Origin of Wind Waves
Wind waves form from wind blowing on the ocean surface. The key factors influencing wave intensity include fetch, wind duration, wind strength, and proximity to wind source area. Wind energy is gradually transferred to the waves forming on a body of water, causing waves to absorb energy and grow in amplitude and period over distance and time (Figure 10.15). The transfer of wind energy to wave energy is not very efficient (only about 2% of the energy is actually transferred) but it is the size of the area that the wind is impacting, as well as how strong the wind is blowing that matters.
|Wind-Wave Input Factors:|
|• Fetch is the length (distance) wind blows over open water. This is the uninterrupted distance over which the wind blows without significant change in direction.|
|• Duration is how long the wind blows. Strong wind that does not blow for a long period will not generate large waves.|
|• Wind strength: The stronger the wind, the bigger the waves. The wind must be moving faster than the wave crests for energy to be transferred.|
|• Proximity: Separation of wave trains by period. Long-period waves move faster than shorter-period waves and will separate and advance before wave trains with shorter periods.|
|Wind-Wave Output Factors: (Waves!)|
|• Wave height increases.|
|• Wavelength increases.|
|• Wave period increases.|
|• Direction - wave travel in the direction that the wind blows.|
Wave Equation: Large Fetch + Long Duration + Strong Winds (wind speed) = Large, Long Period Waves
Fetch is important because the interrelationship between wind speed and duration, both functions of fetch, is predictive of wave conditions.
Figure 10.16. Sea and Swells illustrated. A storm generates winds that impact a region over open water. The area impacted by the wind is called a sea. The waves generated by the storm will move out and away from sea are called swell.