Lab 9 Oceanography .docx
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GEOL101 Dynamics of the Earth – Fall 2023
Name: Emily Thomson
Laboratory 9: Oceanography
Section:
How do ocean waves form?
“All waves are disturbances of a fluid medium through which energy is moved” (Davis, 1997). Ocean
waves travel on the interface between the ocean and the atmosphere, and are produced most
commonly by winds. As shown in Figure 1, the
crest
of a wave is its highest point and the
trough
is
the lowest. The
height
of the wave is the vertical distance between the crest and the trough. The
wavelength
(λ) is the horizontal distance from crest to crest or from trough to trough. The
steepness
is the ratio of its height to its wavelength. When the steepness value reaches ~0.143
(i.e., a height:wavelength ratio of 1:7), the crest of the moving wave becomes unstable and begins to
break forward.
Figure 1. Key characteristics used to describe ocean waves.
Use Figure 1 to answer Questions 1 through 4 below:
Question 1. What is the height of the illustrated wave?
1 meter
Question 2. What is the wavelength?
8 meters
Question 3. What is the steepness?
0.125 steepness
Question 4. Will the wave break given your answer to Question 3? Please briefly explain your
answer.
Not yet because when the steepness value reaches 0.143 that is when the wave becomes unstable
and begins to break and the steepness from question 3 is lower than the breaking point
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Question 5. In the graph below, sketch a wave with a height of 2 m and a wavelength of 10 m.
Label the wavelength, wave height, crest, trough, and steepness.
Question 6. What is the steepness of the wave that you sketched?
steepness is .2
Question 7. Will the wave break?
Yes the wave will break
Waves form when the interface between the oceans and the atmosphere is disturbed by some force.
The most common disturbing force is the friction of wind moving over the water. Once the wave has
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formed gravity acts against this disturbance, and attempts to restore the water/atmosphere interface
back to its flat-water position (i.e., a horizontal state). Hence, wind-generated waves are sometimes
referred to as
gravity waves
. As gravity pulls the crest of a wave downward, momentum carries the
water/atmosphere interface beyond the flat-water position to form a trough. As a result, a buoy will
appear to move up and down without being translated in the direction that the waves appear to be
moving. Such up and down motion will continue as long as the wind is blowing. When the wind stops
blowing, the water/atmosphere interface returns to its normal flat-water state.
The
period
of a wave is the time it takes for one wavelength to pass a reference mark. The periods
for normal ocean waves range from a few seconds to about 15 seconds. Note that this differs from
wave celerity
which is the speed at which a wave advances or propagates. Mathematically, wave
celerity in deep water is simple (i.e., 1.56 / wave period in m/sec or 5.12 / wave period in ft/sec.
Deep water waves are waves that occur in water depth that is greater than one half their
wavelength.
Question 8. If it takes 10 seconds for 1 wavelength of the wave that you sketched in Figure 2
to pass the end of Scripps pier, then what is the period of the wave?
the period is 10 sec
Question 9. What is the celerity of the wave?
0.156
Question 10. If the wave passing the end of Scripps pier had the wavelength of the wave
shown in Figure 1, but nonetheless took 10 seconds for 1 wavelength to pass, then what
would its period be?
the period would be 10 sec
Question 11. What is the celerity of the wave? Did this wave advance faster or slower than
the wave in Figure 2?
1.56. it did not advance faster or slower because they have the same period and celerity
The size of a wave increases as the speed, duration, and fetch of the wind increases.
Fetch
is the
extent of the open water over which the wind travels. A slight breeze over a calm sea can generate a
series of
ripples
(i.e., waves with a period of less than one second). The generation of ripples
provides the wind an elevated surface across which it can push. In doing so it transfers energy to
this surface, and, as a result, the small waves (ripples) begin to grow increasing their height and
period. Wind blowing for many hours and over a great distance (long fetch) will result in very large
and powerful waves.
You can demonstrate the fact that blowing wind can produce waves by blowing a breath of air across
a pan of water. Try it. Your breath is analogous to a large storm in the NW Pacific.
