Ocean surface wave

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This article is about waves in the ocean. For information about the TV Movie, see The Ocean Waves.

Ocean waves

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

1 Wave formation
2 Types of wind waves
3 Science of waves
4 Ocean wave measurement
5 Ocean wave models
6 Gallery
7 References
8 External links

[edit] Wave formation

A wave creating spray.

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

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

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)
Wavelength (from crest to crest)
Period (time interval between arrival of consecutive crests at a stationary point)

Waves in a given area typically have a range of sizes. For weather reporting and for scientific analysis of wind wave statistics, their size over a period of time is usually expressed as "significant wave height." This figure represents the average height 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, the largest individual waves are likely to be twice the reported significant wave height for a particular day or storm.

[edit] Types of wind waves

Three different types of wind waves develop over time:

Ripples appear on smooth water when the wind blows, but die if the wind stops. The restoring force that allows them to propagate is surface tension. Seas are the larger-scale, often irregular motions that form under sustained winds. They tend to last much longer, even after the wind has died, and the restoring force that allows them to persist is gravity. As seas propagate away from their area of origin, they naturally separate according to their direction and wavelength. The regular wave motions formed in this way are known as swells.

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 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.

Spilling type of surf

Three main types of breaking waves 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 on which to surf; 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.

In the context of sediment transport on beachs, ocean surface waves can also be classified as either constructive or destructive:

Constructive waves tend to be low in height (less than 1 meter), and therefore low in energy. As they approach the beach, the wave front steepens only slowly, gently spilling 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).

Destructive waves are tall, toppling waves carrying a lot of energy. As they approach the beach, they rapidly steepen, and when breaking they plunge down and scour the beach. This creates a powerful backwash, as a significant amount of the energy of the wave has not dissipated during breaking and runup. The backwash 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 beaches.

[edit] Science of waves

Motion of a particle in a ocean wave.
A = At deep water.
B = At shallow water (ocean floor is now at B). The circular movement of a surface particle becomes elliptical with decreasing depth.
1 = Progression of wave
2 = Crest
3 = Trough

Ocean surface 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 (also see shallow water equations). 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 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 at the Earth's surface;

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 $λ$ 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 $d$ 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, 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 $ρ$ , 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.

[edit] Ocean wave measurement

A common way of measuring waves is using a buoy that records the motion of the water surface. These movements become the wave climate for that location and statistics are calculated including the significant and maximum wave heights and periods. Early waverider buoys measured wave trains in one direction only. More modern waverider buoys can measure movement in true three dimensions and so also give information about wave direction. For the south east Queensland coastline there are waverider bouys about every 100km along the coast. The waverider buoys are typically positioned off the entrances of major ports or major recreational surfing or swimming beaches. A network of waverider buoys properly positioned can allow the interpolation of the wave climate for that region. Waverider buoy data is a typical input for coastal modelling, the waverider wave train is typically the deep water wave climate that is then refracted across the seabed contours into the wave breaking zone.

A cheaper method is to use a wave pole. An observer stands on the shore in a designated spot and sights the wave alongside a pole positioned between them and the waves. Such poles are often part of weather monitoring stations located along coastlines, particularly those associated with lighthouses.

Ship board observations of waves has been happening for over 130 years. Ship board observations are the longest record of wave climate on the planet and are commonly the best source of data for coastal modelling. A problem with more recent datasets is that they are less likely to contain the waves associated with rarer extreme events.

There are newer methods of measuring wave climate including extracting the information from satellite camera images.

[edit] Ocean wave models

Surfers are very interested in the predicted wave climate. There are many websites that provide predictions of the surf quality for the upcoming days and weeks. The Ocean Wave models are driven by more general climate models that predict the winds and pressures over the oceans.

Ocean wave models are also an important part of examining the impact of shore protection and beach nourishment proposals. For many beach areas there is only patchy information about the wave climate, therefore estimating the effect of ocean waves is important for managing littoral environments.

[edit] Gallery

Breaking waves at Children's Pool, in La Jolla, CA.	A wave just before breaking at Manhattan Beach, CA.	Waves breaking on stones.	Plunging wave or dumper forming a tube.
	Gentle waves at the beach.	Wave surf in the west coast of Portugal	Storm surf

[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