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Freak wave

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This article discusses deep ocean freak waves which occur far out to sea. For tsunami ("tidal" waves) and megatsunami (single giant waves due to impact or landslide in confined areas), see those articles.
The Draupner wave, a single giant wave measured on New Year's Day 1995, finally confirmed the existence of freak waves, which had previously been considered near-mythical
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The Draupner wave, a single giant wave measured on New Year's Day 1995, finally confirmed the existence of freak waves, which had previously been considered near-mythical

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 ocean phenomenon, not rare, but rarely encountered. 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 have been cited in the media as a likely source of the sudden, inexplicable disappearance of many ocean-going vessels. However, although a credible explanation for unexplained losses, there is to date little clear evidence supporting this hypothesis nor any cases where the cause has been confirmed, and the claim is contradicted by information held by [Lloyd's Register].The story that "200 large ships lost to freak waves in the past two decades" was published in The Times (May 2006). The earliest reference seems to be in the press release by the European Space Agency (cited at the page bottom), and first quoted as "200 large ships of 600ft long or more in the past two decades sunk without trace". At the time the claim was made, there had only been 142 ships of that size lost at sea in the time frame, all with clear, known causes (source: [Lloyd's Register - Fairplay]). The main culprits were the Iranian and Iraqi air forces in the 1980s (See: Iran-Iraq war). One of the very few cases in which evidence exists that may indicate a freak wave incident is the 1978 loss of the freighter München, detailed below. In February 2000, a British oceanographic research vessel sailing in the Rockall Trough west of Scotland encountered the largest waves ever recorded by scientific instruments in the open ocean. Holliday, NP, MJ Yelland, RW Pascal, VR Swail, PK Taylor, CR Griffiths, and EC Kent (2006). [Were extreme waves in the Rockall Trough the largest ever recorded?] Geophysical Research Letters, Vol. 33, L05613

Contents

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 somewhat more if some deformation is allowed for, which is about a wave of twenty metres.According to traditional linear models, a 12 meter wave has a force of about 6 MT/m, and ships are therefore designed to withstand around 15 MT/m. However a freak wave may have a force of around 100 MT/m. [link] (PDF)

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 reconsideration of the implications for ocean-going ship design.

These localized freak waves are not the same as tsunami or megatsunami. Tsunami (or tidal waves) 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, tidal waves do not represent a threat to shipping. Megatsunami are also rare events, but only arise in confined spaces, such as inlets and river valleys. Freak waves, by contrast, are localized short-lived water phenomena that most frequently occur far out to sea.

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, resulting an a total imaged 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.

Causes

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:

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 are three categories of freak waves:

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

Encounters

:The Master said it "came out of the darkness" and "looked like the White Cliffs of Dover." [link] (PDF) Newspaper reports at the time described the cruise liner as attempting to "surf" the near-vertical wave in order not to be sunk.
  • Bremen and Caledonian Star (same wave, South Atlantic, 2001)
    • :Bridge windows on both ships smashed, 30 meters above sea level, and all power and instrumentation lost. No adverse currents exist in that part of the world to explain the wave. The First Officer of the Caledonian Star stated it was "just like a mountain, a wall of water coming against us." [link] (PDF)
  • Naval Research Laboratory ocean-floor pressure sensors detected a freak wave caused by Hurricane Ivan in the Gulf of Mexico, 2004. The wave was around 27.7 meters high from peak to trough, and around 200 meters long. [link]
  • Norwegian Dawn, (off the coast of Georgia, 16 April, 2005): [media report]
    • :"The sea had actually calmed down when the [21 meter] wave seemed to come out of thin air... Our captain, who has 20 years on the job, said he never saw anything like it."
  • Norwegian Spirit, (off the coast of Tortola, January, 2006)
  • Aleutian Ballad (Bering Sea, 2006) : from television show "Deadliest Catch".
    • Several sources (including those below) repeat the claim that around 200 large ships have been sunk in recent years by 'freak' waves. That claim is a myth. There are a tiny number of cases in recent years where no obvious explanation has been found, but according to the Lloyd's Register casualty database, fire or poor maintenance are more likely causes.

    Freak waves in literature and film

    • Early in the movie, a "rogue wave" washes over the protagonists' fishing vessel in the film The Perfect Storm, but does not sink the boat. It is merely treated as a bad omen of what is to come in the story. However, it is possible that the giant wave that finally sinks the boat is a similar rogue wave, only much larger.

    See also

    References

    External links

    MaxWave report and WaveAtlas

    Other

     

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