Presented by:
Saira Hashmi
Nov 18th, 2005
Wind Waves….
EPS 131
Introduction to Physical Oceanography
Overview
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Classification of Waves
What are wind waves in the oceans?
What determines the relationship between
the wave length and the period for wind
waves?
why do waves break?
Animations/Simulations
References
Questions
Wave
Classification
Wave
Classification
Period of
Waves
Generation
Method
Water depth
long waves
periods > 5 m
gravity waves
b/w 1 & 30 s
capillary wave
periods < 0.1 se
Seiches
(Standing
Waves
Tides
(Gravity of
moon, Sun)
Impact waves
(Tsunamis)
Wind waves
Shallow Wate
waves
Deep Water
waves
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|_______________________________|________________________________|________________________
0
<λ<
2h
<λ<
20h
<λ
deep water waves
(short waves )
transitional waves
shallow water waves
( long waves)
Wind Wave Generation
http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10/part1.html
http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10/par
t1.html
What are wind waves in the
oceans?
Waves are periodic oscillations in the water surface
Generating force: wind
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Deep Water Waves (from Classification)
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When wind blows over the ocean, surface waves are generated
by transferring some of the wind's energy, in the form of
momentum, from the air to the water
Characteristics:
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Speed
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Duration of the wind
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Length of the fetch (distance over which wind blows)
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Water depth
Waves and Winds……
http://www.seafriends.org.nz/oceano/waves.htm#environment
Parameters of wind waves
Deep Water Waves: D >> L
Wave Height (H)
1 - 15 m
Wave Length (L)
50 - 500 m
Period
5 - 20 sec
(T)
Velocity (speed) of Individual Wave (C) 30 -100 km/hr
depth of orbital motion (wave action) = 1/2 L = 25m
Relationship between
Parameters
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Wave Length , L = (g/2π)T2
(g = gravitational acceleration = 9.8 m/s2)
Velocity , C=L/T
So, C = 1.56T
T is in the units of seconds,
L is in meters,
C is in meters/sec
Dispersion:
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Waves with long periods and wavelengths have higher
speeds
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escape more rapidly from the regions where they are
created which is "dispersion.“
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Waves tends to sort themselves out by T and L as they are
transmitted across the oceans.
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Dispersion accounts for "swell" at sea -- the long, uniform
waves in the open sea that have moved far from their
source area.
Why do waves break?
As wind waves move toward the shore
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their characteristics
& direction change.
Also, encroaching waves produce currents that move
water and sediments parallel to coasts.
Changes in wave parameters . .
 T remains constant
 H increases because wave energy is confined to a
smaller area (as L decreases)
 Waves become "shallow-water" waves when D < L/20.
 C and L become entirely controlled by depth.
Why do waves break?
WAVES BREAK
As waves move further onshore,
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the top of the wave advances more rapidly than the
deep portion because of friction with the sea floor.
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Waves become unstable and break when H/L > 1/7.
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This occurs when the depth of water is
approximately equal to wave height. As waves break
and strike the coast (or run up a beach), their energy
is dissipated.
Surf (breaker) zone on a smooth, gently-sloping shore
Surf on a rocky shore
Changes in wave direction:
Refraction of waves…..
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Refraction occurs whenever waves advance on a
coast at an oblique angle (not parallel to the
coast).
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Refraction Bending of wave "fronts" (crests)
Bending of wave movement direction
Also occurs as waves move over an irregular
sea-floor topography (with shallow and deep
regions) onto an irregular coastline (with
headlands, bays).
Also,refraction results in the divergence of wave
energy from bays.
Longshore transport . .
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is the event of waves advancing on a coastline
at an oblique angle
Waves run-up a beach at an angle "swash."
But water runs back down the beach
("backswash")
The resultant "imbalance" produces a long shore
current parallel to the beach.
Longshore currents are capable of carrying
suspended sediments
3D Water Waves
http://images.google.com/imgres?imgurl=http://www.scitech.technion.ac.il/projects/mat
hematics/proj27.gif&imgrefurl=http://www.scitech.technion.ac.il/projects/mathematics/mathematics.ht
ml&h=529&w=674&sz=2510&tbnid=lg7u-8xHyL0J:&tbnh=
References
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http://www.es.flinders.edu.au/~mattom/IntroOc/lecture09.html
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http://www.seafriends.org.nz/oceano/waves.htm#wind
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http://electron4.phys.utk.edu/141/dec8/December%208.htm
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http://oceanworld.tamu.edu/resources/ocng_textbook/chapter16/chapter16_04.htm
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http://www.es.flinders.edu.au/~mattom/IntroOc/lecture09.html
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http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10/part1.html
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http://seaboard.ndbc.noaa.gov/educate/pacwave.shtml
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http://www.phys.unt.edu/~klittler/demo_room/demo_concepts.html#Waves
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http://www.phys.unt.edu/~klittler/demo_room/demo_concepts.html#Waves_
Questions