Ocean Waves Waves are disturbances in the ocean that transmit energy from one place to another. The most familiar types of waves —the ones that cause boats to bob up and down on the open sea and dissipate as breakers on beaches —are generated by wind on the ocean surface. Other ocean wave types include tsunamis, which are often caused by underwater earthquakes, and internal waves, which travel underwater between water masses. Tides are also a type of wave. Ocean Wave PropertiesA group of waves consists of several crests separated by troughs. The height of the waves is called the amplitude, the distance between successive wave crests is known as the wavelength, and the time between successive wave crests is the period. Waves are classified into types based on their periods. They range from ripples, which have periods of less than 0.5 seconds, up to tsunamis and tides, whose periods are measured in minutes and hours (their wavelengths range from hundreds to thousands of miles). In between these extremes are chop and swell —the most familiar types of surface waves. Oc ean waves behave like light rays: t hey are reflected or refracted by obstacles they encounter, such as islands. When different wave groups meet, they interfere —adding to, or cancelling, each other. Ocean Wave Generation Wind energy is imparted to the sea surface through friction and pressure, causing waves. As the wind gains strength, the surface develops gradually from flat and smooth through growing levels ofroughness. First, ripples form, then larger waves, called chop. The waves continue to build, their maximum size depending on three factors: wind speed, wind duration, and the area over whic h the wind is blowing, called the fetch. When ocean waves are as large as they can get under the current conditions of wind speed and size of fetch, the sea surface is said to be “ fully developed.” The overall state of a sea surface c an be summarized by the significant wave height —defined as the average height of the highest one-third of the waves. For example, in a fully developed sea produced by winds of about 25 mph (40 km/h), the significant wave height is typically about 8 ft. (2.5 m). Wave Propagation on the Ocean In the fetch, many different groups of waves of varying wavelength are generated and interfere. As they disperse away from the fetch, the waves become more regularly sized and spaced. This is because the speed of a wave in open water is closely related to its wavelength. The different groups of waves move at different speeds and so are naturally sorted by wavelength: the largest, fastest- moving waves at the fore, the smal ler, slower-moving ones behind. This produces a regular wave pattern, or swells. Occasionally, groups of waves from separate storms interfere to produce unusually large “rogue” waves. As they propagate across the o pen ocean, wind-generated waves maintain a constant speed, which is unaffected by depth until they reach shallow wat er.
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When out in open water there is little horizontal movement of ocean water, the bulk of the motion
is up and down or vertical. However, this changes slightly when waves approach the coastline. As
the water approaches the coastline it encounters increasing contact with the shelving sea bed, which
exerts a frictional force on the base of the wave. This changes the normal circular orbit of the wave
into an elliptical orbit. As the waves gets closer and closer to the coast the impact of friction grows,
with the top of the wave moving faster than the base of the wave. Eventually a critical point is
reached where the top of the wave (the CREST) curves over and creates a breaking wave. This
breaking wave can be further disrupted by water returning down the coastline back out to sea.
Arrival on Shore
As ocean waves approach a shore, the motion they generate deep down begins to interact with the
sea floor. This slows the waves down and causes the crests in a series of waves to bunch up—an
effect called shoaling. The period of the waves does not change, but they gain height as the energyeach contains is compressed into a shorter horizontal distance, and eventually break.
There are two main types of breakers. Spilling breakers occur on flatter shores: their crests break
and cascade down the front as they draw near the shore, dissipating energy gradually. In a plunging
breaker, which occurs on steeper shores, the crest curls and falls over the front of the advancing
wave, and the whole wave then collapses at once. Ocean waves can also refract as they reach a
coastline. This concentrates wave energy onto headlands and shapes some types of beaches.
PLUNGING BREAKER
"Barrel" or "tube-forming" breakers like this occur when the waves reaching shore have large
amounts of energy. The seabed must be firm and quite steep.
When the swash is bigger than backwash material gets pushed up to the back of beaches rather than
removed. This steepens the beach profile. These waves are called CONSTRUCTIVE WAVES, and
these waves tend to have low waves heights, lower wave frequencies (they break less often) and the
waves are less steep. These waves are created by storms far out to sea which create a large swell
which eventually reaches the coast.
Where backwash is larger than swash more material is being eroded from the beach profile than is
being accumulated. This carries material out to sea and makes for a gentler beach profile. These
waves are called DESTRUCTIVE WAVES which have steeper wave profiles, larger and higher wave
crests and come more frequently.
Wave refraction
Wave refraction refers to what happens to waves when they approach an uneven coastline. Very
few coastlines are perfectly straight, and few sea beds have uniform height and shape. This is
known as the varying Geomorphology of our coastline and it will affect the waves travelling above it
and towards it in differing ways. As waves approach an uneven coastline they reach the headlands
first, this focuses a lot of energy on those forelands and bends the waves into the bays where the
energy expended is less.
Refraction of waves involves a change in the direction of waves as they pass from one medium to
another. Refraction, or the bending of the path of the waves, is accompanied by a change in speedand wavelength of the waves. It is known that the speed of a wave is dependent upon the properties
of the medium through which the waves travel. So if the medium (and its properties) is changed, the
speed of the waves is changed. The most significant property of water that would affect the speed of
waves traveling on its surface is the depth of the water. Water waves travel fastest when the
medium is the deepest. Thus, if water waves are passing from deep water into shallow water, they
will slow down. This decrease in speed will also be accompanied by a decrease in wavelength. So as
water waves are transmitted from deep water into shallow water, the speed decreases, the