Ocean Waves What causes ocean waves to form and move? Generating Force – adds energy to water

1. Ocean Waves • What causes ocean waves to form and move? • Generating Force – adds energy to water • e.g. landslide, earthquake movement, WIND • Restoring Force – returns water to level • surface tension for small waves • gravity for large waves See http://artsandscience1.concordia.ca/GEOG/course_notes/g398/MY_OCEAN_8.ppt#2 see http://www.franksingleton.clara.net/seastate.html

2. Ripples or Capillary Waves restoring force = surface tension Pulses of wind blow across the water surface and friction (drag) between the air and water stretches the water surface making wrinkles which are restored to a smooth surface by surface tension. Wavelengths of approximately centimeters “Cat’s Paws” = dark patches of capillary waves at the front of a wind gust

3. Gravity Waves restoring force = gravity If wind blows steadily, as ripples form, the water surface becomes rougher and rougher, and it is easier for the wind to add energy to the “roughened” surface (think about friction). If the wind blows faster over a longer time and distance, waves become bigger and the restoring force changes from surface tension to gravity.

4. Deep Ocean waves are MOSTLY transferring ENERGY Mostly ENERGY is moved along the wave front in deep water. A very small amount of water is transferred due to the fact that the velocity at the top of the orbit is slightly faster than at the bottom. (hypothetical sine wave shown above)

5. Ocean waves are NOT perfect sine waves

6. Ocean Wave Speed Once a wave is created, the speed may change, but the period remains constant because it was determined by the generating force. Speed of a single wave, celerity, C = λf = λ/T Speed of a group of waves = ½ C = ½ λf = λ/(2T) because the front wave dissipates as a new wave forms behind it. Throw a pebble into water and observe this…

7. Deep Ocean Wave Speed When water depth > ½ λ λ = (gT2) / (2π) since g = 9.8 m/s2 λ = 1.56 T2 since C = λ / T C = 1.56 T C2 = 1.56 λ λ = wavelength in meters T = period in seconds C = single wave speed in m/s

8. Shallow Ocean Wave Speed When water depth < 1/20 λ (or λ > 20 depth) since g = 9.8 m/s2 and C = λ / T and when λ = wavelength in meters T = period in seconds C = single wave speed in m/s depth is in m C = 3.13 depth λ = 3.13 T depth

9. Deep Ocean Wave Height Depends on wind speed: faster = bigger higher waves wind duration: more time = higher waves fetch: longer length of unidirectional wind flow = higher waves All 3 of these are limiting factors – all 3 must be present to create high waves! The highest wind waves on the ocean are about 30 m (or 100 ft) high.

10. Universal Sea State Code The Universal Sea State Code is also called the Beaufort scale of sea state after Sir Francis Beaufort (1806).

11. Waves of uniform wavelength and period, known as swell, approaching the coast.

12. Episodic or Rogue Waves Ocean waves can and do demonstrate rectilinear propagation, reflection, refraction, diffraction, and interference. Sometimes random waves interfere to create unexpected, abnormally high waves call episodic or rogue waves.

13. Fig. 8.9 A giant wave breaks over the 25 m high bow of a supertanker.

14. Deep Ocean Wave Steepness Wave steepness, S = wave height / wavelength When S > 1/7 the wave “breaks” because it is too steep to support itself. (The angle of the wave crest is greater than 120° which is unstable.)

15. Progressive Wind Waves generated by wind (low pressure storm centers), restored by gravity, progress in a particular direction FORCED WAVES are found near the storm continuous energy input makes a jumbled sea surface with a variety of waves FREE WAVES have move far enough from the storm to no longer be wind-driven

16. Progressive Wind Waves DISPERSION, or sorting, is the term used to describe the process whereby faster, longer waves move ahead of slower, shorter ones in the storm center. Longer wavelengths have a greater speed and race out ahead of other waves.

17. reduction of wavelength and speed = increase in height and steepness Ocean Waves in Shallow Water Deep-water waves become intermediate waves, then shallow water waves, as depth decreases and wave motion interacts with the bottom.

18. The Surf Zone BREAKERS are waves in which waves “dragging” along the bottom slow at the bottom, but the top doesn’t slow as much, steepening the wave until it collapses onto itself as a… PLUNGER if on a steep, narrow beach wave curves over the air below and breaks with a sudden splash and loss of energy, if waves approach shore at an angle, a “tube can result SPILLER if on a wider, flatter beach turbulent water bubbles down the wave front, longer surfing ride, but not as exciting! http://video.google.com/videoplay?docid=1041053573910778617&q=wave+surf

19. Tsunamis Seismic sea waves with wavelengths of 100-200 km and periods of up to 20 minutes. http://video.google.com/videoplay?docid=-529794418106164793&q=tsunami Link to pics

20. Storm Surge The high water that comes on shore during a low pressure area making landfall – not really a wave in the traditional sense. Effects can be similar to tsunamis. http://video.google.com/videoplay?docid=-7392209151146546355&q=storm+surge&total=235&start=0&num=10&so=0&type=search&plindex=1

21. Seiches A standing wave created in a basin filled with water. Usually created by landslides or earthquakes because a long wavelength is necessary to sustain them.