Oceanography: Waves  

Properties of Waves. The Anatomy of a Wave
Waves transmit energy, not water mass, across the ocean''s surface

Fundamental Principles
Ideally, waves represent a propagation of energy, not matter.
ѓЬ (but ocean waves are not always ideal).
Three kinds:
ѓЬ Longitudinal (e.g., sound wave)
ѓЬ Transverse (e.g., seismic Ў¡±SЎЁ wave) - only in solids
ѓЬ Surface, or orbital wave
ѓЬ Occur at the interfaces of two different densities.
ѓЬ These are the common wind-generated waves.
Some Definitions
ѓЬ Wave Period: Time it Takes a Wave Crest to Travel one Wavelength (units
of time)
ѓЬ Wave Frequency: Number of Crests Passing A Fixed Location per Unit
Time (units of 1/time)
ѓЬ Frequency = 1/Period
ѓЬ Wave Speed: Distance a Wave Crest Travels per Unit Time (units of
distance/time)
ѓЬ Wave Speed = Wave Length / Wave Period for deep water waves only
ѓЬ Wave Amplitude: Wave Height/2
ѓЬ Wave Steepness: Wave Height/Wavelength

Introductory
wave period
standing wave
wave diffraction
wave refraction
wave reflection
constructive interference
destructive interference
Mixed interference
wave frequency
wave length
wave height
trough
internal wave
swell
sea
surf zone
tsunami
shallow water wave
orbital wave
wave train
plunging breaker
spilling breaker
capillary wave
storm surge
transverse wave
crest
resonating force
wave steepness
Tidal Bore
Sir Isaac Newton
seiche
ebb current
tidal range
tidal period
mixed tide
semidiurnal tide
diurnal tide
neap tide
quarter moon
spring tide
full moon
new moon
vernal equinox
autumnal equinox
winter solstice
summer solstice
apogee
perigee
aphelion
perihelion
gravitational force

Marine science
WAVE ANATOMY
orbital wave
progressive wave
Most waves affect only the ocean''s surface layer. Movement ceases at a depth equal to about half the wave''s wavelenght.

CLASSIFYING WAVES

disturbing force
restoring force
wavelenght (capillary waves, wind wave, seiche, seismic sea wave-tsunami, tide

Arranged from short to long wavelenghths, ocean waves are generated by very small disturbances (capillary waves), wind, rocking of waters in enclosed spaces (seiches), seismic and volcanic activity or other sudded displacement, and gravitation attractions (tides)


DEEP-WATER WAVES AND SHALLOW_WATER WAVES

Physical Oceanography
DEEP-WATER WAVES AND SHALLOW_WATER WAVES
Classification

Progressive waves - classification depends on their wavelength relative to the depth of water deep-water waves - transitional water waves (L/2-L/20) - shallow awter awves


The speed of ocean waves usually depends on their wavelength, with long waves moving fastest.

The behavior of a wave depends largely on the relationship between the wave''s size and the depth of water through which it is moving.

Wind wavws (dep-water waves)
Seismic Sea Waves (Shallow0Water waves)



Shallow-Water waves
 travel in water that is shallower than 1/20 of the wave’s wavelength; Depth
<L/2
&#61548; Wave speed is function of water depth only
&#61548; Waves are not ideal and propagate both Energy and Mass (Stokes Drift)
Intermediate waves
&#61548; neither purely “deep” or “shallow”; L/20 < Bottom Depth < L/2
&#61548; wave speed depends on both wavelength and water depth
Breaking Waves
&#61548; As waves approach the shore, wavelength decreases, while height and
steepness increase.
&#61548; As waves approach the shore, drag of the bottom slows the water motion
near bottom
&#61548; Consequently, there is net forward transport at the surface as the waves
steepens
&#61548; Once height reaches 1/7 wave length, the wave becomes unstable and
breaks.
Wave Refraction & Reflection
&#61548; Shallow-Water Wave change direction due to changes in speed; which are
in turn due to changes in depth
&#61548; The net result is a rotation of wave fronts to become parallel with bottom
depth contours.
&#61548; Wave may also be reflected
&#61548; Consequence of Wave Refraction: wave energy is focused on headlands
and away from bays – nature likes a straight coast!
&#61548; Waves striking beach at an angle produce Longshore transport of sediment
Transport Obstructed by Groins

