Topic 16 Waves

1. Topic 16Waves GEOL 2503 Introduction to Oceanography

2. What is a wave? • Energy moving through some medium

4. How a Wave Begins • How do we put energy into the ocean surface to form waves? • Called the generating force

5. Generating Force • Wind, for most waves • Pebble in a pond • Earthquake • Meteorite impact

6. Restoring Force • Force that causes water to return to its undisturbed level • Surface tension for very small waves (called ripples or capillary waves) • Gravity for large waves

7. Ripples (capillary waves) on top of waves

8. Most Waves of Interest • Surface waves • travel on the water surface • Gravity waves • gravity is the major restoring force • Wind waves • wind is the major generating force • Progressive waves • Moving forward through the water

9. Waves • Energy moving through a medium • Water is the medium • Sound is also energy moving through air, water, walls, etc. • Waves move through the medium, but the medium is not transported

10. Wave Terminology • Crest = highest water level • Trough = lowest water level • Height = H = distance from crest to trough • Amplitude = A = distance from still water level to crest or trough • Period = T = time (in seconds) for two successive crests to pass fixed point • Frequency = F = number of wave crests to pass a fixed point in a given amount of time

11. Direction of wave motion A B Wavelength Height Still water level Trough Crest Frequency: Number of wave crests passing point A or point B each second Orbital path of individual water molecule at water surface Period: Time required for wave crest at point A to reach point B Fig. 10-2, p. 266

12. Wave Motion • Energy moves through the water, but the water is not transported • Individual water particles move in a circular path called a wave orbit

13. 1. As wave trough passes, water particles begin to rise and move backward 2. At the midpoint of crest approach, water particles stop moving backward, continue to rise, and begin moving forward 3. Under crest, particles have stopped rising and are moving forward with speed of the crest 4. As crest passes, particles fall and stop moving forward 5. As trough advances, particles move backward 6. At bottom of trough, maximum backward speed The bird, cork, boat, or water particles have no net forward motion, they move in circular orbits Orbital Wave Motion 1 2 3 4 5 6

14. Note the ½ wavelength depth and orbital diameter

15. Wave Speed The speed of a wave is the distance traveled by its crest per unit time. It is known as celerity, thus the “C.” L (wavelength) • C = —— T (time) C = Celerity, speed at which a wave moves across the sea surface Once a wave is created, its speed may change, but its period remains the same

17. Deep-Water Waves • Waves in water deeper than one-half the wave’s length • DWWs move at a speed controlled by their wavelength • Leads to wave dispersion

18. Storm Centers • Most waves are progressive wind waves • Generated by wind • Restored by gravity • Progress in a particular direction • Formed in local storm centers or in trade wind belts

19. Sea • In the storm center, winds are variable and turbulent • Sea surface is a mixture of waves of all heights, lengths, and periods • Called a “sea” (waves in the area of generation) • Waves move outward in all directions

20. Wave Dispersion • Waves from storm center move at speeds controlled by wavelength • The greater the wavelength, the greater the speed • Faster, longer waves gradually move through and ahead of shorter, slower waves • This is called sorting, or dispersion

21. Wave Dispersion

22. Sea—waves in the area of generation

23. Swell—waves after dispersion

24. In a sea, the waves are sharp-crested, choppy, and irregular, with a mixture of wave heights, lengths, and periods

25. Swell—very regular and long-crested waves

26. Wave Train • As wave train progresses, leading waves lost • Energy used to advance wave form into undisturbed water • Therefore, speed of each individual wave (C) in the group is greater than the speed of the leading edge of the wave train

27. Progress of a wave train

28. Group Speed • Wave train moves at speed of one-half that of the individual waves • Group speed = ½ wave speed = speed of energy transport C V = —— 2

29. Wave Height • Wave height depends on • Wind speed • Wind duration • Fetch (area over which the blows)

32. Wave Terminology • Fully developed sea—when the local wind speed, duration, and fetch have transferred maximum energy, thus creating the largest possible waves for that set of conditions • Maximum wave height—largest recorded • Average wave height—average of all • Significant wave height—average height of the highest one-third of waves over long time periods

34. Wave Interaction • Waves are likely to meet other waves from other storm centers • Waves may intersect at any angle • Form interference patterns

35. 1 2 a b Constructive interference (addition) Destructive interference (subtraction) Constructive interference (addition)

36. Episodic Waves • Also called “rogue waves” • Combination of intersecting waves, changing depths, and currents • May be responsible for sudden disappearance of ships

37. Rogue Waves Usually many different storms, producing waves of different characteristics. They interfere with each other to produce small swell. When they become synchronized they can produce huge waves that can overwhelm small boats. Waves can have amplitudes of between 5 and 15 m.

38. Wave Steepness • There is a maximum possible height for any given wavelength • Ratio of wave’s height to length • S = H/L • Called wave steepness

39. When S approaches 1/7 (wave crest angle of about 120° ) the wave becomes too steep and breaks

40. Shallow-Water Waves • As deep-water waves approach shore, wave orbits interact with sea bottom • Orbits gradually become ellipses (flattened circles) • SWW speed is controlled by water depth • Leads to refraction and wave breaking

41. Video • Watch the learner.org video on Waves, Beaches and Coasts to see more about shallow water waves • http://www.learner.org/resources/series78.html

42. Wave Refraction • Refraction means bending • Waves usually approach shore at an angle • Part in shallower water moves slower • So part of the wave is in deeper water and moves faster • The wave bends or refracts • Waves end up more parallel to shoreline

43. Wave Refraction

44. Wave Refraction Figure 20-5

45. Wave refraction around a rocky point

46. Wave refraction around a rocky island

47. Wave refraction into a small bay

48. Wave Refraction

49. Wave Breaking • The surf zone is the shallow area along the coast where waves slow, steepen, break, and expend their energy • Waves in the surf zone do transport water forward

50. Deep-water waves transitioning to surf