WIND / SWELL WAVES
WARFARE
APRIL 2006
Ocean Waves
• Characteristics
- Wave crest - the highest portion of the wave
between two successive troughs
- Wave trough - the lowest portion of the wave
between two successive crests
- Wave amplitude - 1/2 of the wave height
- Wave length - the horizontal distance between
two successive crests or troughs and measured
in feet
WARFARE
APRIL 2006
Ocean Waves
• Characteristics (cont)
- Wave period - the time interval between
successive wave crests or wave troughs as they
pass a fixed point and measured in seconds
- Wave speed - the rate at which the wave
moves through the water and measured in
knots
- Wave frequency - the number of waves
passing a given point per unit time
NOTE: Wave frequency and wave height are inversely proportional.
Question: The lower the wave heights = the higher the wave frequency
WARFARE
APRIL 2006
OCEAN WAVE
WAVE
MOTION
2
1
3
4
1.
2.
3.
4.
5.
WARFARE
Crest
Wave Length
Amplitude
Wave Height
Trough
STILL
WATER
LEVEL
5
APRIL 2006
Sea Waves
Sea waves (aka wind waves)
• Wind generated waves are generated by the local wind field
within a “fetch area”
fetch area = area of constant wind direction and speed
• Beaufort Scale is used to predict wind generated waves
WARFARE
APRIL 2006
WARFARE
APRIL 2006
BF: 0
WIND: 0-1KT
WAVES: 0FT
BF: 1
WIND: 1-3KT
WAVES: 0FT
BF: 2
WIND: 4-5KT
WAVES: 1FT
BF: 3
WIND: 7-10KT
WAVES: 2-3FT
WARFARE
APRIL 2006
BF: 4
WIND: 11-16KT
WAVES: 4-5FT
BF: 5
WIND: 17-21KT
WAVES: 6-8FT
BF: 6
WIND: 22-27KT
WAVES: 9-13FT
BF: 7
WIND: 28-33KT
WAVES: 14-19FT
WARFARE
APRIL 2006
BF: 8
WIND: 34-40KT
WAVES: 18-25FT
BF: 9
WIND: 41-47KT
WAVES: 23-32FT
BF: 10
WIND: 48-55KT
WAVES: 29-41FT
BF: 11
WIND: 56-63KT
WAVES: 37-52FT
WARFARE
APRIL 2006
BF: 12
WIND: > 64KT
WAVES: > 46FT
The scale was created in 1806 by
Sir Francis Beaufort, an Irish-born
British admiral and hydrographer.
The scale was made a standard for
ship's log entries on Royal Navy
vessels in the late 1830s and was
adapted to non-naval use in the
1850s.
The Beaufort scale was extended in 1946, when Forces 13 to 17
were added. However, Forces 13 to 17 were intended to apply
only to special cases, such as tropical cyclones. Nowadays, the
extended scale is only used in Taiwan and mainland China,
which are often affected by typhoons.
WARFARE
APRIL 2006
Swell Waves
Swell waves (aka surface waves)
• Waves that began as wind generated waves and
propigated out of the fetch area
• Can travel distances up to thousand’s of miles
• Swells generally have periods of 10 seconds or greater
WARFARE
APRIL 2006
Ocean Waves
Development
• Max sea height for each wind speed (scientific
study) is based on wind speed, time, and length of
the fetch area
– Numerous variables cause lower heights
- variable wind speed/direction
- depth of water
- ocean currents
- islands
– Based on wind speed, wave reaches top height - known
as “Fully Arisen Sea”
– North Wall is an exception
– Maximum sea height can physically reach up to a height
of 212 feet based on mathematics
WARFARE
APRIL 2006
Ocean Waves
Development
• Growth determined by tangential stress and pressure
transfer
» Forces of gravity and surface tension causes wavelets or
ripples with 2-3 knots of wind
» Transfer of energy from wind to the sea is known as
pressure transfer
» The wind generated surface current will develop at 45°
to the right of the predominate wind direction (due to
the Ecmond Spiral)
WARFARE
APRIL 2006
Ocean Waves
Decay
• Dispersion
– As a swell wave leaves the fetch area it leaves at an angle
to the wind direction, which causes a decrease in height
» Air resistance
» Gravity
• Angular Spreading
– The highest swells will be plus or minus 30 from the
predominate wind direction
– Swells lose approximately 1/3rd of their height each time
they travel a distance in nautical miles equal to their
length in feet
WARFARE
APRIL 2006
WARFARE
APRIL 2006
BF: 9
WIND: 41-47KT
WAVES: 23-32FT
WW3
WARFARE
APRIL 2006