Freaks

[bilbobaggins]

A study by oceanographers in the US suggests that freak wave 'hot spots' exist and may be capable of prediction.

Apparently, "the theoretical study shows that coastal areas with variations in water depth and strong currents are hot spots for freak waves" and "sandbanks and strong currents may cause waves to change direction and speed. This concentrates wave energy into a single point .....a "wave focal zone."

Ignoring for now the BBC's confusion between a 'point' and a 'zone', I recall that a deeply experienced YM instructor/examiner once told me that, from his own experience, the area off Milford Haven/St Davids Head was one such zone of concern, and there should certainly be others around our coasts. It is commonly held that the vicinity of the edge of the continental shelf off the west coast of France is another such problem area ( 'Heavy Weather Sailing' ) - of significance to those transiting the Bay of Biscay.

I also recall comment in an earlier edition of the usually-understated 'Admiralty Pilot' which, referring to the Pentland Firth, stated "in conditions of a developed westerly gale and a weather-going tide, the seas have to be seen to be believed."

Bearing in mind that a 'rogue' or 'freak wave' is considered to be over 2 times higher than the average of the one-third largest waves ( Significant Wave Height, or Hs ), and that seas also increase in height and steepness when impeded by shoaling water - and then are likely to 'break' or 'jet' - it is certainly of interest to us who may seek to identify and avoid such areas.

The 'Draupner freak wave' recording, which proved that such waves occurred 'in the wild'.

What do others think?

 

[Tommyrot]

I remember watching a TV prog a few years ago about it, I think they were looking at a stretch of the coast somewhere around southern Africa.
Similar conditions applied there, strong currents and sea bottom contours causing "freak" waves.

 

[ParaHandy]

Apparently, "the theoretical study shows that coastal areas with variations in water depth and strong currents are hot spots for freak waves"

I was munching grub with a professor on the subject (funded by various bodies so, clearly, somebody's worried) ... the ESA have a pretty accurate radar satellite which can detect not the absolute sea level but the relative heights between adjacent waves. It is "turned on" to detect rogue waves every now and then when somebody's paid them. That's where most scientific data comes from. Anyway, the prof reckons that your average rogue wave will have come from sometimes many thousands of miles away .. I can tell you no more as the pud was excellent ...

Coincidentally, recent MAIB reports have been on ships damaged by sea conditions and, in a few, the MAIB have gone to some length in trying to explain the phenomena of rogue waves and, which probably explains why the prof gets his money from, the changes which might have to be made to ship design for them to survive such conditions.

Another coincidence is the second passenger jet leaving South America in recent times which has encountered unusually violent conditions in the Atlantic. Logically, if especially violent conditions can exist on the surface there might be every reason to believe they exist well above the surface ....

 

[photodog]

There is a lesson to be learnt here...

When she's on her best behaviour, Dont be tempted by her favours,

Never turn your back on mother earth.

 

[sarabande]

big conference a couple of years ago:

http://www.waveworkshop.org/10thWaves/ProgramFrameset.htm

See Session N, for papers. LoOts in previous sessions, too.

Also in New Scientist recently
http://www.newscientist.com/article/dn7793

For animated radar views of transient rogues
http://www.oceanwaves.de/
and
http://www.oceanwaves.org/technicals/radar_animation.HTML

You can odd red blips appearing from nowhere, and the relationship between a deep trough and a high peak is chillingly graphic.

But your point about rogues being generated from a long distance links to a more recent article in NS about surf forecasting (see the second half)

http://www.newscientist.com/article...up-learning-to-forecast-the-waves.html?page=2

Years ago, I did some work on flooding on the north Somerset Coast (Porlock, Porlock Bay, and just beyond Hinkley Point.) The purpose was to cost a chain of public address systems, using electrostatic loudspeakers of huge power, to be used as warning systems for a radioactive problem from Hinkley Nuclear Power Station. The speakers could be used for signal and voice, and remote controlled from the County war room.

I saw some work from commercial hydrologists about the funnelling effect of the Severn Estuary. Potentially even more frightening than the North Sea surges. It's not unthinkable that the Levels could be flooded again (are you there, Somerset ? Time to get some floats for your van )

 

[sarabande]

There's some intriguing work on the role of thermoclines on sub-surface waves here
www.norway.org/NR/rdonlyres/6F8E14CA-B8F0-40F6-8B38-163CB417BE71/23700/GRUEweb.pdf

Really good graphics BTW, and fascinating stuff about suspended pipelines, which I never knew existed !

Also we have had discussions on the forum about subsurface waves in the Straits of Messina about a couple of years ago.

 

[Bilgediver]

Three places for which there are piccies on the web in various places are.


Off Newfoundland...


Dogger Bank when a northerly gale blows



Off East London South Africa .


All these share the combination.


However there are instances in mid Pacific and mid Atlantic where the depth variation is so far below surface as not really an issue????


The only one I experienced was on a flat calm day in th Pacific...it was a trough and not a wave an travelling at phenominal speed. Estimated about 75/100ft deep!!!!! We survived though the engines coughed and spluttered and turbos surged.

Good thing the indian crew had gone of deck for lunch or we.d still be looking for them.