Ocean surface wave

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Ocean surface waves are surface waves that occur in the upper layer of the ocean. They usually result from distant winds or geologic effects and may travel thousands of miles before striking land. They range in size from small ripples to huge tsunamis. There is surprisingly little actual forward motion of individual water particles in a wave, despite the large amount of forward energy it may carry.

1 How waves are formed and measured
2 Types of waves
3 Science of waves
4 Gallery
5 See also
6 References
7 External links

[edit] How waves are formed and measured

Image:WaterSpraySurf.jpg The great majority of waves one sees on an ocean beach result from distant winds. Three factors influence the formation of "wind waves":

Windspeed
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 determine the size and shape of ocean waves. The greater each of the variables, the larger the waves. Waves are measured by:

Height (from trough to crest)
Length (from crest to crest)
Steepness, or slope (either the angle between crest and trough, or the "steepness ratio," of the wave's height to its length)
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, regardless of how long the wind blows.

Both in theory and in reality, waves are never created in one uniform height. They fall into a systemic pattern of varying size. For weather reporting and for scientific analysis of wave behavior, their size over a period of time is usually expressed as "significant wave height." This figure represents the average of the highest one-third of the waves in a given time period (usually twelve hours) or in a specific wave or storm system. Given the variability of wave size, it might be expected that individual waves are likely to be twice the reported significant wave height for a particular day or storm.

[edit] 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.

Three different types of waves develop over time:

Ripples
Seas
Swells

Ripples appear on smooth water when the wind is light, but die if the wind stops. 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. They may be thought of as seas that persist long after the wind that produced them has stopped.

Some waves undergo a phenomenon called "breaking". 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 kilopascals (4 to 9 short tons per square yard), enough force to crush the hull of a ship. A wave breaks when it runs into shallow water, or when two wave systems oppose and combine forces. When the slope, or steepness ratio, of a wave is too great, breaking is inevitable. A 1:24 slope may be a long, shallow swell found in deep waters. A 1:14 and higher slope is a wave that is too steep to remain coherent. Waves can also break if the wind grows strong enough to blow the crest off the base of the wave. The wind speed required for this to occur is unpredictable, 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 lifesavers. Their varying characteristics make them more or less suitable for surfing, and present different dangers.

Spilling, or rolling: these are the safest to surf on; they can be found in relatively sheltered areas.

Plunging, or dumping: these break suddenly and can "dump" swimmers—pushing them to the bottom with great force. Strong winds can cause dumpers; they can also be found where there is a sudden rise in the sea floor.

Surging: these 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.

Waves can also be classified as either constructive or destructive:

Image:Constructive Wave Diagrams.png

Constructive waves are usually associated with long fetch.They tend to be low in height (under 1 metre), have a long wavelenth (up to 100 metres) and have a low frequency (around six-eight per minute). As they approach the beach, the wave front steepens only slowly, giving a gently spill on the beach surface. Swash rapidly loses volume and energy as water percolates through the beach material. This tends to give a weak backwash that has insufficient force to pull sediment off the beach or to impede swash from the next wave. As a consequence, material is slowly, but constantly, moved up the beach, leading to the formation of ridges (or berms).

Image:Destructive Wave Diagrams.png

Destructive waves tend to occur when the fetch is shorter. They are tall, toppling waves with a steep form and a high frequency (10–14 per minute). As they approach the beach, they rapidly steepen, and when breaking they plunge down and scour the beach. This creates a powerful backwash, as there is little forward movement of water. It also inhibits the swash from the next wave. Very little material is moved up the beach, leaving the backwash to pull material away. Destructive waves are commonly associated with steeper beach profiles. The force of each wave may project some shingle well towards the rear of the beach where it forms a large ridge known as the storm beach.

[edit] Science of waves

Image:Wave motion-i18n.png

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 as it corresponds to the speed of the shape of the wave, note that this is not directly related to the speed of the water particles. The celerity is well approximated by

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

where

c = phase speed;

λ = wavelength;

d = water depth;

g = acceleration due to gravity;

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 <math>1</math>, <math>c</math>, in m/s, approximates <math>1.25\sqrt\lambda</math>, when <math>\lambda</math> is measured in meters. This expression tells us that waves of different wavelengths travel at different speeds. 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 wavelength 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.^{[citation needed]} 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, tsunamis that are caused by underwater earthquakes or landslides, and waves generated by underwater explosions or the fall of meteorites.

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 <math>\rho</math>, gravity acceleration <math>g</math> and the wave height <math>h</math> (which is equal to twice the amplitude, <math>a</math>):

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

[edit] Gallery

Breaking waves at Children's Pool, in La Jolla, CA.	A wave just before breaking at Manhattan Beach, CA.	Waves breaking on rocks.	Plunging wave or dumper forming a tube.

[edit] See also

[edit] 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