1. Waves

2. Waves
Wave – a disturbance caused by the movement of energy from a source through some medium (solid, liquid, or gas)

3. Waves Disturbing force - Energy that causes ocean waves to form
Wind is the most common force that creates waves, but they can also be due to seismic disturbances (tsunamis) and gravitational forces (tides).
Restoring force – dominant force that returns the water surface to flatness after a wave has formed in it.

4. Free waves vs. Forced waves
Free waves – a wave that forms and then continues without any further influence from the force that formed it (wind waves, tsunami)
Forced waves – a wave which is maintained by its disturbing force (tides)

5. Parts of a Wave Crest – highest point on a wave
Trough – lowest point on a wave; valley between two waves
Wavelength (L) – horizontal distance between 2 consecutive crests or troughs (measured in cm)

6. Parts of a Wave Cont.
Wave Height (H) – vertical distance between a wave crest and trough (measured in m)
Wave Amplitude – vertical distance between the still-water line and a wave crest or trough. (1/2 the wave height)

7. Diagram of a Wave

8. Diagram of a Wave

9. Wave Characteristics
Wave Period (T) – time it takes for 2 consecutive crests or troughs (one wavelength) to pass a stationary point (measured in s).
Long Period = big waves
Short Period = small waves

10. Wave Characteristics
Wave steepness (S) = measure of how high a wave can be compared to its length. S=H/L
If wave steepness is greater than 1/7, the wave breaks
Wave frequency = the number of waves passing a fixed point per second
inverse of period (1/T)
Wave speed (celerity) (C) = the distance a wave travels divided by the time it takes to travel that distance. Calculated by dividing the wavelength by the period. C=L/T

11. Waves (Continued)

12. Wave motion All waves transmit energy Particles may move
Up and down (transverse wave)
Back and forth (longitudinal wave)
Around in circles (orbital wave)

13. Wave motion
In the ocean (water waves), water particles move in a circle (cyclic motion) called an orbital path

14. Wave motion
In the ocean (water waves), water particles move in a circle (cyclic motion) called an orbital path

15. Wave Orbital
The circular motion of water particles within a wave.

16. Circular orbital motion
Water particles move in circle
Movement up and down AND Back and forth
Fig. 8.4

17. Fig. 8.3C
Orbital motion
Diameter of orbital motion decreases with depth of water, until there is no movement
Wave base = level below which there is little water movement due to waves (depth = ½ wavelength)
Hardly any motion below wave base due to wave activity

18. Wave Motion Water in a wave does not move horizontally.
The water is not moving toward shore; only the energy is
Its vertical motion is called an orbital.
Orbital motion changes shape as wave approaches shallower water.

19. Deep-water waves Water depth is greater than ½ wavelength (d/L>1/2)
Orbital shape is circular
The orbits will not touch the bottom.
Wave speed (celerity) proportional to wavelength

20. Shallow-water wave
Water depth is less than or equal to 1/20 wavelength (d/L<1/20)
Orbital shape becomes elliptical
Orbitals will touch the bottom and be compressed.
Celerity proportional to depth of water
Fig. 8.5b

21. Transitional waves
Characteristics of both deep and shallow-water waves
Water depth is greater than 1/20 but less than 1/2 of wavelength
Orbitals begin to become elliptical in shape
Celerity depends on both water depth and wavelength

22. Waves (Continued)

23. Waves approach shore Wave speed decreases (waves get slower)
Deep-water waves  Transitional waves  Shallow-water waves
Wave speed decreases (waves get slower)
Wavelength decreases (waves get closer together)
Wave height increases (waves get taller)
Wave steepness increases (waves get steeper)
Period remains constant
Waves break

24. Shoaling waves
Fig. 8.15

25. What makes a wave break?
The 1:7 ratio – when a wave’s height is 1/7th its wavelength, the wave will break.
What is the ratio of wave height to wavelength called?
Wave steepness (S) = measure of how high a wave can be compared to its length. S=H/L
If wave steepness is greater than 1/7, the wave breaks

26. How a Wave Breaks
The wave approaches the shore and becomes a shallow water wave. Its orbitals flatten to ovals.
The wave’s energy is packed into less water depth so the wave crests become peaked instead of rounded.
The wave approaches the 1:7 ratio.
The bottom of the wave slows down because of friction with the ocean floor, while the crest of the wave continues at a faster speed.

