Ocean surface wave

Ocean surface waves are surface waves which occur at the surface of an ocean.

Science of Waves

Wave_motion-i18n.png|thumb|350px|right|A particle motion in a ocean wave.
A=At deep water.
B=At shallow water. The circular movement of a surface particle becomes elliptical with decreasing depth.
1= Progression of wave
2= Crest
3= Trough ]] Ocean Waves are mechanical waves that propagate along the interface between water and air; the restoring force is provided by gravity, and so they are often referred to as surface gravity waves. As the wind blows, pressure and friction forces perturb the equilibrium of the ocean surface. These forces transfer energy from the air to the water, forming waves. In the case of monochromatic linear plane waves in deep water, particles near the surface move in circular paths, making ocean surface waves a combination of longitudinal (back and forth) and transverse (up and down) wave motions. When waves propagate in shallow water, (where the depth is less than half the wavelength) the particle trajectories are compressed into ellipses. As the wave amplitude (height) increases, the particle paths no longer form closed orbits; rather, after the passage of each crest, particles are displaced a little forward from their previous positions, a phenomenon known as Stokes drift. A good illustration of the wave motion is given by *Prof. Robert Dalrymple Java applet

As the depth into the ocean increases, the radius of the circular motion decreases. By a depth equal to half the wavelength λ, the orbital movement has decayed nearly to 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

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

where

c = phase speed;

λ = wavelength;

d = water depth;

g = acceleration due to gravity;

In deep water, where $d \ge \frac{1}{2}\lambda$ , so $\frac{2\pi d}{\lambda} \ge \pi$ and the hyperbolic tangent approaches $1$ , $c$ , in m/s, approximates $1.25\sqrt\lambda$ , when $\lambda$ is measured in meters. 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 $d$ 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 density (per unit area) of regular sinusoidal waves depends on the water density $\rho$ , gravity acceleration $g$ and the wave height $h$ (which is equal to twice the amplitude, $a$ ):

E=\frac{1}{8}\rho g {h}^2=\frac{1}{2}\rho g a^2.

The velocity of propagation of this energy is the group velocity.

How Waves are Formed

There are three factors that influence the formation of wind waves:

Wind speed
Length of time the wind has blown over a given area
Distance of open water that the wind has blown over; called fetch

All of these factors work together to create waves. The greater each of the variables, the greater the waves. Waves are measured by:

Height (from trough to crest)
Length (from crest to crest)
Steepness (angle between crest and trough)
Period (length of time between crests)

There are theoretical limitations, however, for each variable. The smaller the fetch, the smaller the largest wave can be for a given wind speed, no matter how long the wind blows.

Both in theory and in reality, waves are never created in one uniform height. Waves fall into a systemic pattern of varying size. Significant wave height is the average of the highest 1/3 of the waves in a system. This is how weather reports specify wave height.

Types of waves

Waves take time to develop; they do not spontaneously erupt from the ocean. It takes a certain speed of wind to blow over a certain distance for a considerable length of time to create lasting waves.

There are three different types of waves that develop over time:

Ripples
Seas
Swells

Ripples appear on smooth water when the wind is light, but if the wind dies, so do the ripples. Seas are created when the wind has blown for a while at a given velocity. They tend to last much longer, even after the wind has died. Swells are waves that have moved away from their area of origin and are unrelated to the local wind conditions -- in other words, seas that have lasted long beyond the wind.

Somewaves break while others do not.

A breaking wave is one whose base can no longer support its top, causing it to collapse. A very large breaking wave can impart a pressure of up to 50 to 100 thousand pascals (5 to 10 tons per square yard), which can be enough force to crush the hull of a ship. When the steepness ratio (or slope) of a wave is too great, it must break. This happens when a wave runs into shallow water, or when two wave systems oppose and combine forces. The steepness ratio is expressed as the height to the length. A 1:24 is a long, shallow swell found in deep waters. A 1:14 and up is a wave that is too steep to remain coherent. Waves can also break if the wind grows strong enough to blow the top (crest) off the base of the wave. The wind-speed required for this to occur is upredictable, as it varies as a function of the slope of the wave. (As well as the force of gravity, and the density and surface tension of seawater.)

There are three main types of waves that are identified by surfers or surf lifesaversTheir varying characteristics make them more or less suitable for surfing and present different dangers.

Spilling, or Rolling
Plunging, or Dumping
Surging

Spilling waves are the safest to surf on. They Can be found in relatively sheltered areas.

Plunging break suddenly and can ‘dump’ swimmers - push them to the bottom with great force, which can create serious back and neck injuries. Strong winds can cause dumpers, and they can also be found where there is a sudden rise in the sea floor.

Surging waves may never actually break as they approach the water's edge, as the water below them is very deep. These waves can knock swimmers over and drag them back into deeper water.

Gallery

Image:Waves lajolla.jpg|Breaking waves at Children's Pool, in La Jolla, CA Image:Manhattan beach wave.JPG|A wave just before breaking at Manhattan Beach, CA Image:Rompiente.jpg|Waves breaking on rocks Image:Wave_kils.jpg|Plunging wave or dumper forming a tube Image:Boelge stor.jpg

References

"Anatomy of a Wave" Holben, Jay boatsafe.com captured 5/23/06
Carr, Michael "Understanding Waves" Sail Oct. 1998: 38-45.
Rousmaniere, John. The Annapolis Book of Seamanship New York: Simon & Schuster 1989
National Weather Service

External links

Physical oceanography | Water waves