by keith meldahl

© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Lesson: Waves - Part I

By Keith Meldahl

Corresponding to Chapter 10: Wave Dynamics &

Wind Waves

Waves transmit energy across the ocean’s surface. Several types

of forces cause ocean waves. Waves produced by WIND are

covered in this lesson and Chapter 10. Waves produced by other

types of forces are covered in the next lesson and in Chapter 11.


Summary of Important Concepts

• Waves are classified according to the type of force that disturbs the ocean to cause the wave.

• Waves transmit energy, not water mass, across the ocean’s surface. As a wave moves from one place to another, the water does not go with the wave, but rather moves in a circular or elliptical path called an “orbit”.

• The behavior of a wave depends on the relation between the wave’s size and the depth of water through which it is moving. A wave can be either a “deep water wave” or a “shallow water wave”.

DEEP WATER WAVE = any wave traveling in water deeper than 1/2 its wavelength. In this case the wave’s speed is controlled by its wavelength. Longer waves are faster than shorter waves. The different speeds of waves in deep water causes them to separate out from one another as they travel: a process called dispersion.


Summary of Important Concepts, continued

SHALLOW WATER WAVE = any wave traveling in water shallower than 1/2 its wavelength. In this case the wave’s speed is controlled by the water depth. Waves go slower in shallower water, because friction with the bottom slows down the wave.

• Waves can interfere with one another in ways that either increase or decrease their size.

DESTRUCTIVE INTERFERENCE occurs when the crest of one wave coincides with the trough of another. This creates a smaller wave.

CONSTRUCTIVE INTERFERENCE occurs when the crests of two or more waves coincide with one another. This creates a larger wave.

Occasionally the crests of several large waves may coincide, creating a short-lived giant wave, called a rogue wave. Rogue waves have caused accidents and capsizing of ships at sea!


Summary of Important Concepts, continued

• Waves can change direction by refraction. As waves approach shore, the part of the wave in shallower water slows down more than the part in deeper water. This causes the wave to bend and change direction.

• As waves approach shore bottom friction slows them down and causes them to break. The force of breaking waves is important for shaping our coastlines (a topic discussed in Chapter 12).


Classifying Waves

Waves are classified according to the type of force that disturbs the water to

make the wave. This table summarizes the type of wave and the

corresponding disturbing force. This chapter (Chapter 10) considers wind

waves, while Chapter 11 covers the other types of waves.


Ocean Waves

Ocean waves are orbital waves:

waves in which the particles of water

move in closed circles as the wave

passes.

Ocean waves are also progressive

waves, meaning they travel across

the ocean surface from one place to

another.

It is important to realize that the

motion of the WAVE is not the same

as the motion of the WATER through

which the wave travels. As this figure

illustrates, the wave progresses from

one place to another, while the water

moves in a circular path called an

orbit.


Ocean Waves

The water in a wave moves in a circular path, or orbit. But with each

orbit the water makes a small amount of progress in the direction of

the wind. The figure on the right below shows that there is a small

net movement of water in the direction of the wind. This movement,

called Stoke’s Drift, creates a flow of water in the direction of the

wind, and is partly responsible for the prevailing surface currents you

learned about in Chapter 9.


Ocean Waves

To measure and study waves,

oceanographers refer to the

following:

• Wavelength - the horizontal

distance from crest to crest.

• Wave height - the vertical

distance from crest to trough.

• Wave period - the time

between one crest and the

next crest.

• Wave frequency - the

number of crests passing by a

certain point in a certain

amount of time.


Ocean Waves

This figure illustrates the

movement of water as a

wave passes. Note that:

• The water moves in

orbits.

• The water in the crest

part of the orbit moves

in the same direction

as the wave, while the

water in the trough

moves in the opposite

direction.

• The orbits get smaller the deeper you go.

• At a depth equal to 1/2 the wavelength, there is no more water motion.


Deep Water versus Shallow Water Waves

The behavior of a wave depends on the

wavelength compared to the depth of water

through which the wave is moving.

DEEP WATER WAVE - when a wave is traveling

in water deeper than 1/2 its wavelength. In this

case the wave’s speed is controlled by its

wavelength. Longer waves are faster than

shorter waves.

SHALLOW WATER WAVE - when a wave is

traveling in water shallower than 1/2 its

wavelength. In this case the wave’s speed is

controlled by the water depth. Waves go

slower in shallower water, because friction with

the bottom slows down the wave.

Note: the figure also shows “transitional waves”,

but for our purposes we will treat transitional

waves as equal to shallow water waves.


Deep Water Waves

This graph shows the connection between wave speed and wavelength:

longer waves are faster waves.


Deep Water Waves: Dispersion & Swell

The previous slides explained the factors

that control a wave’s speed.

In deep water, waves of different

wavelengths travel at different speeds.

Waves with the longest wavelengths move

the fastest and leave an area of wave

formation sooner. Because of their

different speeds, waves separate out from

one another into groups with similar

wavelength.This process is called

dispersion.

Dispersion causes groups of waves with

the same wavelength to travel together,

causing a very regular, undulating ocean

surface called swell.


The following three factors control the size of wind waves:

1. Wind strength

2. Wind duration

3. Fetch - the uninterrupted distance over which wind blows

without changing direction.

Factors Affecting Wind Wave Development


When waves from different areas meet up, they will interfere with

one another. Wave interference can be:

Destructive interference – occurs when the crest of one wave coincides

with the trough of another. This creates a smaller wave or no wave.

Constructive interference – occurs when the crests of two or more

waves coincide with one another. This creates a larger wave.

Rogue waves are exceptionally large waves that can form when

several wave crests all coincide at once. These giant waves are rare

but potentially destructive. They have capsized large ships, and may

be responsible for some unexplained ship disappearances. (See Box

10.2.)

Wave Interference


Wave Interference

This figure illustrates the concept of interference.

In area (1), the two waves shown on line “a” are coinciding (crest with

crest and trough with trough). The result is a larger wave (line “b”).

In area (2), the two waves shown on line “a” are out of phase: the crest of

one coincides with the trough of another. The result is a smaller wave

(line “b”).


Waves Approaching Shore

As waves move into shallow water, they begin to slow down when the

depth becomes less than 1/2 of the wavelength. As the water gets

shallower the wave slows, builds up in height, becomes “peaked” in

shape, and eventually breaks.


Waves break against the shore in several ways.

Plunging waves break violently against the shore, leaving an air-filled

tube, or channel, between the crest and foot of the wave. Plunging

waves are formed when waves approach a shore over a steeply sloped

bottom.

Spilling waves occur on gradually sloping bottoms. The crest of a

spilling wave slides down the face of the wave as it breaks.

The force of breaking waves is important for shaping our coastlines -- a

topic discussed in detail in Chapter 12.

Waves Approaching Shore


A wave will bend and change direction when one part of it goes slower or faster

than another part.

Wave refraction is the bending of waves in shallow water. As waves approach

shore, the part of the wave in shallower water slows down more than the part in

deeper water. This causes the wave to bend and change direction.

Wave Refraction


Internal Waves

Waves that occur at the boundaries of water layers with different

densities (such as at the pycnocline) are called internal waves.

by keith meldahl

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