WIND / SWELL WAVES

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: 3WIND: 7-10KTWAVES: 2-3FT

WARFARE APRIL 2006





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


WIND / SWELL WAVES

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