wind waves…. presented by: saira hashmi nov 18 th, 2005 eps 131 introduction to physical...
TRANSCRIPT
Wind Waves….
Presented by:
Saira Hashmi
Nov 18th, 2005
EPS 131Introduction to Physical Oceanography
Overview
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
Classificatio
n
Water d
epth
Gen
eration
M
etho
d
Perio
d o
f W
aves
Deep
Water
waves
Sh
allow
Water
waves
Win
d w
aves
Imp
act waves
(Tsu
nam
is)
Tid
es(G
ravity of
mo
on
, Su
n)
Seich
es(S
tand
ing
W
aves
capillary w
aves p
eriod
s < 0.1 sec
gravity w
aves b
/w 1 &
30 s
lon
g w
aves p
eriod
s > 5 m
in
WaveWave
ClassificationClassification
|_______________________________|________________________________|________________________
0 < λ < 2h < λ < 20h < λ
deep water waves transitional waves shallow water waves
(short waves ) ( long waves) Wind Wave Generation
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What are wind waves in the oceans?Waves are periodic oscillations in the water surface
Generating force: wind
- Deep Water Waves (from Classification)
- 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:
Speed
Duration of the wind
Length of the fetch (distance over which wind blows)
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 (T) 5 - 20 sec
Velocity (speed) of Individual Wave (C) 30 -100 km/hr
depth of orbital motion (wave action) = 1/2 L = 25m
Relationship between Parameters
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: Waves with long periods and wavelengths have higher
speeds
escape more rapidly from the regions where they are created which is "dispersion.“
Waves tends to sort themselves out by T and L as they are transmitted across the oceans.
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
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,
the top of the wave advances more rapidly than the deep portion because of friction with the sea floor.
Waves become unstable and break when H/L > 1/7.
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 occurs whenever waves advance on a coast at an oblique angle (not parallel to the coast). 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.
Refraction of waves…..
Longshore transport . .
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/mathematics/proj2-7.gif&imgrefurl=http://www.scitech.technion.ac.il/projects/mathematics/mathematics.html&h=529&w=674&sz=2510&tbnid=lg7u-8xHyL0J:&tbnh=
References http://www.es.flinders.edu.au/~mattom/IntroOc/lecture09.html
http://www.seafriends.org.nz/oceano/waves.htm#wind
http://electron4.phys.utk.edu/141/dec8/December%208.htm
http://oceanworld.tamu.edu/resources/ocng_textbook/chapter16/chapter16_04.htm
http://www.es.flinders.edu.au/~mattom/IntroOc/lecture09.html
http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10/part1.html
http://seaboard.ndbc.noaa.gov/educate/pacwave.shtml
http://www.phys.unt.edu/~klittler/demo_room/demo_concepts.html#Waves
http://www.phys.unt.edu/~klittler/demo_room/demo_concepts.html#Waves_
Questions