Though many students think that water is moving in the direction of wave motion it turns out that this
perception is not correct. If you think about it, you actually have observed things like fishing boats
remaining stationary as a swell passes beneath them, or a fishing bob moving up and down as small
waves or ripples pass beneath along a lake or bay shoreline. If the water was moving with the
waves, then each of these items should have been translated along with the wave. What actually
happens is that water particles within the wave are moving forward on the crest and backward on the
trough, with vertical motion occurring between the two. The exact path that they follow is circular. For
example, shown in Figure 3 are 11 different positions of a wave that is moving from right to left. The
position of a particle of water time 1 (T
1
) is shown at the crest of the wave. As the wave moves to the
left at times 2 (T
2
) – 11 (T
11
) that same particle will appear to move downward in a circular path.
Question 12. Use your colored pencils to show the positions a water particle will occupy if it
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starts out in the trough of the wave at time 1 (T
1
) in Figure 3 below.
Figure 3. Circular path taken by a water particle as a wave moves
through a column of water from right to left.
In the open ocean the circular paths followed by water particles decrease systematically downward
until a depth of 0.5*λ. This depth is known as wave base (Figure 3). Below wave base water
particles do not feel the wave. In contrast, as waves near the shore, the water depth decreases and
results in a dramatic change in the wave character.
What happens to waves as they encounter shallow water?
As deep water waves move toward adjacent shores they progressively encounter shallower waters.
4
Eventually, they will begin to feel the bottom when the depth of water is less than one half their
wavelength. At this depth, the circular orbits that water particles follow come into contact with the
seabed and the wave begins to travel slower as wavelength is reduced, and wave height increased.
In contrast, the wave period remains constant.
The friction produced by the wave encountering the seabed causes the circular paths of water
particles to be squeezed into elliptical forms, and as water depth continues to decrease these paths
eventually become simple back and forth motions parallel to the seaward dipping seabed. Because
the friction between the advancing wave and the seabed is greatest lower in the wave than higher in
the wave, the speed of advancement of the wave at this depth is more greatly reduced than it is
higher in the wave. The contrast in speed of advancement with depth leads to an unstable waveform
with the upper part outracing the lower part, a circumstance that eventually leads to collapse in a
breaker.
When water depth is about 1/25 the wavelength (0.04 * λ) waves are said to be in shallow water. At
this depth water particles follow strongly elliptical back-and-forth motions (see Figure 4). The celerity
or speed of advance of a shallow water wave in the metric system is 3.13 times the square root of
the water depth (m) (m/sec). In the English system celerity is 5.66 times the square root of the water
depth (ft) (ft/sec).
Figure 4. The change in particle paths as waves encounter shallow water.
Question 13. If the wavelength is 10 m, wave height is 1 m, and the period 10 sec, then in
Figure 4 at what depth is the wave base?
5m
Question 14. At what depth would the wave be considered to be in shallow water?
5m would be considered shallow
What are tsunamis and how do they differ from normal ocean waves?
The term tsunami means “harbor wave” in Japanese, the location where the Japanese most feared
them. Tsunamis are sometimes referred to as tidal waves. Tsunamis originate when a large
magnitude earthquake or landslide suddenly shifts or displaces a large amount of seawater. The
result is a train of waves migrating away from the center of disturbance, a configuration that
resembles the concentric waves emanating outward from the disturbance created by tossing a stone
in a lake. Tsunamis move at great speeds sometimes up to 800 km/hour. Wavelengths are on the
order of 150 km or more while wave height is about half a meter or so.
Question 15. If the wavelength of a tsunami is 1500 m, then at what depth would it begin to
feel the seabed?
750m
Tsunamis that begin to steepen at great distances from the shore will continue to steepen as they
5
advance toward the continental mainland or island in its path. Some tsunamis are known to have
grown to 25 m or more. Prior to December 26, 2004 the most life destroying tsunami on record hit
Awa, Japan in 1703, killing 100,000 people.
You will have to do some web-based research to answer the following questions:
Question 16. What is the current death toll resulting from the December 26, 2004 tsunami that
originated in Sumatra?
7000 people
Question 17. What is the largest wave height reported for the December 26 tsunami?
30m (100 ft)
Question 18. What is the estimated speed of the December 26 tsunami?
500 mph
Question 19. What was the wavelength of the December 26 tsunami?
800 km
Question 20. What was the wave base for the December 26 tsunami?
225 km
Question 21. Please provide your references to the above questions in the space below.
google
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