Freak wave

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Image:Drauper freak wave.png Freak waves, also known as rogue waves or monster waves, are relatively large and spontaneous ocean surface waves which can sink even large ships and ocean liners. In oceanography, they are more concisely defined as waves that are more than double the significant wave height (SWH), which is itself defined as the mean of the largest third of waves in a wave record.

Once thought to be only legendary, they are now known to be a natural (although relatively rare) ocean phenomenon. Anecdotal evidence from mariners' testimonies and damages inflicted on ships suggested they occurred; however, their scientific measurement was only positively confirmed following measurements of a freak wave at the Draupner oil platform in the North Sea on January 1, 1995. During this event, minor damage was inflicted on the platform, confirming that the reading was valid.

In the course of the Project MaxWave, researchers from the GKSS Research Centre, using data collected by ESA satellites, identified a large number of radar signatures that may be evidence for freak waves. Further research is underway to verify the method that translates the radar echoes into sea surface elevation.

Freak waves are a likely source of the sudden inexplicable disappearance of many ocean-going vessels.

1 History
2 Frequency of occurrence
3 Possible causes of freak waves
4 Encounters with freak waves
5 See also
6 External links
- 6.1 The MaxWave report and WaveAtlas
- 6.2 Other

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History

It is common for mid-ocean storm waves to reach 7 metres (23 feet) in height, and in extreme conditions such waves can reach heights of 15 metres (50 feet). However, for centuries maritime lore told of the existence of vastly more massive waves — veritable monsters up to 30 metres (100 feet) in height (approximately the height of a 12-story building) — that could appear without warning in mid-ocean, against the prevailing current and wave direction, and often in perfectly clear weather. Such waves were said to consist of an almost vertical wall of water preceded by a trough so deep that it was referred to as a "hole in the sea"; a ship encountering a wave of such magnitude would be unlikely to survive the tremendous pressures of up to 100 tonnes/m² (980 kPa) exerted by the weight of the breaking water, and would almost certainly be sunk in a matter of seconds. Usual ship design allows for rounded storm waves up to 15 m and pressures around 15 tonnes/m² (147 kPa) without damage, and up to twice that if some deformation is allowed for, which is about a wave of twenty metres.

Scientists long dismissed such stories, asserting that mathematical models indicated that ocean waves of greater than 15 metres in height were likely to be rare "once in 10,000 years" events. However, satellite imaging has in recent years confirmed that waves of up to 30 metres in height are much more common than mathematical probability would predict based on a linear model of wave size. In addition, pressure readings from buoys moored in the Gulf of Mexico at the time of Hurricane Katrina also indicate the presence of such large waves at the time of the storm. In fact, they seem to occur in all of the world's oceans many times every year. This has caused a re-examination of the reason for their existence, as well as calling into serious question many long-accepted principles of maritime engineering.

Note: The localized freak waves discussed here are not the same as tsunami, formerly called "tidal waves". Tsunami are displacement waves which travel at high speed and are more or less unnoticeable in deep water; they only become dangerous as they approach the shoreline. In the deep sea, tsunami do not represent a threat to shipping. Freak waves, in contrast, are localized short-lived water phenomena that most frequently occur far out to sea.

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Frequency of occurrence

The MaxWave project studied the ocean surface with radar over a 3-week period in 2001. They took 30,000 images each of a 10 x 5 km section of ocean in that time, a total area of 1.5 million km². Giant waves were detected in 10 of these, or one per 150,000 km². A short-lived wave in a section of ocean this size is an extremely rare occurrence in its own right. [1]

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Possible causes of freak waves

The phenomenon of freak waves is still a matter of active research, so it is too early to say clearly what the most common causes are or whether they vary from place to place. The areas of highest predictable risk appear to be where a strong current runs counter to the primary direction of travel of the waves; the area near Cape Agulhas off the southern tip of Africa is one such area. However, since this thesis does not explain the existence of all waves which have been detected, several different mechanisms are likely, with localised variation. Suggested mechanisms for freak waves include the following:

Diffractive focusing by, perhaps, coast shape or seabed shape
Focusing by currents
Nonlinear effects (similar to solitons)

According to some research, it is completely feasible to have a freak wave occur by natural, nonlinear processes from a random background of smaller waves. In such a case, it is hypothesised, an unusual, unstable wave type may form which 'sucks' energy from other waves, growing to a near-vertical monster itself, before becoming too unstable and collapsing shortly after. This is modelled by a wave equation known as the nonlinear Schrödinger equation (NLS), in which a normal and perfectly accountable (by the standard linear model) wave begins to 'soak' energy from the waves immediately fore and aft, reducing them to minor ripples compared to other waves. Such a monster, and the abyssal trough commonly seen before and after it, may last only for some minutes before either breaking, or reducing in size again.

It is important to note that the spatio-temporal focussing seen in the NLS equation can also occur when the nonlinearity is removed. In this case, focussing is primarily due to different waves coming into phase, rather than any energy transfer processes. Further analysis of freak waves using a fully nonlinear model by R.H. Gibbs (2005) brings this mode into question, as it is shown that a typical wavegroup focusses in such a way as to produce a significant wall of water, at the cost of a reduced height.

There seem to be three categories of freak waves:

"Walls of water" travelling up to 10 km through the ocean
"Three Sisters", groups of three waves (Endeavour or Caledonian Star report March 2 2001, Template:Coor dm)
Single, giant storm waves, building up to fourfold the storm's waves height and collapsing after some seconds (MS Bremen report Feb 22 2001, Template:Coor dm)

A comprehensive paper describing the ways that freak waves could form, complete with layman descriptions, photos and animations, can be found here.

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