GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 16: Waves
POWERPOINT SLIDE SHOW NOTES
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Topic 16. Waves
What is a wave? We are talking about waves in water, but in an earlier Topic we talked
about sound waves traveling through water and through Earth. There are many different
types of waves.
It’s easy to visualize the power of the water, of waves, that is battering this ship.
How does energy get into the water?
However it happens, the process is called the generating force.
The opposing force, trying to restore the water surface to its flat state, is called the
restoring force.
The tiny waves are ripples (also called capillary waves), the large waves are gravity waves
There are other ways to classify waves besides the type of generating and restoring
forces. When you go to the beach, the waves you see rolling in can properly be called
wind, gravity, surface, progressive waves. They are generated by the wind, gravity is the
restoring force, they move on the surface of the water, and the progress, or move
forward. These are the most common types of waves, though there are others. Wind
waves are created by friction. Not really different from the trade winds creating ocean
currents, except wave generation is at a much smaller scale. Friction transfers energy
from the wind into the water surface.
A very important thing about waves is that even though energy is being transported, the
medium is not transported. Just like air is the medium that transport the energy of my
voice, the air is not transported. Same thing with wave energy. The water does not move
forward, only the energy.
Wave terminology
Wave terminology illustrated
As a wave moves, the water moves in circles but there is no net forward motion. The
circles are called wave orbits.
Orbital wave motion illustrated. Watch a cork or seagull or small boat as waves pass
under them. The move in circular motion ending up where they started.
Water particles make one complete orbit in one wavelength. All the water is rotating, but
only a few orbits are highlighted. There is a loss of with depth because of transferring
energy through the water column. Thus, wave orbitals decrease in diameter. At a depth
of one-half a wavelength, there is essentially no orbital motion. For example, imagine a
wave with a wavelength of 100 meters. One-half of 100 = 50. So the wave orbitals extend
down to about 50 meters.
Speed is a measurement of length divided by time. Miles per hour, for example. Wave
speed is wavelength (usually measured in meters) divided by the wave period (usually
measured in seconds). The term “celerity” is used to define wave speed.
There is something very interesting about progressive waves. Depending on the water
depth and wavelength, their behavior can be quite different. A wave traveling in water
depth equal to or greater than 1/2 wavelength is called deep-water wave. A wave
traveling in water depth equal to or less than 1/20 wavelength is called shallow-water
wave. Let’s take our 100-meter wavelength wave. If it is travelling in water depths of 50
meters or greater, it is a deep-water wave. If it is travelling in water depths of 5 meters or
less, it is a shallow-water wave. What’s the big deal? It’s all a matter of whether or not
the wave orbitals interact with the sea bottom.
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Note the difference in the orbitals between deep- and shallow-water waves. We’ve
already seen deep-water waves. Circular orbitals decrease in diameter in depth until their
diameter is zero at a water depth of L/2. The wave orbitals in shallow water interact with
the sea bottom so much that they are deformed into ellipses. The ellipses have a
constant width axis with depth, but the height axis decreases with depth until at the
seafloor the water is not orbiting so much as simply swashing back and forth. Perhaps
you’ve noticed this at the beach in very shallow water.
And the big deal about all this is the difference in behavior between deep- and shallowwater waves.
Deep-water waves progress at a speed controlled by their wavelength. The longer the
wavelength, the faster they move. This leads to a process called wave dispersion.
Deep-water waves move away from their area of generation.
Another use of the word “sea.” Waves in the area of generation.
Wave dispersion is a sorting out of waves by wavelength as they progress from the area of
generation. Long wavelength waves moving faster than shorter wavelength waves.
Wave dispersion. A sea surface in the area of generation and with very irregular wave
heights, lengths, and periods, eventually is sorted out by wave dispersion into a very
regular sea surface. These sorted-out waves are called swell. The area of ocean surface
over which the wind is blowing (a storm or trade winds, for example) is called the fetch.
Sea
Swell. Swell is what we usually experience when we go to the beach. Nice regular waves
with more or less constant wave lengths and wave periods.
A ship in a storm.
Swell
Waves don’t occur by themselves. The wind creates many waves, called wave trains. The
lead waves lose energy as they deform the sea surface ahead of them. Thus, the leading
waves are lost and new waves are catching up with the group to take its place.
The speed of an individual wave is greater than the speed of the wave train.
Group speed is the speed of the wave train and is equal to 1/2 of the wave speed
(celerity).
Wave height is a representation of how much energy the wind can transfer to the water
surface. The energy transfer depends on three factors: wind speed, wind duration, and
fetch.
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Wind blows across the water surface, transferring as much energy as possible based on
the three controlling factors. When the largest possible waves for that set of conditions
are created, the situation is called a fully developed sea. Check out this site where you
can vary the factors and see the resulting waves{
http://www-tc.pbs.org/wnet/savageseas/multimedia/wavemachine.swf
For a wind speed of 58 mph, notice the increase in wave heights wavelength, period, and
speed with increasing fetch.
Wave terminology. Significant wave height is used more frequently than maximum or
average wave height to describe local wave conditions, especially for marine engineering.
It’s a better representation of the energy of the system during storms than a maximum
wave’s height which may be large but also rare.
Some conditions for a fully developed sea
Waves can be generated by more than one storm at the same time. As they move, the
waves may interact.
When waves interact, if crests and troughs match up, constructive interference occurs
and there is an addition of energy resulting in larger waves. When two waves are offset
by about one-half a wavelength, the crests and troughs cancel out each other through
destructive interference and smaller waves result.
