EAS/BIOEE 154
Lecture 12
Introduction to Oceanography
Waves
Fundamental Principles
Ideally, waves represent a propagation of energy, not matter.
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(but ocean waves are not always ideal).
Three kinds:
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Longitudinal (e.g., sound wave)
Transverse (e.g., seismic “S” wave) - only in solids
Surface, or orbital wave
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Occur at the interfaces of two different densities.
These are the common wind-generated waves.
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Some Definitions
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Wave Period: Time it Takes a Wave Crest to Travel one Wavelength (units
of time)
Wave Frequency: Number of Crests Passing A Fixed Location per Unit
Time (units of 1/time)
 Frequency = 1/Period
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Wave Speed: Distance a Wave Crest Travels per Unit Time (units of
distance/time)
 Wave Speed = Wave Length / Wave Period for deep water waves only
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Wave Amplitude: Wave Height/2
Wave Steepness: Wave Height/Wavelength
Wave Interference
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Crossing waves can “interfere” to create either a bigger wave (constructive
interference) or smaller wave (destructive interference).
Generation of Waves
Most surface waves generated by wind; therefore called wind waves
Waves are also generated by
 Earthquakes, landslides — tsunamis
 Atmospheric pressure changes (storms)
 Gravity of the Sun and Moon — tides
Height of Wind-Generated Waves depends on:
 Wind Speed
 Duration of Wind Event
 Fetch - the distance over which wind can blow without obstruction
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Waves and Swell
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Once generated, waves can propagate as swell without wind
Dispersion: Large waves travel faster than small ones
Because waves are not ideal, energy is dissipated and waves die out
Small ones die out, or damp out, faster.
Wave Speed and Water Depth
Deep-Water waves
 travel in water that is deeper than 1/2 the wave’s wavelength; Depth > L/2
 Speed is function of wavelength only – long wavelengths move faster
 Waves have nearly ideal shape and thus propagate energy but very little
mass
EAS/BIOEE 154
Lecture 2
Shallow-Water waves
 travel in water that is shallower than 1/20 of the wave’s wavelength; Depth
<L/2
 Wave speed is function of water depth only
 Waves are not ideal and propagate both Energy and Mass (Stokes Drift)
Intermediate waves
 neither purely “deep” or “shallow”; L/20 < Bottom Depth < L/2
 wave speed depends on both wavelength and water depth
Breaking Waves
 As waves approach the shore, wavelength decreases, while height and
steepness increase.
 As waves approach the shore, drag of the bottom slows the water motion
near bottom
 Consequently, there is net forward transport at the surface as the waves
steepens
 Once height reaches 1/7 wave length, the wave becomes unstable and
breaks.
Wave Refraction & Reflection
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Shallow-Water Wave change direction due to changes in speed; which are
in turn due to changes in depth
The net result is a rotation of wave fronts to become parallel with bottom
depth contours.
Wave may also be reflected
Consequence of Wave Refraction: wave energy is focused on headlands
and away from bays – nature likes a straight coast!
Waves striking beach at an angle produce Longshore transport of sediment
Transport Obstructed by Groins
Other Waves
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Tsunamis
Storm Surges & Sieches
Internal Waves
Planetary Waves
Tsunamis
Tsunamis (sometimes improperly called tidal waves) are large amplitude,
long wavelength waves that propagate on the ocean surface
Tsunamis can be generated by
 Earthquakes
 Landslides
 Volcanic Eruptions
 Meteorite/Asteroid Impact
Properties of Tsunamis
 Tsunamis have long periods and long waves lengths (as long as 1 hour and
100 km respectively).
 For tsunamis, wavelength is always greater than twice the water depth.
 Therefore, tsunamis always behave as shallow water waves (speed
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Lecture 2
depends on depth).
In water of average depth (4000 m), a tsunami will travel at 700 km/hr.
The December, 26 2004 Sumatran Earthquake and Tsunami
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Generated by a magnitude 9.3 earthquake as the Indian Plate thrust under
the Sunda Plate
Calculated vertical displacements were as much as 5 meters
The fault ruptured along more than 1200 km
One of the largest earthquakes in past 100 years.
May have generated submarine landslides
Extensive damage and loss of life throughout the Eastern Indian Ocean
Can we predict tsunami’s and save lives?
Generating a Tsunami Warning
 Earthquake occurs
 Seismic waves travel through the Earth (at about 8 km/sec) to seismometers
 Earthquake detected by seismometers
 Determine location - did it occur in the sea?
 Determine size - is it big enough to generate a tsunami?
 Pacific Tsunami Warning Center issues bulletin
 About 75% are false alarms - most earthquakes don’t generate damaging
tsunamis
 Governments must then take action to warn & evacuate
 Detect tsunamis using seabed detectors - NOAA’s DART system: only 6
detectors deployed so far – most in the northeast Pacific; 32 planned for
the Pacific by 2007 (at a cost of over $1 million a piece).
What areas are vulnerable to tsunamis?
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Pacific Rim is most vulnerable
Hawaii is particularly vulnerable
Japan, Alaska, S. America
Pacific Northwest
The plate boundary system in the Pacific NW is behaves very similarly to the
Indonesia one, with very large, very infrequent earthquakes. Geologic
evidence and Japanese historical records indicate a very large tsunami
generated there in 1700.
Indian Ocean
Atlantic and Caribbean
Many of the Caribbean islands are subduction zone volcanoes - in addition
to earthquakes, volcanic eruptions and volcanic landslides could generate
tsunamis
Eastern Mediterranean
Both volcanically and seismically active
Salt beds beneath Mediterranean are particularly subject to landslides.
Other Waves
Seiches – resonant oscillations
Internal Waves – waves that propagate along density boundaries within
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the ocean
Kelvin and Rossby waves:
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Low amplitude, long wavelength waves related to wind changes – notably
El Niño
Some Study Questions
How deep must the water be for a wave with a 100 m wavelength
to behave as a strictly deep water wave?
How shallow must the water be for that same wave to behave as a
strictly shallow water wave?
What is the restoring force for capillary waves?
If a wave has a period of 10 seconds, what is the frequency of that
wave?
Describe what happens as waves move out from a storm? How
does the distribution of short and long wavelength waves
change?
Why would wave refraction cause headlands to erode, and
sediment to be deposited in bays?
Explain how waves can transport sediment along the coast in
“longshore drift”.
Why are tsunamis considered “shallow water waves”?
About how long did it take the Dec. 26 tsunami to reach Sri
Lanka?
What role do NOAA’s DART buoys play in the Pacific Tsunami
Warning System?
How do the DART instruments detect a tsunami?
How does water motion differ between tsunamis and wind-driven
waves?
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