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Wind Waves….

Wind Waves…. Presented by: Saira Hashmi Nov 18 th, 2005. EPS 131 Introduction 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?

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Wind Waves….

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  1. Wind Waves…. Presented by: Saira Hashmi Nov 18th, 2005 EPS 131Introduction to Physical Oceanography

  2. 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

  3. Wave Classification

  4. |_______________________________|________________________________|________________________ 0 < λ < 2h < λ < 20h < λ deep water waves transitional waves shallow water waves (short waves ) ( long waves) Wind Wave Generation http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10/part1.html

  5. http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10/part1.htmlhttp://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10/part1.html

  6. 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

  7. Waves and Winds…… http://www.seafriends.org.nz/oceano/waves.htm#environment

  8. 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

  9. 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

  10. 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. Dispersion:

  11. 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?

  12. 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.

  13. Surf (breaker) zone on a smooth, gently-sloping shore Surf on a rocky shore

  14. Changes in wave direction: Refraction of waves….. • 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.

  15. 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

  16. 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=

  17. 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_

  18. Questions

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