Answers to STUDY BREAK Questions
Essentials 5th
Chapter 9
WAVES
1. I wrote that an ocean wave is, in a sense, an illusion. What’s actually moving in an ocean
wave?
You can point to a wave crest and follow its progress, but only energy is moving long
distances in ocean waves, not water mass.
2. Draw an ocean wave, and label its parts. Include a definition of wave period.
Draw the anatomy of a wind wave, and then check Figure 9.2.
3. Make a list of ocean waves, arranged by disturbing force and wavelength.
In wavelength order (shortest to longest): capillary waves, wind waves, seiches,
tsunami, tides.
4. What is a gravity wave?
A wave with wavelength greater than 1.73 centimeters (0.68 inch) whose restoring
forces are gravity and momentum.
5. What is restoring force?
Restoring force is the dominant force that returns the water surface to flatness after a
wave has formed in it.
6. What defines a deep-water wave? Are there any waves that can never be in deep water, no
matter how distant the seabed above which they are moving?
Deep-water waves move through water deeper than half their wavelength. Only
capillary waves and wind waves can be in “deep” water.
7. What is the mathematical relationship between celerity (speed), wavelength, and wave
period for deep-water waves? For shallow-water waves?
Page 1 of 5
Answers to STUDY BREAK Questions
Essentials 5th
C

For deep-water waves, celerity (speed) may be expressed as:

For shallow-water waves, celerity may be expressed as: C  gd
L
T
8. How are wind waves formed? What’s a fetch?
Wind waves form when a water surface irregularity deflects wind upward, slows it,
and causes some of the wind’s energy to be transferred into the water to drive the wave crest
forward. Fetch is the distance wind blows across the ocean to generate sets of wind waves.
9. How does the wavelength of a wind wave affect its speed?
Wavelength and wave speed are proportional in deep-water waves – the longer the
wavelength, the greater the wave speed (C).
10. What is a fully developed sea?
In a fully developed sea the maximum wave size theoretically possible for a wind of a
specific strength, duration, and fetch is reached.
11. How is group velocity different from the velocity of an individual wave within the group?
Though each individual wave moves forward with a speed proportional to its
wavelength in deep water (C), the group velocity is only half that speed.
12. Can constructive or destructive interference ever be seen on a casual visit to the beach?
Constructive and destructive interference is often seen at wave-swept beaches. Do
you notice that every 9th wave is the largest (or 12th, or 5th)? The cause is shown in Figure
9.13.
13. What’s a rogue wave? Are rogue waves potentially dangerous?
A rare confluence of crests at sea can form a rogue wave which would be much larger
than any noticed before or after, and would be higher than the theoretical maximum wave
capable of being sustained in a fully developed sea. Rogue waves have broken many large
ships.
Page 2 of 5
Answers to STUDY BREAK Questions
Essentials 5th
14. When does a wind wave become a shallow-water wave as it approaches shore?
A wave ceases to be in “deep” water when it moves over a seabed shallower than half
the wave’s wavelength.
15. What factors influence the breaking of a wind wave?
A wave break is influenced by wavelength, bottom depth, bottom contour, bottom
texture, wind conditions, and shore steepness.
16. What might cause waves approaching a shore at an angle to bend to break nearly
parallel with the shore?
A wave approaching shore at an angle does not break simultaneously along its length
because different parts of it are in different depths of water. The part of the wave line in
shallow water slows down, but the attached segment still in deeper water continues at its
original speed; so the wave line bends, or refracts.
17. Are the wave speed and period of an internal wave comparable to those of a wind wave?
A tsunami?
Because the density difference between the joined media in internal waves is very
small, their speed and period are very slow. They are not comparable to wind waves or
tsunami.
18. Are internal waves dangerous?
Internal waves are not dangerous at the ocean surface, and pose a threat only to
delicate oceanographic sensing equipment or (rarely) submariners.
19. Is there really any such thing as a true “tidal wave?”
True “tidal waves” cause the tides. You’ll learn about these largest of all waves in
the next chapter.
20. What causes a storm surge? Why is a storm surge so dangerous?
Page 3 of 5
Answers to STUDY BREAK Questions
Essentials 5th
The low atmospheric pressure associated with a great storm will draw the ocean
surface into a broad dome which accompanies the storm to shore, becoming much higher as
the water gets shallower at the coast. They are dangerous because of their height and sudden
onslaught.
21. Can a storm surge be predicted?
Storm surge can be predicted from a tropical cyclone’s wind speed, probable path,
and atmospheric pressure.
22. Lake Michigan is long and narrow and trends north-south. Could a seiche develop in this
lake? Do seiches tend to be dangerous?
Seiches form in Lake Michigan as persistent north winds drive water toward the
southern shore. When the wind ceases, parts of the Lake rocks rhythmically for a day or two.
Seiches are generally not dangerous, but there are exceptions. A rare confluence of high
wind and low atmospheric pressure generated a seiche in Lake Michigan in 1954 that
drowned eight fishermen.
23. What would a standing wave look like in a rectangular swimming pool?
As any active kid knows, swimming pools can easily be seiched. Stand in the middle
with a kick-board (surfboard, etc.) and make waves by pushing the board away from you and
pulling it back. Find the natural resonance of the pool and reinforce it (that is, time your
pushes as you would push a child on a swing). Soon the center of the pool will be high when
the edges are low, and vice versa. Add more energy and watch the fun!
24. What causes tsunami? Do all geological displacements cause tsunami?
Tsunami are caused by the rapid displacement of ocean water.
25. How fast does a tsunami move?
Tsunami move at high speeds – 750 kilometers (470 miles) per hour is typical.
26. Is a tsunami a shallow-water wave or a deep-water wave?
Tsunami are shallow-water waves. Half their wavelength would be 100 kilometers
(62 miles), and even the deepest ocean trenches do not exceed 11 kilometers (7 miles) in
depth.
Page 4 of 5
Answers to STUDY BREAK Questions
Essentials 5th
27. What is the wave height of a typical tsunami away from land? Are tsunami dangerous in
the open sea?
A ship on the open ocean that encounters a tsunami with a 16-minute period would
rise to a crest only 0.3 to 0.6 meter (1 or 2 feet) above average sea level. Tsunami are not
dangerous in the open ocean.
28. Does a tsunami come ashore as a single wave? A series of waves? Does a tsunami wave
break like a surfing wave?
Unless the location is very close to the causal epicenter, tsunami typically come
ashore as a series of wave at regular intervals.
29. What caused the most destructive tsunami in recent history? Where was the loss of life
and property concentrated?
The 2004 Indonesian event (described in the chapter’s opener) was the most lethal
earthquake in 5 centuries. The numbers of dead exceeded 176,000 with another 67,000
missing.
30. How might one detect and warn against tsunami?
Tsunami warning systems depend on seabed seismometers, and submerged devices
and satellites that watch the shape of the sea surface.
Page 5 of 5