Episodic or rogue waves can be caused by constructive interference. Accounts of large
waves “coming out of nowhere” are numerous, and may account for many lost ships.
Rogue waves
Wave steepness
Steepness of a newly-formed wind wave
Now to shallow-water waves. The intense interaction of wave orbitals with the sea
bottom makes shallow-water wave behavior quite different from that of deep-water
waves. The speed of a shallow-water wave is controlled by water depth. The shallower
the water, the slower the wave. This leads to the processes of wave refraction and wave
breaking.
Watch the Waves, Beaches and coasts video in the “Earth Revealed” series on learner.org.
Wave refraction
Wave crests bend as waves move into shallow water.
On irregular coasts, wave refraction results in a concentration of wave energy on
headlands, speeding their erosion.
Waves refract, or bend, around this rocky point
Wave refraction around a rocky island
Wave refraction into a small bay
Wave refraction into a small coastal bay
Wave breaking. We’ve come from sea, to swell, and now to surf. The end of wave, the
final dissipation of energy on the shore. A big change occurs in the surf zone. Surf zone
waves do transport water forward.
Deep-water waves transitioning to surf. In shallow water the back of a wave is moving
faster than the front, causing water to pile up and eventually fall forward as a breaking
wave. Water moving up above sea level due to the forward momentum of the wave is
called the swash zone. Gravity pulls the water back down to sea level. The surf zone is
where waves transition to water moving forward. That is why we can surf on breaking
waves, but not on deep-water waves.
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Notice how the wave orbitals are deformed forward in shallow water. Recall that once a
wave is formed, the wave period remains constant, and that a wave orbital completes one
revolution in one wave period. So orbital speed (controlled by wave period) is constant,
but celerity decreases as water depth decreases. At some point, orbital velocity becomes
greater than wave speed and the water of the wave crest is thrown out ahead of the
wave, causing the wave to break.
A breaking wave, often called a breaker, throws water up above sea level. That is called
wave swash. Water pulled back down into the ocean by gravity is called backwash.
The type of breaking wave depends on the steepness of the seafloor.
Longshore current
Wave swash carries sand up the beach, and backwash carries it back down. The zigzag
motion is caused because waves usually approach the beach at an angle, and the sand is
pushed up the beach at that angle. But gravity pulls the backwash water and sand
straight back down the beach. The sand thus moved is called beach drift. The longshore
current also moves sand parallel to the beach. The movement of sediment along the
shore by longshore current and beach drift is called longshore transport.
Breaking waves push water against the shore. One result is driving the longshore current.
Another is creating rip currents. Water pushed above sea level by breaking waves can
sometimes return to the sea in narrow flows called rip currents.
Rip currents are dangerous, but they are small and can be avoided with a little knowledge.
Rip currents may be only 10 meters or so wide, and 100 meters long, depending on the
size of incoming waves.
Rip currents are visible here as narrow zones of no breaking waves.
Swimmers can get caught in rip currents and carried out to sea. Frequently the swimmers
will panic and try to swim back to shore against the current. Panic or fatigue can have
deadly consequences.
Fortunately, there is a very simple way to get out of a rip current. Simply swim parallel to
shore beyond the rip current. Then you can safely swim back to shore. Often, swimmers
do not know this technique, or are too panicked to remember it.
Tsunamis are surface, gravity, progressive waves, but are not generated by the wind. The
generating force is some type of seismic event, usually an undersea earthquake.
Generation of tsunamis.
Tsunami characteristics.
Example of a tsunami caused by a fault.
The 2004 Indonesian tsunami killed over 250,000 people. It was caused by a massive
undersea earthquake.
Indonesian tsunami, December 26, 2004. Check out NOAA’s Center for Tsunami Research
page for all kinds of neat information: http://nctr.pmel.noaa.gov/index.html. An
animation of the 2004 Indonesian tsunami can be found here:
http://nctr.pmel.noaa.gov/Mov/TITOV-INDO2004.mov
Let’s look at wave speed for a tsunami. Recall that the formula for wave speed is
wavelength divided by period, or L/T. Tsunamis can have wavelengths of hundreds of
kilometers and wave periods of tens of minutes. Let’s use L = 200 km and T = 10 minutes.
200 km ÷ 10 minutes = 20 km/min or 1200 km/hour which is equal to 745 mph. Tsunamis
move faster than the planes flying overhead!
The city of Banda Aceh was one of the hardest hit by the 2004 tsunami.
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Banda Aceh Shore, Indonesia: after
Gleebruk Village (1): before
Gleebruk Village (1): after
Internal waves
Internal waves diagram
Standing waves
Standing waves diagram. At nodes, the water level doesn’t change. At antinodes the
water level is maximum at the crest of a standing wave, and minimum at the trough. This
animation gives you an idea of what might happen when an incoming wave is reflected off
a seawall or rocky coast:
http://faraday.physics.utoronto.ca/IYearLab/Intros/StandingWaves/Flash/standwave.swf
A seiche (pronounced “saysh” or “seesh”) is a special kind of standing wave occurring
within an enclosed basin. The initial displacement could be caused by a chunk of a glacier
falling into a lake, wind pushing water to one side of a lake, or an earthquake. Jump into a
small swimming pool and you can create your own seiche. Check out this site:
http://earthguide.ucsd.edu/earthguide/diagrams/waves/swf/wave_seiche.html
The spectrum of wave types. Wind waves are so common, that collectively they contain
more energy than any other type of waves. Most wind waves have periods of 1-30
seconds. We’ll talk about tides next.