Ocean Wave Refraction

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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 Properties

A 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. Ocean

waves behave like light rays: they 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 of

roughness. 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 which 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 can 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 smaller, 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 open ocean, wind-generated waves

maintain a constant speed, which is unaffected by depth until they reach shallow water.



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.



Constructive and Destructive Waves

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

wavelength decreases, and the direction changes.



This boundary behaviour of water waves can be observed in a ripple tank if the tank is partitioned

into a deep and a shallow section. If a pane of glass is placed in the bottom of the tank, one part of

the tank will be deep and the other part of the tank will be shallow. Waves traveling from the deep

end to the shallow end can be seen to refract (i.e., bend), decrease wavelength (the wave fronts get

closer together), and slow down (they take a longer time to travel the same distance). When

traveling from deep water to shallow water, the waves are seen to bend in such a manner that they

seem to be traveling more perpendicular to the surface. If traveling from shallow water to deep

water, the waves bend in the opposite direction.

Refraction vs. Diffraction

A Physic’s definition of refraction involves a change in the direction of waves as they pass from one

medium to another. This is due to energy conservation when a wave changes its wavelength, and

thus wave speed, as it enters a new medium (like in light or sound waves traveling from air to

water). Now, in terms of ocean waves, refraction also involves a change of direction when a wave

experiences a shift in wavelength and wave speed. However, this shift generally occurs due to

changes in ocean depth. As a wave travels from deep to shallow water, the wavelength shortens, the

wave speed slows down, and the wave will refract, or bend, toward the shallow area in order to

conserve its energy.

Diffraction occurs when a wave encounters an obstruction in its path and will change direction, or

wrap around it. In ocean waves, we see this occur when a wave encounters an object like a jetty and

the wave rotates around it (sometimes diffraction also occurs when a waves moves through a small

opening in a seawall or between or two islands). The 'wrapping' or turning potential of a wave is

larger in waves with a longer wavelength (i.e. longer period).

Wave refraction due to bottom shoaling

The simplest description of waves propagating over a slowly changing sea bottom is given by the

geometric optics approximation (Massel 1989). A wave ray, being basic quantity in this

approximation, is a curve which is tangent to the local wave number k vector at every point along

the curve. The wave number should satisfy the following conservation energy:

d(kSin(P))/dx =d(kSin(P))/dy =0 ….(1)

in which K=|k|, and P is an angle between normal to local bottom contours and wave direction.

For bottom contours parallel to the y-direction, equation (1) reduces to :

k(Sin(P))=k*(Sin(P*)) = constant …..(2) {Snell’s Law}

Refraction of ocean waves when approaches the coastline forms the white foam of waves breaking

on a rock or shore (surf).



(Pictures showing refraction of ocean waves as they approach the coastline)

References

1. Oceana.org/en/explore/marine-science/ocean-waves

OCEAN WAVES

09-04-201212:41am

2. Advanced series on Ocean Engineering-Volume 11

OCEAN SURFACE WAVES: THEIR PHYSICS AND PREDICTION

Stanislaw R. Massel, World Scientific, Chapter 6, page 256

Ocean Wave Refraction

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