Ocean surface wave

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Image:Waves lajolla.jpg Image:Rompiente.jpg Image:Wave kils.jpg Ocean surface waves are surface waves which occur at the surface of an ocean. That is, a wave that is guided along the interface between water and air. As the wind blows, pressure and friction forces associated with the wind perturb the equilibrium of the ocean surface. The wind actually transfers some of its energy into the water. The water is able to gain energy from the wind because of the friction between the wind and the water. This causes the surface particles to move in an elliptical motion, which is a combination of longitudinal (back and forth) and transverse (up and down) wave motions. A good illustration of the wave motion is given by *Prof. Robert Dalrymple Java applet.

Image:Elliptical trajectory on ripples.png

As the depth into the ocean increases, the radius of the elliptical motion decreases. At a depth equal to half the wavelength λ, the orbital movement is zero. The speed of the surface wave is also called celerity or phase velocity because it corresponds the speed of the shape of the wave, but is different from the speed of the water particles. This celerity is well approximated by

<math>c=\sqrt{\frac{g \lambda}{2\pi} \tanh \left(\frac{2\pi d}{\lambda}\right)}</math>

where

<math>c</math> = phase speed in m/s;

<math>\lambda</math> = wavelength in m;

<math>d</math> = water depth in m;

<math>g</math> = gee in m/s^2.

In deep water, where <math>d \ge \frac{1}{2}\lambda</math>, so <math>\frac{2\pi d}{\lambda} \ge \pi</math> and the hyperbolic tangent approaches to <math>1</math>, <math>c</math> approximates <math>1.25\sqrt\lambda</math>. This expression tells us that waves of different wavelengths travel at different speeds: waves disperse. The fastest waves in a storm are the ones with the longest wavelength. As a result, when after a storm waves arrive on the coast, the first ones to arrive are the long swells.

When several wave trains are present, which is always the case in the ocean, the waves form groups. In deep water the groups travel at a group velocity which is half of the phase velocity. Following a single wave in a group one can see the wave appearing at the back of the group, growing and finally disappearing at the front of the group.

As the water depth <math>d</math> decreases towards the coast, this will have an effect on the speed of the crest and the trough of the wave; the crest moves faster than the trough. This causes surf, a breaking of the waves.

Individual "freak waves" (also "rogue waves", "monster waves" and "king waves") sometimes occur in the ocean, often as high as 30 metres. Such waves are distinct from tides, caused by the moon and sun's pull, and tsunamis that are caused by underwater earthquakes or landslides.

The movement of ocean waves can be captured by wave energy devices. The energy of regular sinusoidal waves contained in 1 m² of sea depends on the water density <math>\rho</math>, gravity acceleration <math>g</math> and the wave height h (= 2·amplitude (<math>a</math>):

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Types of waves

Image:Breaking wave cot valley 800px.jpg When waves "break", they have different characteristics depending on factors including the structure of the ocean floor. There are three main types that are identified by surfers or surf lifesavers: plunging waves (also known as "dumpers"), spilling waves and surging waves. Their varying characteristics make them more or less suitable for surfing and present different dangers. Surfers often call these characteristics a "haydon" or "haydon's"

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