WARFARE APRIL 2006
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
21
5
4 STILL WATER LEVEL
WAVE MOTION
OCEAN WAVE
3
1. Crest2. Wave Length3. Amplitude4. Wave Height5. Trough
WARFARE APRIL 2006
Sea WavesSea 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: 0WIND: 0-1KTWAVES: 0FT
BF: 1WIND: 1-3KTWAVES: 0FT
BF: 2WIND: 4-5KTWAVES: 1FT
BF: 3WIND: 7-10KTWAVES: 2-3FT
WARFARE APRIL 2006
BF: 4WIND: 11-16KTWAVES: 4-5FT
BF: 5WIND: 17-21KTWAVES: 6-8FT
BF: 6WIND: 22-27KTWAVES: 9-13FT
BF: 7WIND: 28-33KTWAVES: 14-19FT
WARFARE APRIL 2006
BF: 8WIND: 34-40KTWAVES: 18-25FT
BF: 9WIND: 41-47KTWAVES: 23-32FT
BF: 10WIND: 48-55KTWAVES: 29-41FT
BF: 11WIND: 56-63KTWAVES: 37-52FT
WARFARE APRIL 2006
BF: 12WIND: > 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 WavesSwell 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 WavesDevelopment• 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 WavesDevelopment
• 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 WavesDecay• 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
WW3
BF: 9WIND: 41-47KTWAVES: 23-32FT