Wave Generation. WIND WAVES
Generation of Waves
Ć Most surface waves generated by wind; therefore called wind waves
Waves are also generated by
ƒÜ Earthquakes, landslides ¡X tsunamis
Ć Atmospheric pressure changes (storms)
ƒÜ Gravity of the Sun and Moon ¡X tides
Height of Wind-Generated Waves depends on:
Ć Wind Speed
Ć Duration of Wind Event
Ć Fetch - the distance over which wind can blow without obstruction
Waves and Swell
Ć Once generated, waves can propagate as swell without wind
Ć Dispersion: Large waves travel faster than small ones
Ć Because waves are not ideal, energy is dissipated and waves die out
Ć Small ones die out, or damp out, faster.
Wave Speed and Water Depth
Deep-Water waves
ƒÜ travel in water that is deeper than 1/2 the wave¡¦s wavelength; Depth > L/2
ƒÜ Speed is function of wavelength only ¡V long wavelengths move faster
Ć Waves have nearly ideal shape and thus propagate energy but very little
mass


swell
wind strength
wind duration
fetch

Wind strength and duration determine the wavelength and speed of wind waves.

wave steepness
Global wave heigth
wavelength records

Interference
Destructive interference
constructive interference

Crossing waves can ¡§interfere¡¨ to create either a bigger wave (constructive
interference) or smaller wave (destructive interference).

the freak wave=a rogue wave
waves can interfere with one another, resulting in larger or smaller waves.

Wind waves Approaching Shore
surf zone
plunging waves

WAVE REFRACTION

Most wave change shape and speed as they approach shope. They may plunge or spill at the surf zone and bend to breack nearly parallel to shore.

LONG WAVES

TIDAL WAVES
STORM WAVES
storm tides
dome of water

OTHER WAVES
Other Waves
&#61548; Tsunamis
&#61548; Storm Surges & Sieches
&#61548; Internal Waves
&#61548; Planetary Waves
Tsunamis
&#61548; Tsunamis (sometimes improperly called tidal waves) are large amplitude,
long wavelength waves that propagate on the ocean surface
Tsunamis can be generated by
&#61548; Earthquakes
&#61548; Landslides
&#61548; Volcanic Eruptions
&#61548; Meteorite/Asteroid Impact
Properties of Tsunamis
&#61548; Tsunamis have long periods and long waves lengths (as long as 1 hour and
100 km respectively).
&#61548; For tsunamis, wavelength is always greater than twice the water depth.
&#61548; Therefore, tsunamis always behave as shallow water waves (speed
depends on depth).
&#61548; In water of average depth (4000 m), a tsunami will travel at 700 km/hr.
The December, 26 2004 Sumatran Earthquake and Tsunami
&#61548; Generated by a magnitude 9.3 earthquake as the Indian Plate thrust under
the Sunda Plate
&#61548; Calculated vertical displacements were as much as 5 meters
&#61548; The fault ruptured along more than 1200 km
&#61548; One of the largest earthquakes in past 100 years.
&#61548; May have generated submarine landslides
&#61548; Extensive damage and loss of life throughout the Eastern Indian Ocean
Can we predict tsunami’s and save lives?
Generating a Tsunami Warning
&#61548; Earthquake occurs
&#61548; Seismic waves travel through the Earth (at about 8 km/sec) to seismometers
&#61548; Earthquake detected by seismometers
&#61548; Determine location - did it occur in the sea?
&#61548; Determine size - is it big enough to generate a tsunami?
&#61548; Pacific Tsunami Warning Center issues bulletin
&#61548; About 75% are false alarms - most earthquakes don’t generate damaging
tsunamis
&#61548; Governments must then take action to warn & evacuate
&#61548; 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?
&#61548; Pacific Rim is most vulnerable
&#61548; Hawaii is particularly vulnerable
&#61548; Japan, Alaska, S. America
&#61548; Pacific Northwest
&#61548; 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.
&#61548; Indian Ocean
&#61548; Atlantic and Caribbean
&#61548; Many of the Caribbean islands are subduction zone volcanoes - in addition
to earthquakes, volcanic eruptions and volcanic landslides could generate
tsunamis
&#61548; Eastern Mediterranean
&#61548; Both volcanically and seismically active
&#61548; Salt beds beneath Mediterranean are particularly subject to landslides.
Other Waves
Seiches – resonant oscillations
Internal Waves – waves that propagate along density boundaries within
the ocean
Kelvin and Rossby waves:
&#61548; Low amplitude, long wavelength waves related to wind changes – notably
El Ni&#241;o
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