Ocean WavesOcean Waves

CapillaryCapillary

GravityGravityWind Wind

generatedgenerated

TidesTides

TsunamisTsunamisSeichesSeiches

Capillary waves are driven by the surface tension produced by electrically polarized water moleculeproduced by electrically polarized water moleculeCapillary waves are driven by the surface tension produced by electrically polarized water molecule produced by electrically polarized water molecule

San Pedro LighthouseSan Pedro Lighthouse

WavesWaves are alternate rises and falls, are alternate rises and falls,

describable as simple/complex describable as simple/complex sinusoidalssinusoidals

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

0 45 90 135 180 225 270 315 360

Crest

Am

plitu

de

Amplitude

Height

Trough

Wave period or Wavelength

As a wave travels through the waver, the particles travel in clockwise circles. The radius of the circles decreases as the depth into the water increases. The following animation shows a water wave traveling from left to right in a region where the depth of the water is greater than the wavelength of the waves.

http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html

WavesWaves are alternate rises and falls, are alternate rises and falls,

describable as simple/complex describable as simple/complex sinusoidalssinusoidals

only add-up, alwaysonly add-up, always

i.e., wave interference can be i.e., wave interference can be constructive and/or destructiveconstructive and/or destructive

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0

0.5

1

0 90 180 270 360

AABB

CC

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

0 45 90 135 180 225 270 315 360

AABB

C = A+BC = A+B

-1

-0.5

0

0.5

1

0 90 180 270 360

AABB

CC

-4

-3

-2

-1

0

1

2

3

4

0 100 200 300 400 500 600 700

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-3

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-1

0

1

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0 100 200 300 400 500 600 700

Waves interference is always additiveWaves interference is always additive

This is the algebraic sum of these

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0

1

0 360

Wave interference can be Wave interference can be constructive or destructiveconstructive or destructive

Constructive Constructive interferenceinterference

Constructive Constructive interferenceinterference

Constructive Constructive interferenceinterference

Destructive Destructive interferenceinterference

Destructive Destructive interferenceinterference-1

0

1

Second Harmonic

Standing Wave Pattern

First Harmonic Standing Wave Pattern

Third Harmonic Standing Wave Pattern

http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/swf.html

Waves and wave activity

• Standing waves

As waves can be thought of as single or complexsinusoids,we can lookat waves ascomprisingone or moreharmonics.

Waves and wave activity

• Traveling waves

Waves travel in groups,and thegroupvelocityis one-half the velocity of individual waves.

http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/swf.html

WavesWaves are alternate rises and falls, are alternate rises and falls,

describable as simple/complex describable as simple/complex sinusoidalssinusoidals

only add-up, alwaysonly add-up, always

i.e., wave interference can be i.e., wave interference can be constructive and/or destructiveconstructive and/or destructive

carry energy, not mattercarry energy, not matter

light is an exception, it travels light is an exception, it travels in waves and as particlesin waves and as particles

10-1 1 101 102 105104103 106 107

104

103

102

10

1

Wavelength (cm)

Wav

e sp

eed

or

velo

city

(cm

/s)

Cap

illar

y C

apill

ary

wav

esw

aves

Gravity wavesGravity wavesin deepwater,in deepwater,

V V 1.25 1.25 L L

Output from a shallow water equation model of water in a bathtub. The water experiences five splashes which generate surface gravity waves that propagate away from the splash locations and reflect off of the bathtub walls.

http://en.wikipedia.org/wiki/Ocean_surface_wave

Waves carry energy, not matterThe orbital motion of representative

water molecules: orbital size decreases with depth, with negligible water motion

at depth ½ wavelength

Circular path: waves of oscillation

Elliptical path: waves of translation

Waves break on reaching the shore. Why?

Waves break as the succeeding waves catch up with preceding waves

Spilling breakers form when the bottom slopes gradually

Spilling breakers form when the bottom slopes gradually

Plunging or surging breakers form when the bottom slope is steep

Plunging or surging breakers form when the bottom slope is steep

Three factors affect wind wave development:

(a) Wind speed, (b) Wind duration,

and (c) Fetch

How wind affects the wave height

WindWindspeedspeed

19 km/hr19 km/hr(10 knots)(10 knots)

37 km/hr37 km/hr(20 knots)(20 knots)

56 km/hr56 km/hr(30 knots)(30 knots)

74 km/hr74 km/hr(40 knots)(40 knots)

92 km/hr92 km/hr(50 knots)(50 knots)

FetchFetch

19 km19 km

139 km139 km

518 km518 km

1313 km1313 km

2627 km2627 km

WindWinddurationduration

2 hr2 hr

10 hr10 hr

23 hr23 hr

42 hr42 hr

69 hr69 hr

Wind ConditionsWind Conditions

AverageAverageheightheight

0.27 m0.27 m

1.5 m1.5 m

4.1 m4.1 m

8.5 m8.5 m

14.8 m14.8 m

AverageAverageLengthLength

8.5 m8.5 m

33.8 m33.8 m

76.5 m76.5 m

136 m136 m

212 m212 m

AverageAverageperiodperiod

3.0 sec3.0 sec

5.7 sec5.7 sec

8.6 sec8.6 sec

11.4 sec11.4 sec

14.3 sec14.3 sec

Wave SizeWave Size

Conditions conducive of a fully developed seaConditions conducive of a fully developed sea

20 50 100 200 500 1000

Rel

ativ

e w

ave

ener

gy

Wavelength (m)

75 km/hr

55 km/hr

37 km/hr

Wave energy versus wavelength for fully developed sea: Stronger winds generate waves that are both longer and more energetic, on average

http://www.oceanweather.com/data/global.html