Rogue wave (oceanography)

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This article discusses deep ocean freak waves which occur far out at sea. For tsunami and tidal wave phenomena, see those respective 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
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

Rogue waves, also known as freak waves, are relatively large and spontaneous ocean surface waves which are a threat even to large ships and ocean liners. In oceanography, they are more precisely 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 rogue 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 rogue 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 this is 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.[1],[2] One of the very few cases in which evidence exists that may indicate a freak wave incident is the 1978 loss of the freighter MS 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. [3]

Contents

[edit] History

Merchant ship labouring in heavy seas as huge wave looms behind. Huge waves are common near the 100-fathom curve on the Bay of Biscay. Published in Fall 1993 issue of Mariner's Weather Log. Credits: NOAA Photo Library
Merchant ship labouring in heavy seas as huge wave looms behind. Huge waves are common near the 100-fathom curve on the Bay of Biscay. Published in Fall 1993 issue of Mariner's Weather Log. Credits: NOAA Photo Library

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 (around 20 m) if some deformation is allowed for.[4]

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 theory would predict using a Rayleigh probability distribution of wave heights. 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.

Rogue waves are also known to occur on the inland Great Lakes, which are more like large inland seas. Perhaps most famously such inland freak waves are believed, according to some reconstructions, to be responsible for the sinking of the Edmund Fitzgerald in November 1975 (see below). However other causes have been advanced; the matter is far from settled.

A rogue wave is not the same as a tsunami. Tsunamis are mass displacement generated waves which propagate at high speed and are more or less unnoticeable in deep water; they only become dangerous as they approach the shoreline and do not present a threat to shipping (the only ships lost in the 2004 Asian tsunami were in port). A rogue wave, on the other hand, is a spatially and temporally localized event that most frequently occurs far out at sea.

[edit] 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 10×5 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.[5]

[edit] 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 that 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
  • Constructive interference — In this theory, several smaller wave trains meet in phase. Their crest heights combine to create a freak wave.[6]
  • Focusing by currents — Storm forced waves are driven into an opposing current. This results in shortening of wavelength, causing shoaling (i.e., increase in wave height), and oncoming wave trains to compress together into a rogue wave.[6]
  • Nonlinear effects — It seems possible to have a freak wave occur by natural, nonlinear processes from a random background of smaller waves.[7] 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. One simple model for this is 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. The NLS is only valid in deep water conditions, and in shallow water an alternative such as the Boussinesq equation is used.
  • Normal part of the wave spectrum — Rogue waves are not freaks at all but are part of normal wave generation process, albeit a rare extremity.[6]
  • Wind waves — While it is unlikely that wind alone can generate a rogue wave, its effect combined with other mechanisms may provide a fuller explanation of freak wave phenomena. As wind blows over the ocean, energy is transferred to the sea surface. Phillips and Miles[8] provide some insight into the problem, though it still remains a tricky one.

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 rogue 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:

  • "Walls of water" travelling up to 10 km through the ocean
  • "Three Sisters", groups of three waves[9]
  • Single, giant storm waves, building up to fourfold the storm's waves height and collapsing after some seconds[10]

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

A research group at the Umeå University, Sweden in August 2006 showed that normal stochastic wind driven waves, all of a sudden can give rise to monster waves. The nonlinear evolution of the instabilities was investigated by means of direct simulations of the time-dependent system of nonlinear equations.[11]

[edit] Encounters

  • On 10 October 1903, RMS Etruria was only four hours out of New York when, at 2:30 p.m., the ship was struck by a freak wave. The wave was reported to be at least 50 feet (15 meters) high and she struck the ship on the port side. The wave carried away part of the fore bridge and smashed the guardrail stanchions. There were a number of first-class passengers sitting in deck chairs close to the bridge and they caught the full force of the water. One passenger was fatally injured and several other passengers were hurt.
  • In 1933 in the North Pacific, the U.S. Navy oiler USS Ramapo encountered a huge wave. The crew triangulated its height at 112 feet (34 meters).[12]
  • In 1942 while carying 15,000 American troops 700 miles from Scotland during a gale, RMS Queen Mary was broadsided by a 28-meter wave and nearly capsized. Queen Mary listed briefly about 52 degrees before the ship slowly righted itself.
  • In 1966, the Italian cruise ship Michelangelo was steaming toward New York City when a giant wave tore a hole in its superstructure, smashed heavy glass 80 feet (24 meters) above the waterline, and killed a crewman and two passengers.[12] The matter is related by Daniel Allen Butler in his book The Age of Cunard and by Walter Ford Carter in No Greater Sacrifice, No Greater Love.
  • SS Edmund Fitzgerald was a lake freighter that sank suddenly during a gale storm on November 10, 1975, while on Lake Superior, on the Canada–United States border. The ship went down without a distress signal in Canadian waters about 17 miles (15 nm; 27 km) from the entrance to Whitefish Bay (at 46°59.9′N 85°6.6′W). At the location of the wreck the water is 530 feet (162 m) deep. All 29 members of the crew perished. A coast guard report blamed water entry to the hatches, which gradually filled the hold, or alternatively errors in navigation or charting causing damage from running onto shoals. However, another nearby ship, the Anderson, was hit at a similar time by two rogue waves, and this appeared to coincide with the sinking around ten minutes later — or at least contributed to the sinking if the Edmund Fitzgerald was already in trouble as suggested. A Discovery Channel reconstruction pointed the finger towards freak waves as the cause. The matter is far from settled.
  • The Wilstar, a Norwegian tanker, suffered structural damage from a rogue wave in 1974.[12]
  • In October 1977, the tanker Stolt Surf ran into a rogue wave on a voyage across the Pacific from Singapore to Portland, and the engineer took photos of a wave higher than the 22-meter bridge deck.[13]
  • The six-year-old, 37,134-ton barge carrier MS München was lost at sea in 1978. At 3am on 12 December 1978 she sent out a garbled mayday message from the mid-Atlantic, but rescuers found only "a few bits of wreckage". This included an unlaunched lifeboat, stowed 20 metres above the water line, which had one of its attachment pins "twisted as though hit by an extreme force". The Maritime Court concluded that "bad weather had caused an unusual event". It is thought that a large wave knocked out the ship's controls (the bridge was sited forward), causing the ship to shift side-on to heavy seas, which eventually overwhelmed it. Although more than one wave was probably involved, this remains the most likely sinking due to a freak wave.[7]
  • Draupner wave (North Sea, 1995): first confirmed scientific evidence
  • RMS Queen Elizabeth 2 (North Atlantic, 1995), 29 meters, during bad weather in the North Atlantic.
The Master said it "came out of the darkness" and "looked like the White Cliffs of Dover." [14] Newspaper reports at the time described the cruise liner as attempting to "surf" the near-vertical wave in order not to be sunk.
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."[14]
"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."[16]
"The water exerted enough force to shear off the welds for the aluminum rail supports on the [ninth and tenth level] balconies of two cabins, allowing the teak balcony rails to break loose and crash into the cabin windows. The broken glass filling the drains compounded the water damage by allowing a large amount of water to enter the two cabins and damage the carpets in 61 other cabins. The ship’s operating at reduced speed when the waves hit probably limited the damage."[17]

