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Introduction to

Oceanography

Dynamic Oceanography: Waves


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Overview

* Waves transmit energy, not water mass, across the ocean’ssurface.

* Waves are classified by several characteristics.

* The behavior of a wave depends on the relation between thewave’s size and the depth of water through which it is moving.

* Wind waves form when energy is transferred from wind to water.

* Waves can change direction by refraction and diffraction, caninterfere with one another, and reflect from solid objects.


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Waves are the undulatory motion of a water surface.• Parts of a wave are, Wave crest,Wave trough, Wave height (H),

Wave Amplitude, Wave length (L),and Wave period (T).

• Wave period provides a basis for the wave classifications!apillar" waves, !hop, #well, Tsunamis, #eiches.

7-1Properties of Ocean Waves

Most of the waves present on the

ocean’s surface are wind-generated waves.• #i$e and t"pe of wind%generated waves are controlled b"

Wind velocit", Wind duration, &etch, and 'riginal state ofsea surface.

• As wind velocit" increases wave length, period and height

increase, but onl" if wind duration and fetch are sufficient.• &ull" developed sea is when the waves generated b" the

wind are as large as the" can be under current conditions ofwind velocit" and fetch.

• #ignificant wave height is the average wave height of thehighest * of the waves present and is a good indicator of

potential for wave damage.


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Ocean Waves

What are the parts of an ocean wave?


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© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Classifying Waves


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Ocean Waves

Orbital waves are waves in which the particles of water move inclosed circles as the wave passes.

Progressive wavesare waves of moving energy in which the waveform moves in one direction along the surface (or junction) of thetransmission medium. Orbital waves are a type of progressive wavebecause the waveform moves forward


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Progressive waves are waves that moveforward across the surface.

• As waves pass, wave form and wave energ" move rapidl"forward, not the water.

• Water molecules move in an orbital motion as the wave passes.

• +iameter of orbit increases with increasing wave si$e anddecreases with decreasing water depth.

• Wave base is the depth to which a wave can move water.

• f the water is deeper than wave base, orbits are circular andthere is no interaction between the bottom and the wave, but ifthe water is shallower than wave base, orbits are elliptical andbecome increasingl" flattened towards the bottom.

7-2Wave Motions


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11/38© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Ocean Waves

Note that the water molecules in the crestof the wave move in the same direction asthe wave, but molecules in the trough movein the opposite direction.


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• There are three t"pes of waves defined b" water depth +eep%

water wave, ntermediate%water wave, and #hallow%water wave.

• !elerit" is the velocit" of the wave form, not the water.

! - LT

• The celerit" of a group of waves all traveling at the same speedin the same direction is less than the speed of the waves within

the group.

7-2

Wave Motions



Deep Water Waves, hallow Water Waves

Note the importance of therelationship between wavelength anddepth in determining wave type.



What factors affect wind wave development?

Wind strength - wind must be moving faster than the wave crests forenergy transfer to continue

Wind duration- winds that blow for a short time will not generate largewaves

Fetch -the uninterrupted distance over which the wind blows withoutchanging direction

!actors "#ecting Wind WaveDevelopment


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!actors "#ecting Wind WaveDevelopment


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!etch is the area of contact $etween the wind and the water and is where wind-generated waves $egin.

• #eas is the term applied when the fetch has a chaotic umbleof new waves.

• Waves continue to grow until the sea is full" developed orbecomes limited b" fetch restriction or wind duration.

• Wave interference is the momentar" interaction betweenwaves as the" pass through each other. Wave interferencecan be constructive or destructive.

• /ecause celerit" increases as wave length increases, longerwaves travel faster than short waves.

7-3 Life History of Ocean Waves


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Wind Waves

Wind waves are gravity waves formed by the transfer of wind energyinto water. Wind forces convert capillary waves to wind waves.


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What happens when waves from different storm systems exist

simultaneously?

When waves meet up, theyinterfere with one another.

