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

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ClassificationClassification

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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