It has also been suggested that these types of waves may be responsible for the loss of several low-flying aircraft, namely U.S. Coast Guard helicopters on Search and Rescue missions.[21]

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.[2] There are a tiny number of cases in recent years where no obvious explanation has been found, but according to the Lloyd's Register-Fairplay casualty database, fire or poor maintenance are more likely causes. The claim first appeared in the terms of reference for the EU's Max Wave project in 2001, without any supporting evidence. It was phrased as "200 supertankers or containerships of 200m and over sunk in the past 20 years". According to Lloyd's Register, only 143 ships of this size were lost from 1981–2001. The claim achieved wider currency after it was picked up by the European Space Agency in its 2004 press release about freak waves observed from space (see External Links below).

[edit] Freak waves in literature and film

The rogue wave that struck the Poseidon in the 2006 film Poseidon
The rogue wave that struck the Poseidon in the 2006 film Poseidon
  • In Paul Gallico's 1969 novel The Poseidon Adventure, and in the 2006 film Poseidon, based on the novel, a rogue wave capsizes an ocean liner. (In the 1972 film, The Poseidon Adventure, the wave is described as a tsunami generated by a sub-sea earthquake).
  • Early in the 2000 movie The Perfect Storm, a "rogue wave" washes over the protagonists' fishing vessel, 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.
  • Freak waves are a major theme in Clive Cussler's novel Polar Shift.
  • Stephen Colbert listed rogue waves in one edition of his Threat Down.
  • The Apocalypse by Tim Bowler also has freak waves crashing upon the island of Skår.

[edit] See also

[edit] References

  1. ^ Lloyd's Register–Fairplay
  2. ^ a b 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 143 ships of that size lost at sea in the time frame, according to Lloyd's Register, all with clear, known causes. The main culprits were the Iranian and Iraqi air forces in the 1980s; Iran–Iraq war.
  3. ^ 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
  4. ^ 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 rogue wave may have a force of around 100 MT/m (Beacon #185, Skuld, June 2005)
  5. ^ Freak waves spotted from space. BBC News Online (22 July 2004). Retrieved on May 8, 2006.
  6. ^ a b c Rogue Waves. Ocean Prediction Center. National Weather Service (April 22, 2005). Retrieved on May 8, 2006.
  7. ^ a b Freak Wave, BBC.co.uk programme summary for Horizon episode aired on 14 November 2002
  8. ^ Phillips and Miles, 1957, Journal of Fluid Mechanics
  9. ^ Endeavour or Caledonian Star report, March 2, 2001, 53°03′S 63°35′W
  10. ^ MS Bremen report, February 22, 2001, 45°54′S 38°58′W
  11. ^ P. K. Shukla, I. Kourakis, B. Eliasson, M. Marklund and L. Stenflo: "Instability and Evolution of Nonlinearly Interacting Water Waves" nlin.CD/0608012, Physical Review Letters (2006)
  12. ^ a b c Rogue Giants at Sea, Broad, William J, New York Times, July 11, 2006
  13. ^ The Storm: Stolt Surf in the North Pacific, 1977, Petersen, Karsten, December 8, 2003; retrieved July 11, 2006.
  14. ^ a b Freak wavesPDF (1.07 MiB), Beacon #185, Skuld, June 2005
  15. ^ Hurricane Ivan prompts rogue wave rethink, The Register, 5 August 2005
  16. ^ Freak wave pummels cruise ship, Sydney Morning Herald, April 18, 2005
  17. ^ NTSB Marine Accident Brief: Heavy-weather damage to Bahamas-flag passenger vessel Norwegian Dawn, National Transportation Safety Board, April 16, 2005, reference NTSB/MAB-05/03
  18. ^ Captain of Picton Castle says a rogue wave swept woman overboard off U.S., CBC, December 9, 2006
  19. ^ Woman on Nova Scotia ship swept overboard, Canada.com, December 9, 2006
  20. ^ Air search for missing sailor to resume Sunday, CTV, December 9, 2006
  21. ^ Monster waves threaten rescue helicoptersPDF (35.7 KiB), U.S. Naval Institute, December 15, 2006

[edit] External links

[edit] MaxWave report and WaveAtlas

[edit] Other

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