Extreme (Rogue) Waves

Waves that are more than twice the size of the significant wave height, which itself is defined as being approximately equal to the average of the highest one-third of the waves in a wave record. Those waves are also popularly known as rogue, monster, or freak waves. For decades, and even hundreds of years, extreme waves have been thought of as large-scale fisherman’s tales on the part of mariners, in other words, tall tales that wouldn’t bear too much scrutiny. However, the first serious questions were raised in 1978 when the newly constructed, state-of-the-art cargo ship München went down in the mid-Atlantic Ocean with only a garbled SOS message. An extensive search and rescue effort found limited debris and a single lifeboat that had obviously sustained severe structural damage while still attached to the München some 65 feet above the water line. However, despite rampant speculation, no definitive answers were found and scientific fingers continued to be pointed in the direction of more tall tales on the part of mariners.

But on January 1, 1995, all that changed when the height of a monster wave was measured at 80 feet by a precise laser instrument at the securely stationed Draupner oil platform in the North Sea off the coast of Norway in a background of waves averaging 35 feet in height. That wave has since become known as the “New Year Wave” and had a maximal wave height that was much more than twice the significant wave height. Almost as important to a sea captain, located at the base of the extreme wave crest was an almost equally dramatic wave trough. So a ship’s captain approaching such a wave would be confronted not only with a massive wall of foaming, boiling water towering about two or three times higher than all other waves in the area but also a huge trough or “hole in the sea” below it where the ocean should be, a truly terrifying situation.

Immediately after the 1995 “New Year Wave” scientists started paying a lot more attention to the nature and propagation of ocean waves since reliable calculation of high return period wave heights is of critical importance in the design and construction of offshore structures as well as of large ships. The largest wave naval engineers and architects were required to accommodate in the design strength calculations was 15 meters from trough to crest. If that design parameter was determined to be incorrect, especially in light of the Draupner platform reading, the world’s shipping and oil platform industries would be confronted with grave difficulties.

Mid-ocean storm waves commonly reach seven meters (23 feet) in height and in severe storm conditions can attain 15 meters (50 feet). But if extreme waves up to 30 meters (100 feet) in height could appear without warning in mid-ocean, against the prevailing current and wave direction, and often in perfectly clear weather those waves could break over the bow of a ship, generating tremendous pressures of up to 100 tons per square meter and could easily inflect severe structural and equipment damage or even sink the vessel instantaneously or in a matter of minutes. Ship design standards anticipated rounded storm waves of up to 15 meters (trough to crest) and pressures around 15 tons per square meter without the ship sustaining damage and up to twice that pressure if the design allowed for some structural deformation, which would indicate a wave height of about 20 meters.

A linear mathematical model had long been used by oceanographers, meteorologists, naval engineers/architects, and the shipping industry to predict the height of ocean waves. That model assumed that storm waves were smooth and gently sloping. Consequently, ships of all sizes were designed to deal with undulating waves. The model also assumed that waves varied in a normal distribution around the significant (average) wave height. As a result, in an ocean storm with a significant wave height of 12 meters, the model suggested a wave higher than 15 meters would be very rare and one of 30 meters had an extremely slight statistical probability of occurrence, about once every 10,000 years. But the model failed to correspond with the reality of 20 vessels having been struck by larger than normal waves off the South African coast since 1990 or with the Draupner platform data.

A number of oceanographers, especially Marten Grundlingh of the Southern African Data Centre for Oceanography (Council for Scientific and Industrial Research), who were interested in the high number of South African incidents determined that all the ships damaged by the large waves had been located at the edge of the Agulhas Current, where the warm Indian Ocean water mixed with colder Atlantic flow. Satellite radar imagery confirmed that wave heights at the edge of that Current could grow well beyond the linear model’s predictions, especially if the dominant wind was blowing in the opposite direction of current flow. The obvious solution to the problem required ship captains to avoid certain ocean currents in specific weather conditions.

However, research performed in 2001 by the German Aerospace Center (known as MaxWave, a three-week project in August and September 2001) using global satellite radar imagery collected by the European Space Agency verified the existence of these waves in other parts of the world ocean by analyzing 30,000 radar images of the ocean surface and finding three waves of enormous size (100-foot, 90-foot, and 85-foot waves) and at least ten others with crests and troughs measuring more than 81 feet in height but less than 85 feet. In other words, more extreme waves were found than anyone had expected, taking the entire scientific community by surprise. Although definitive conclusions have yet to be reached, three types of extreme waves may be cruising the oceans: 1) walls of water traveling at speeds of up to six miles per hour; 2) groupings that mariners call Three Sisters; and 3) short-lived, single giant waves building up to four times the storm’s normal wave height and collapsing soon thereafter.

Over the past ten years, extreme waves have gradually been recognized as normal ocean phenomena that have several possible causes.

Constructive Interference (superposition): Several wave trains meet that are characterized by differing speeds and directions. Their crest heights combine so that when high waves are included in the wave trains an extreme wave can result. Since the wave trains continue to separate and move apart this wave effect is normally of short duration.

Focusing of Wave Energy by Strong Currents: When storm-forced waves are driven into an opposing ocean current an interaction can take place that results in a shortening of wave frequency, causing oncoming wave trains to compress together into an extreme wave that tends to have a long duration. The Gulf Stream and Agulhas Current are examples of locations where those multidirectional and multidimensional conditions are found with some regularity.

Normal Wave Generation Process: Water wave generation results in a spectrum of wave heights distributed from the smallest capillary waves to very large waves. Within that spectrum is a possibility of wave heights occurring from the most common (small waves) to the least likely (extreme waves). The random, nonlinear nature of waves implies that individual waves can be substantially higher than the significant wave height, thus, forming extreme waves.

Nonlinear Effects Similar to Solitons: According to research in Chaos Theory and quantum physics (especially by physicist Alfred R. Osborne, University of Turin; Efim Pelinovsky of the Institute of Applied Physics in Nizhny Novgorod, Russia; and mathematician Krystian Dysthe of the University of Bergen in Norway) it is feasible to have extreme waves occurring by natural, nonlinear processes from a random background of smaller waves.

Frequency Modulation and Resonance: Other research seeks to determine whether frequency or amplitude modulation and resonance may create extreme waves. An analogy is easily demonstrated in a bathtub by using your hand to slosh water back and forth. If you’re lucky, at just the right period a larger than normal wave will be produced.

Real World Examples include the first recorded evidence provided in 1995 by the Draupner platform wave in the North Sea; also in 1995 an extreme wave that was part of Hurricane Luis in the North Atlantic Ocean and estimated at 29-30 meters (by the ship’s officers who observed it as 95 feet in height) struck the RMS Queen Elizabeth II in the North Atlantic causing structural damage as high as the 10^th floor; the cruise ship Caledonian Star was hit by a wave in the South Atlantic in 2001 that was estimated by the captain as being 30 meters or more in height that struck “without any prediction, any expectations,” destroying much of the ship’s bridge, including the radar, gyro compasses, echo sounders, sonar, and parts of the radio communication system; in 2001 the German cruise liner Bremen was hit by an extreme wave in the South Atlantic in the same storm that had devastated the Caledonian Star only two days earlier, destroying the radar equipment, weather faxes, ventilator, alarms, and engines with the steering gear failing completely — for almost two hours the ship was not maneuverable and was dead in the water, a condition of extraordinarily high hazard in a storm; and in April 2005 the cruise ship Norwegian Dawn was hit by an 80- to 90-foot-high wave off the coast of Georgia, sustaining damage as high as its 10^th floor. And let’s not forget the famous upside down Hollywood example, Poseidon Adventure, or its remake, Poseidon.