Wave interference can be:

Destructive interference –two waves that cancel each other out,resulting in reduced or no wave

Constructive interference –additive interference that results in

waves larger than the original waves

Rogue waves - these freak waves occur due to interference andresult in a wave crest higher than the theoretical maximum

%nterference "nd &ogue Waves


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well !ormation and Dispersion

Wave separation, ordispersion, is a

function of wavelength. Waves withthe longest wavelength move thefastest and leave the area of waveformation sooner. The smoothundulation of ocean water caused by

wave dispersion is calledswell.

(left) The process known as awave

train.


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'he shallower the water, the greaterthe interaction $etween the wave andthe $ottom alters the wave properties,eventually causing the wave to collapse.

• !elerit" decreases as depth decreases.

• Wave length decreases as depth decreases.

• Wave height increases as depth decreases.

• Troughs become flattened and wave profile becomes e0tremel"

as"mmetrical.

• Period remains unchanged. Period is a fundamental propert" ofa wave

• 1efraction is the bending of a wave into an area where it travelsmore slowl".



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Wave steepness (sta$ility) is a ratio of wave height divided $y wave length (*+).• n shallow water, wave height increases and wave length

decreases.

• When HL is larger than or e2uals 3 (HL 3), the wavebecomes unstable.

• There are three t"pes of brea4ers, #pilling brea4ers, Plungingbrea4ers, and #urging brea4ers.



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What different ways can waves break against the shore?

Plunging waves break violently against the shore, leaving an air-filledtube, or channel, between the crest and foot of the wave. Plungingwaves are formed when waves approach a shore over a steeply slopedbottom.

Spilling waves occur on gradually sloping ocean bottoms. The crest ofa spilling wave slides down the face of the wave as it breaks on shore.

Wind Waves "pproaching hore


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What can affect the way that waves travel?

Wave refraction - the slowing and bending of waves in shallow water.

Wave diffraction -propagation of a wave around an obstacle

Wave reflection - occurs when waves “bounce back” from an obstaclethey encounter. Reflected waves can cause interference withoncoming waves, creatingstanding waves.

Wave &efraction, Di#raction, and&eection



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What happens when wind waves break against the shore?


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torm surge is the rise in sea levelresulting from low atmosphericpressure associated with storms andthe accumulation of water drivenshoreward $y the winds.• Water is deeper at the shore area, allowing waves to progress

farther inland.

• #torm surge is especiall" severe when superimposed upon ahigh tide.



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tanding waves or seiches consist of a water surface /seesawing0 $ac1 andforth.• A node is an imaginar" line across the surface which

e0periences no change in elevation as the standing waveoscillates. t is the line about which the surface oscillates.

• Antinodes are where there is the ma0imum displacement ofthe surface as it oscillates and are usuall" located at the edgeof the basin.

• 5eometr" of the basin controls the period of the standing

wave. A basin can be closed or open.• #tanding waves can be generated b" storm surges.

7-4 Standing Waves


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%nternal waves form within the watercolumn on the pycnocline.

• /ecause of the small densit" difference between the watermasses above and below the p"cnocline, wave properties aredifferent compared to surface waves.

•nternal waves displa" all the properties of surface progressivewaves including reflection, refraction, interference, brea4ing,etc.

• An" disturbance to the p"cnocline can generate internal waves,including &low of water related to the tides., &low of watermasses past each other, #torms, or #ubmarine landslides.

• 1esonance amplifies the displacement at the nodes and occurswhen the period of the basin is similar to the period of the forceproducing the standing wave.

7-5 Other Types of Progressive

Waves

It lW


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Internal Waves

Waves that occur at the boundaries of water layerswith different densities are called internal waves.


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Oth T f P i


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'sunamis were previously called tidal

waves, $ut are unrelated to tides.• Tsunamis consist of a series of long%period waves characteri$ed

b" ver" long wave length (up to 66 4m) and high speed (up to376 4mhr) in the deep ocean.

•

/ecause of their large wave length, tsunamis are shallow%waterto intermediate%water waves as the" travel across the oceanbasin.

• The" onl" become a danger when reaching coastal areas wherewave height can reach 6 m.

•

Tsunamis originate from earth2ua4es, volcanic e0plosions, orsubmarine landslides.

7-5 Other Types of Progressive

Waves


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