A 'rogue wave' is large, unexpected, and dangerous
freak wave!
Rogue, freak, or killer waves have been part of
marine folklore for centuries, but have only been
accepted as a real phenomenon by scientists over the
past few decades.
Rogues, called 'extreme storm waves' by scientists,
are those waves which are greater than twice the
size of surrounding waves, are very unpredictable,
and often come unexpectedly from directions other
than prevailing wind and waves.
Most reports of extreme storm waves say they look
like "walls of water." They are often steep-sided
with unusually deep troughs.
Since these waves are uncommon, measurements and
analysis of this phenomenon is extremely rare.
Exactly how and when rogue waves form is still under
investigation, but there are several known causes:
-
Constructive
interference. Extreme waves
often form because swells, while traveling
across the ocean, do so at different speeds and
directions. As these swells pass through one
another, their crests, troughs, and lengths
sometimes coincide and reinforce each other.
This process can form unusually large, towering
waves that quickly disappear. If the swells are
travelling in the same direction, these
mountainous waves may last for several minutes
before subsiding.
-
Focusing of wave
energy. When waves formed by
a storm develop in a water current against the
normal wave direction, an interaction can take
place which results in a shortening of the wave
frequency. This can cause the waves to
dynamically join together, forming very big
'rogue' waves. The currents where these are
sometimes seen are the Gulf Stream and Agulhas
current. Extreme waves developed in this fashion
tend to be longer lived.
How high is the highest wave?
Cinemagoers will be
familiar with the thrill of giant waves. But what
most people don't know is that the film The
Poseidon Adventure is based on an incident
involving the Queen Mary in WWII. The famous liner
was hit by a giant 'wall of water' while she was
carrying 15,000 American troops to Britain in 1942.
The ship listed to an astonishing 52 degrees and
almos capsized. More recently, The Perfect Storm
will be familiar to most, providing an account of
the sinking of the Andrea Gail south of Newfoundland
in 1991.
Such giant waves are rare, and seldom recorded by
reliable oceanographic instruments. However, on 1
January 1995 a sensor on a platform in the central
North Sea recorded a giant 60ft high wave crest, so
'freak' waves are not just tall tails. Giant waves
can have disastrous consequences even for the
largest ships and offshore structures.
Such waves are thought to be very rare but just how
rare? What physics drives such waves? Is a 'wall of
water' plausible? How should engineers design
structures to survive rare but potentially
catastrophic events?
Within the last years a high number
of large ships has been lost. The causes of
accidents are in many cases believed to be 'rogue
waves'. These are individual waves of exceptional
wave height or abnormal shape
It is well known that extreme waves often occur in
areas were waves propagate into a strong opposing
current. A well known example where many large ships
have encountered difficulties is the Agulhas current
outside South Africa. The strong current going south
meets strong swell from storms in the Antarctic
Ocean.
"We were in a storm and the tanker was running
before the sea. This amazing wave came from the aft
and broke over the deck. I didn't see it until it
was alongside the vessel but it was special, much
bigger than the others. It took us by surprise. I
never saw one again." Philippe Lijour, first mate of
the oil tanker Esso Languedoc, describing the huge
wave that slammed into the ship off the east coast
of South Africa in 1980
LIJOUR and his shipmates are lucky to be alive.
They were struck by a rogue wave, a monstrous wall of
water that rose out of nowhere and slammed onto the
deck like the fist of god. Ships often don't survive
an onslaught like that. Many sink before anyone on
board knows what's hit them. (from the New
Scientist 30 June 2001)
In areas where waves from storms in the open
ocean approach shallower waters (e.g. several
locations along the Norwegian coast), the waves will
be refracted and diffracted as shown in the picture
below (Aerial photo of an area near Kiberg on the
coast of Finnmark, taken 12 June 1976 by Fjellanger
Widerøe A.S.)
There may be focusing of wave energy in certain
areas such that the probability of encountering
large waves is much greater than in other areas.
Such refraction and diffraction of waves, either due
to currents or bathymetry, can be computed. In a
certain sense these waves may therefore be
predicted.
It is far more difficult to avoid, as well as to
explain, extreme waves occurring in the open ocean
far from variable bathymetry or ocean currents. On
January 1st 1995 an extreme wave was measured under
the Draupner platform (16/11-E) in the North Sea
providing indisputable evidence that such waves do
indeed exist. This wave has been known in the
international scientific community as the "new year
wave". The maximal amplitude of 18.5 m is more than
three times the significant amplitude for the wave
train! The maximal wave height of 25.6 m is much
more than twice the significant wave height of about
10.8 m. The time series is reproduced below with the
surface elevation in meters as a function of the
time in seconds.
|
Esso
Languedoc, during a storm off Durban in South Africa
in 1980
Rogue Wave Capsizes Coast Guard Ship During Training
in California
A giant wave in the Bay of Biscayne, in an image
published in Fall 1993 issue of Mariner's Weather
Log. Credit: NOAA
Soliton Esso being hit by a Rogue Wave
A Freak Wave took out the whole
forepeaktank of the Norwegian tanker "Wilstar",
1974.
In the
book Oceanography and
Seamanship, William G. Van Dorn provided an
example of what the wave heights would be if a
steady 33 mph (30 knots) wind blew for 24 hours over
a fetch of 340 miles.
-
10% of all waves will be less than 3.6
ft (1
m).
-
The most frequent wave height will be 8½ ft (2½
m).
-
The average wave height will be 11 ft (3 m).
-
The significant wave height will be 17 ft (5 m).
-
10% of all waves will be higher than 18 ft (5
m).
-
The average wave height of the highest 10% of
all waves will be 22 ft (7 m).
-
A
5% chance of encountering a single wave higher
than 35 ft (11 m) among every 200 waves that
pass in about 30 minutes.
-
A
5% chance of encountering a single wave higher
than 40 ft (12 m) among every 2,600 waves that
pass in about five hours.
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