than those surrounding it occurs. Statistically, this is not uncommon if we take the number of waves on the ocean into account.
Rogue waves are believed to have been the cause of numerous marine accidents and fatalities. Sailors' logs show that these uncharacteristically large waves occur with no warning, especially along exposed coastlines. Annually, at least one person is swept off the rocks by rogue waves on the southwest coast of Vancouver Island when the seas are reasonably calm. In April 1991, a man was swept from the rocks and drowned by large waves at South Beach near Tofino, BC, during a time when there was no unusual wave activity.

These are obviously very unusual events; however, rogue or freak waves are regularly reported during all sorts of weather including calm days. The unusual confluence of wave trains usually last only several moments, but they are totally unpredictable. The fact that they sometimes come in groups probably contributes to the popular belief that every seventh wave is the highest. Research has shown that this is not the case, but that large waves come in groups of various lengths. In fact, groups of waves of random lengths would be more likely to cause one single freak wave larger than all its peers than if every seventh wave was the largest.

New Evidence   In May 2000 researchers set out to sea from the Virginia coast hoping to find answers to a new scientific riddle: Could tsunamis, our popularly misnamed tidal waves that have terrorized Japan and the rest of the Pacific for centuries, pose a threat to the mid-Atlantic coast of the US as well?

The scientists looking into this have just returned from a follow-up research cruise, during which they found seafloor formations that appear to have been formed by blowouts of surprisingly huge quantities of gas trapped in the sediments off the Virginia coast. Could huge deposits of gas beneath the sea somehow be released?

Some of the craters left by the gas releases are said to be large enough ''to contain an area as large as New York's Central Park,'' said Jeffrey Weissel of Columbia University's Lamont-Doherty Earth. Weissle goes on to say that these historical gas releases ''must have been very vigorous to have created craters of this size. "These are the biggest we've seen. We were a little taken aback by the amount of gas,'' said team leader Neal Driscoll of the Woods Hole Oceanographic Institution.

Craters like this blue hole could be associated with gas releases that may have created large waves.

The discovery raises the possibility that a tsunami could strike the US East Coast. Even though the data from the fact finding cruise will not be analyzed for a while, one thing is already clear, the researchers say: Any tidal wave that might be triggered by these continental-shelf fissures is nothing to get excited about nor enough to pack up and move inland. The waves would be no more severe or widespread than those produced by hurricanes, which happen far more frequently. But the waves caused by one of those minor gas releases could have been recorded as a rogue wave by previous mariners trading along the coast.  

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