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://www.franksingleton.clara.net/seastate.html

See http://artsandscience1.concordia.ca/GEOG/course_notes/g398/MY_OCEAN_8.ppt#2

Ripples or Capillary Wavesrestoring 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

Gravity Wavesrestoring 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.

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)

Ocean waves are NOT perfect sine waves

Ocean Wave SpeedOnce 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…

Deep Ocean Wave SpeedWhen water depth > ½ λ

λ = (gT2) / (2π)

since g = 9.8 m/s2

λ = 1.56 T2

since C = λ / TC = 1.56 TC2 = 1.56 λ

λ = wavelength in metersT = period in secondsC = single wave speed in m/s

Shallow Ocean Wave SpeedWhen water depth < 1/20 λ

(or λ > 20 depth)since g = 9.8 m/s2 and C = λ / T

and when λ = wavelength in metersT = period in secondsC = single wave speed in m/sdepth is in m

C = 3.13 depth

λ = 3.13 T depth

Deep Ocean Wave HeightDepends on

wind speed: faster = bigger higher waveswind duration: more time = higher wavesfetch: 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.

Universal Sea State Code

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

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

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.

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

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.)

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

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.

Deep-water waves become intermediate waves, then shallow water waves, as depth decreases and wave motion interacts with the bottom.

reduction of wavelength and speed = increase in height and steepness

Ocean Waves in Shallow Water

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

TsunamisSeismic sea waves with wavelengths of

100-200 km and periods of up to 20 minutes.

Link to pics

http://video.google.com/videoplay?docid=-529794418106164793&q=tsunami

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

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.