Waves in the Ocean

1. Waves in the Ocean

2. Waves are the undulatory motion of a water surface. • Parts of a wave are, Wave crest,Wave trough, Wave height (H), Wave Amplitude, Wave length (L),and Wave period (T). • Wave period provides a basis for the wave classifications: Capillary waves, Chop, Swell, Tsunamis, Seiches.

3. Sketch of a harmonic or ideal (sinosoidal) wave. H is exaggerated against l for clarity. The narrow vertical bar gives the wave amplitude A (the distance between mean water level and wave crest); the heavy vertical bar gives the wave height H (the difference between wave trough and wave crest, or twice the wave amplitude).

4. Most of the waves present on the ocean’s surface are wind-generated waves. • Size and type of wind-generated waves are controlled by: Wind velocity, Wind duration, Fetch, and Original state of sea surface. • As wind velocity increases wave length, period and height increase, but only if wind duration and fetch are sufficient. • Fully developed sea is when the waves generated by the wind are as large as they can be under current conditions of wind velocity and fetch. • Significant wave height is the average wave height of the highest 1/3 of the waves present and is a good indicator of potential for wave damage.

9. Progressive waves are waves that move forward across the surface. • As waves pass, wave form and wave energy move rapidly forward, not the water. • Water molecules move in an orbital motion as the wave passes. • Diameter of orbit increases with increasing wave size and decreases with decreasing water depth. • Wave base is the depth to which a wave can move water. • If the water is deeper than wave base, orbits are circular and there is no interaction between the bottom and the wave, but if the water is shallower than wave base, orbits are elliptical and become increasingly flattened towards the bottom.

10. 7-2 Wave Motions • There are three types of waves defined by water depth: Deep-water wave, Intermediate-water wave, and Shallow-water wave. • Celerity (speed) is the velocity of the wave form, not the water, C= L/T.

13. Particle movement in deep and shallow water waves The diagrams use vertical exaggeration for clarity. First diagram: deep water waves. Second diagram: shallow water waves. In deep water waves particles move on circles, in shallow water waves particles move on very flat ellipses. Particle movement decreases rapidly (exponentially) with depth in deep water waves but remains essentially the same over the entire water depth in shallow water waves. In both cases, particles under wave crests move in the direction of wave propagation; particles under wave troughs move against the direction of wave propagation.

16. 7-3 Life History of Ocean Waves Fetch is the area of contact between the wind and the water and is where wind-generated waves begin. • Seas is the term applied when the fetch has a chaotic jumble of new waves. • Waves continue to grow until the sea is fully developed or becomes limited by fetch restriction or wind duration. • Wave interference is the momentary interaction between waves as they pass through each other. Wave interference can be constructive or destructive. • Because celerity increases as wave length increases, longer waves travel faster than short waves.

17. Great Lakes Open Ocean

18. Sketch of the relative amounts of energy as a function of wave frequency in ocean waves. The top line (green) gives the classification based on period, the line below (gold) the classification based on the wave-generating force, and the bottom line (blue) the classification based on the restoring force.

20. 7-3 Life History of Ocean Waves The shallower the water, the greater the interaction between the wave and the bottom alters the wave properties, eventually causing the wave to collapse. • Celerity decreases as depth decreases. • Wave length decreases as depth decreases. • Wave height increases as depth decreases. • Troughs become flattened and wave profile becomes extremely asymmetrical. • Period remains unchanged. Period is a fundamental property of a wave • Refraction is the bending of a wave into an area where it travels more slowly.

21. 7-3 Life History of Ocean Waves Wave steepness (stability) is a ratio of wave height divided by wave length (= H/L). • In shallow water, wave height increases and wave length decreases. • When H/L is larger than or equals 1/7 (H/L  1/7), the wave becomes unstable. • There are three types of breakers:, Spilling breakers, Plunging breakers, and Surging breakers.

27. 7-3 Life History of Ocean Waves Storm surge is the rise in sea level resulting from low atmospheric pressure associated with storms and the accumulation of water driven shoreward by the winds. • Water is deeper at the shore area, allowing waves to progress farther inland. • Storm surge is especially severe when superimposed upon a high tide.

31. Refraction along California

32. Gibraltar

36. 7-4 Standing Waves Standing waves or seiches consist of a water surface “seesawing” back and forth. • A node is an imaginary line across the surface which experiences no change in elevation as the standing wave oscillates. It is the line about which the surface oscillates. • Antinodes are where there is the maximum displacement of the surface as it oscillates and are usually located at the edge of the basin. • Geometry of the basin controls the period of the standing wave. A basin can be closed or open. • Standing waves can be generated by storm surges.

37. 7-4 Standing Waves • Resonance amplifies the displacement at the nodes and occurs when the period of the basin is similar to the period of the force producing the standing wave.

38. A first order seiche. The undisturbed sea level is indicated by the broken yellow line. Three water particles are shown as an indication of water movement in the seiche. Note the node in the centre and that water under the node moves only horizontally, while water at both ends of the basin moves vertically.

39. A first order seiche in a basin open to the deep ocean. The undisturbed sea level is indicated by the broken yellow line. Two water particles are shown as an indication of water movement in the seiche. Note the node at the connection of the basin to the deep ocean, and that water under the node moves only horizontally, while water at the end of the basin moves vertically

40. 7-5 Other Types of Progressive Waves Internal waves form within the water column on the pycnocline. • Because of the small density difference between the water masses above and below the pycnocline, wave properties are different compared to surface waves. • Internal waves display all the properties of surface progressive waves including reflection, refraction, interference, breaking, etc. • Any disturbance to the pycnocline can generate internal waves, including: Flow of water related to the tides., Flow of water masses past each other, Storms, or Submarine landslides.

41. An internal wave propagating on the interface between two layers. The undisturbed sea level is indicated by the yellow line. Water particles are shown as yellow and magenta dots. Yellow dots sit in the middle of the water column and move only up and down. Magenta dots sit at the top and bottom of the water column and move only in the horizontal. By watching a yellow dot you can see how a water particle in the middle of the water column moves up and down, but does not move horizontally, as the wave passes through.

43. An internal wave traveling up the Derwent River estuary in Hobart, Tasmania. The effect of the wave is visible by streaks of smooth water produced by the convergences above the wave troughs The most common internal waves are of tidal period and manifest themselves in a periodic lifting and sinking of the seasonal and permanent thermocline at tidal rythm. In some ocean regions their surface expressions, produced by convergence over the wave troughs, is visible in satellite images

44. Internal Waves - Indian Ocean

45. 7-5 Other Types of Progressive Waves Tsunamis were previously called tidal waves, but are unrelated to tides. • Tsunamis consist of a series of long-period waves characterized by very long wave length (up to 100 km) and high speed (up to 760 km/hr) in the deep ocean. • Because of their large wave length, tsunamis are shallow-water to intermediate-water waves as they travel across the ocean basin. • They only become a danger when reaching coastal areas where wave height can reach 10 m. • Tsunamis originate from earthquakes, volcanic explosions, or submarine landslides.