27. Breakers in surf zone
Top of wave topples over base because of decrease in wave speed due to friction with sea floor
Wave form not sustained (wave breaks)
Different types of breakers associated with different slope of sea floor

28. What determines how big a wave will become?
The strength/speed of the wind.
Wind Duration – how long the wind blows
Fetch – the distance over water that the wind blows in a single direction

29. Wave Interactions - Reflections
When waves bounce from a surface back toward the source.
Can produce a “seiche.”

30. Wave Interactions - Reflections
Seiche - A wave that sloshes back and forth in an enclosed or partially enclosed body of water.

31. Wave Interactions - Reflections
Waves and wave energy bounced back from barrier
Reflected wave can interfere with next incoming wave
Fig. 8.18

32. Wave Interactions - Refraction
When waves come into shallow water and begin to “feel” the bottom, the angle of their direction changes.

33. Wave Interactions - Refraction
As waves approach shore, they bend so wave crests are nearly parallel to shore
Wave speed proportional to depth of water (shallow-water wave)
Different segments of wave crest travel at different speeds

34. Wave Interactions - Refraction

35. Wave energy distribution at shoreline
Wave Interactions - Refraction
Wave energy distribution at shoreline
Energy focused on headland
Headland eroded
Energy dissipated in bay
Bay filled up with sediment

36. Wave Interactions - Diffraction
When waves go through a small opening, they will bend around the edge of the opening.

37. Wave Interactions – Wave Interference
When 2 waves meet, their energy combines to make either:
Constructive interference
In-phase wave trains with about the same wavelengths (waves get bigger)
Destructive interference
Out-of-phase wave trains with about the same wavelengths (waves cancel each other)
Mixed interference
Two swells with different wavelengths and different wave heights (unpredictable/mixed results)

38. Wave interference patterns
Fig. 8.13

39. Interference
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

40. A Few More Types of Waves
Standing Wave (seiche) – shallow water waves that reflect back on themselves (bays & seas).
Rogue Wave – an abnormally high wave not related to local conditions; A particularly big wave appearing in a set of smaller waves.
The result of constructive interference.
Storm Surge - a large wave of water created from and driven by the energy of a storm

41. A Few More Types of Waves
Tides – regular rise and fall of sea level due to the gravitational forces of moon & sun
Tides are shallow water waves because their wavelength is so long.
Tsunamis –very long waves created by earthquakes on the seafloor that travel at ~400 mph and build in height when they reach coastal areas.
Tsunamis are shallow water waves because their wavelength is so long.
NOT tidal waves!
March 2011 tsunami

42. Tsunami or seismic sea wave
Sudden changes in sea floor caused by seismic activity (Earthquakes, submarine landslides, volcanic eruptions)
Very long wavelengths (> 200 km or 125 m)
Shallow-water wave
Speed proportional to water depth so very fast in open ocean
Sea level can rise up to 40 m (131 ft) when tsunami reaches shore

43. Tsunami or seismic sea wave
Fig. 8.20a

44. Tsunami or seismic sea wave
Most occur in Pacific Ocean (more earthquakes and volcanic eruptions)
Damaging to coastal areas
Increasing damage to property as more infrastructure constructed near shore
Loss of human lives
Example: Aura, Japan (1703) tsunami killed 100,000 people
Example: Sumatra, Indonesia (2004) tsunami killed more than 225,000 people in eleven countries (one of the deadliest natural disasters in history)

45. Wave Classification

47. Tsunami watches and warnings
Pacific Tsunami Warning Center
Uses data from pressure sensors on bottom of ocean, seismographs, and tidal times to forecast possible tsunami
Evacuate people from coastal areas and send ships from harbors

48. Waves as a source of producing electricity
Lots of energy associated with waves
Mostly with large storm waves
How to protect power plants
How to produce power consistently
Environmental issues
Building power plants close to shore
Interfering with life and sediment movement

49. Wave power plant at Islay, Scotland
Fig. 8.25b

50. Global coastal wave energy resources
Fig. 8.26