Freak waves, also known as rogue waves or monster waves, are relatively large and spontaneous ocean surface waves which can sink even medium-large ships. Once thought to be only legendary, they are now known to be a natural (although relatively rare) ocean phenomenon. Their existence was known anecdotally from mariners' testimonies and damages inflicted on ships, however their scientific measurement was only positively confirmed following measurements of a freak wave at the Drauper oil platform in the North Sea on January 1, 1995.
Disputes as to their existence were finally laid to rest in 2004, when Project MaxWave , and the GKSS Research Centre , using data collected by ESA satellites, identified many dozens of such waves during a study.
They are a likely source of the sudden inexplicable disappearance of many ocean-going vessels.
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 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 15m and pressures around 15 tonnes/m2)
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" type probability 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, based on a linear model of wave size, indicates. In fact they 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 that 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 which most frequently occur far out to sea.
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:
According to some research [1], 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.
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 02.03.2001, )
- Single giant storm-waves, building up to fourfold the storm's waves height and collapsing after some seconds (MS Bremen report 22.02.2001, 45°54′ S 38°58′ W)
A comprehensive paper describing the ways that freak waves could form, complete with layman descriptions, photos and animations, can be found here.
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. It should be noted that a short lived wave in a section of ocean this size is still an extremely rare occurrence in its own right. [2]
Related articles
Reports of freak waves
External links
The MaxWave report and WaveAtlas
Other
