Lecture #10
Week #9
Waves
Waves
¾ Manifestation of energy traveling through the
water
Chapter 10
¾ Progressive waves – energy actually travels after
being disturbed
¾ Disturbance can be anything – usually wind,
earthquakes, crashing icebergs, etc…
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Anatomy of
a wave:
Free waves vs Forced waves
¾ Free waves: Disturbed once, and then on their
Height
own (e.g. surfing waves)
¾ Forced waves: tides, capillary waves under
continuous windage
Trough
Crest
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Anatomy of a
wave (cont.):
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Waves classified by Wavelength
¾ Period: Time a wave takes
to go by a given point
¾ Frequency: Number of
waves in a specific amount
of time
Period
Frequency
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A.K. Morris, Ph.D.
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Lecture #10
Week #9
Wave H2O Movement
Figure 3.22
¾ How do water molecules move in a wave?
¾ What if you could follow a molecule?
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Gull on the Waves
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Wave H2O Movement
¾ “Orbital” Wave Motion
¾ H2O particles don’t actually go anywhere but in
circles
¾ Waves carry E but do not transport water
¾ Example: Rope
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Velocity vs Wavelength
Wave Theory
¾ Once a wave is generated, period doesn’t change
C=
¾ For Deep Waves: speed (C) determined by wave
length (L) and period (T)
C=
L
T
L
T
19.4 meters
233 meters
=
seconds
12 seconds
Æ
¾ Gravity (g) and pi (π) are constant, so:
C = 1.251√L
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A.K. Morris, Ph.D.
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Lecture #10
Week #9
Wave Behavior Influenced by
Water Depth
Wave Theory
¾ DEEP WATER WAVES:
Depth must be > ½ L
¾ Orbital diameter determined
entirely by wavelength
¾ Orbitals gets progressively
smaller with depth
¾ SHALLOW WATER WAVES:
Depth must be < ½0 L
¾ Elliptical orbits since bottom
“gets in the way”
¾ TRANSITIONAL WAVES:
Depth Between ½ – ½0 L
¾ Elliptical
¾ Deep-shallow animation…
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Wave Energy
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What determines wave
intensity?
¾ Wave energy is a function of 3 things – water
density (ρ), gravity (g), and wave height (H).
E = 1/8 ρ g H2
¾ Notice: gravity doesn’t change.
¾ Notice: compared to height, density doesn’t
change nearly as much.
¾ So E determined by wave height squared! Double
the wave height = quadruple the wave energy.
Twice the height
Four times the E
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Wave Interference
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Wave Interference
¾ Three VERY general ways waves interact
¾ By affecting wave height, can effectively increase or
¾ Waves can originate from
decrease the energy
different sources
e.g. storms off Greenland
and out of the tropics
¾ This means waves can
interfere with each other.
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A.K. Morris, Ph.D.
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Lecture #10
Week #9
Synthetic Aperture
Radar
Wind and Waves
¾ Waves grow as more E is transferred by winds
¾ Used to detect sea surface
roughness
Restoring force is
gravity for these
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Wind and Waves
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Wind and Waves
¾ Wave steepness: Water is viscous – push it too hard and H
will exceed the “wave steepness limit” (approx 1/7 L)
¾ 1/7 = 0.142 (beginning
threshold)
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White Caps
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Wind and Waves
¾ Wind velocity (speed)
¾ Winds can essentially “throw” water if it is
blowing fast enough. ≈ 10-12 mph
¾ Fetch: Span of open water
over which wind blows
¾ More fetch means larger
wave length = more E
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A.K. Morris, Ph.D.
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Lecture #10
Week #9
Generally: Wave height in feet equals
about ½ wind speed in MPH
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© W.M. Hamner
Wind speed = 67 mph
Gusts to 78
Wave Height = 112 Feet
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Dispersion
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Pop Quiz
Why do waves break at the beach?
¾ Ultimately favors longer wave
lengths. These are the ones
that reach distant shores
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A.K. Morris, Ph.D.
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Lecture #10
Week #9
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This dude
is nuts
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Waves break and refract
against the shore
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Why does this happen?
Faster here
Refraction
Slower here
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A.K. Morris, Ph.D.
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Lecture #10
Week #9
TSUNAMIS
¾ Not “tidal waves”.
¾ Generated by submarine movements in
lithosphere (earthquake, volcano, landslide)
¾ Very long wavelength (100 miles)
¾ Period from 5-60 minutes
¾ Speed over 400 mph
¾ Height less than 1/2 meter in open ocean
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April 1, 1946 – Aleutian Islands, Alaska. Magnitude 7.8 earthquake resulted in a big tsunami
00:00
Hilo Bay, Hawaii
4:55 hrs later
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May 22, 1960 - Chile, South America. Magnitude 9.5 earthquake resulted in a huge tsunami
Hilo Bay Tsunami, Hawaii
April 1, 1946
Seismic Sea Wave Warning System
Japan
22:00 hrs later
Northern California
15:30 hrs later
(aka the Pacific Tsunami Warning System)
Hilo Bay, Hawaii
14:45 hrs later
U.S. Navy
Valdivia, Chile
00:00
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A.K. Morris, Ph.D.
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Lecture #10
Week #9
May 22, 1960 - Chile, South America. Magnitude 9.5 earthquake resulted in a huge tsunami
Tsunami Indonesia
¾ Tsunami Video…
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Banda Aceh, Indonesia 12/23/04
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Banda Aceh, Indonesia 12/26/04
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Tsunami - Indonesia
A.K. Morris, Ph.D.