Alaskan Killer Waves

Every once in awhile, the forces of the sea and wind combine to produce the rogue wave. A wall of water a hundred or more feet in height, the rogue wave can easily mash the bow of a large ship or break the ship in two.

Writing in Smithsonian Magazine, Peter Britton cites a study of giant waves in the Gulf of Alaska. Waves there are found to be no worse than in the northern North Sea, where extreme wave heights are near 100 feet. However, the study indicates that the maximum possible wave height in the Gulf of Alaska is a terrifying 198 feet.

In a sense, rogue waves are statistical accidents. They form when the crests of several different trains of waves, each with its own speed and direction, all come together at the same time. The wave height at that time is the sum of the heights of the individual waves. Since the individual wave trains are moving at different speeds, the rogue wave cannot last long. Either the waves of which it is made separate or the extreme wave may destroy itself by breaking over at the crest.

Storms at sea are the basic causes of large waves that can merge together to create monster waves. Gusting winds start waves growing, and if the wind continues for several tens of hours the waves will steadily grow until a maximum height proportional to the square of the wind speed develops. A 40-knot wind blowing for 45 hours will create 30-foot waves.

Changes in wind direction, reflection of waves off steep shorelines and modification to the wave trains by changes in undersea topography (especially near continental shelf boundaries) can all combine to bring several of the larger storm waves together for a brief minute. If there also happens to be a ship at the point of intersection, that brief minute will be one of terror.