Chapter 10 Waves Chapter 11 Tides

1. Oceanography An Invitation to Marine Science, 7th Tom Garrison Chapter 10 Waves Chapter 11 Tides

2. 10.1: Anatomy of a Wave • A wave is the transmission of energy through matter. • Key word is “through.” • When energy moves through matter as a wave, matter moves, but overall doesn’t shift forward or backward. It transmits the energy to adjacent matter, allowing the energy to continue.

3. 10.1: Ocean Waves Move Energy across the Sea Surface • Waves: caused by the movement of energy through some medium (solid, liquid, or gas) • Ocean waves are visual proof of the transmission of energy across the surface of the ocean. • Energy moves, not the water • Ex: seagull on a wave

4. 10.1: Ocean Waves Move Energy across the Sea Surface • Progressive waves: waves of moving energy in which the wave form moves in one direction along the surface of the transmission medium.

5. 10.1: Ocean Waves Move Energy across the Sea Surface • 3 types of progressive waves: • Longitudinal waves: move through all states of matter, occur when matter moves in the same direction that the energy travels. • Example: Slinky, sound waves

6. 10.1: Ocean Waves Move Energy across the Sea Surface • 3 types of progressive waves: 2. Transverse waves only transmit through solids, occur when the energy motion is perpendicular to the travel direction. • Example: Rope

7. 10.1: Ocean Waves Move Energy across the Sea Surface • 3 types of progressive waves: 3. Orbital waves only transmit through fluids, occur when the energy moves the fluid in a circular motion as it passes. • Example: Ocean waves Orbital waves are waves in which the particles of water move in closed circles as the wave passes.

8. 10.1: Anatomy of a Wave • Crest – highest point above average water level. • Trough – lowest point below average water level. • Height (H) – vertical measurement from trough to crest. • Wavelength (L) – horizontal distance between the identical point of two waves. • Period (T) – time it takes for the same spot on two waves to pass a single point. • Frequency – the number of waves that pass a fixed point in one second. • Speed = wavelength / period • This is sometimes abbreviated: S = L / T

9. 10.1 Concept Questions • Explain the movement of energy in a wave. • Define a longitudinal wave and give an example. • Define a transverse wave and give an example. • Define an orbital wave and give an example. • Draw a typical wave and label the crest, trough, wavelength, and height. • Explain the difference between the frequency and the period of a wave. • How do you calculate wave speed?

10. 10.2: Waves are Classified by their Physical Characteristics • Waves are classified by the force that creates them and the force that tries to flatten them. • Disturbing forces: energy that cause waves • Wind, gravity, seismic activity, and landslides. • Restoring forces: energy that returns the surface to being flat • Surface tension, cohesion, and gravity.

11. 10.2: Wavelength Is the Most Useful Measure of Wave Size Waves transmit energy across the ocean’s surface. Wave energy in the ocean as a function of the wave period. As the graph shows, most wave energy is typically concentrated in wind waves. However, large tsunami, rare events in the ocean, can transmit more energy than all wind waves for a brief time. Tides are waves – their energy is concentrated at periods of 12 and 24 hours.

12. 10.3: Wave Behavior Is Influenced by Water Depth • Classification depends on their wavelength relative to the depth of water through which they are passing. • Deep-water waves: occur in water that is deeper than half their wavelength • The bottom of the ocean does not affect their orbital motion

13. 10.3: Wave Behavior Is Influenced by Water Depth 2. Shallow-water waves: occur in water that has a depth of 1/20 of the wavelength • The bottom creates drag that affects their orbital motion.

14. 10.3: Wave Behavior Is Influenced by Water Depth

15. 10.4: Many Factors Influence Wind Wave Development • What factors affect wind wave development? • Wind strength - wind must be moving faster than the wave crests for energy transfer to continue • Wind duration - winds that blow for a short time will not generate large waves • Fetch - the uninterrupted distance over which the wind blows without changing direction

16. 10.4: Many Factors Influence Wind Wave Development • Wave size increases with increased wind speed, duration, and fetch. • A strong wind must blow continuously in one direction for nearly three days for the largest waves to develop fully.

17. 10.4: Many Factors Influence Wind Wave Development • Global wave height acquired by a radar altimeter aboard the TOPEX/Poseidon satellite in October 1992. • In this image, the highest waves occur in the southern ocean, where waves were more than 6 meters (19.8 feet) high (represented in white). • The lowest waves (indicated by dark blue) are found in the tropical and subtropical ocean, where wind speed is lowest.

18. 10.2 – 10.4 Concept Questions • Explain a disturbing force and give two examples. • Explain a restoring force and give two examples. • What is the most useful measure of wave size? • Explain the difference between a shallow water wave and a deep water wave. • Explain the three factors that contribute to maximum wave size. • Where are wave heights the smallest? Why? • Where are wave heights the highest? Why?

19. 10.5: Interference Produces Irregular Wave Motions • What happens when waves from different storm systems exist simultaneously? • When waves meet, they interfere with one another. • Wave interference can be: • Destructive interference – two waves that cancel each other out, resulting in reduced or no wave • Constructive interference – additive interference that results in waves larger than the original waves • Rogue waves - these freak waves occur due to interference and result in a wave crest higher than the theoretical maximum

20. 10.5: Interference Produces Irregular Wave Motions (a) Two overlapping waves of different wavelength are shown, one in blue and one in red. Note that the wave show in blue has a slightly longer wavelength. (b) If both are present in the ocean at the same time, they will interfere with each other to form a composite wave. At the position of line 1, the two waves in (a) will constructively interfere to form very large crests and troughs, as shown in (b). At the position of line 2, the two waves will destructively interfere, and the crests and troughs will be very small (again shown in b).

21. 10.5: Interference Produces Irregular Wave Motions

22. 10.6: Deep-Water Waves Change to Shallow-Water Waves As They Approach Shore • In deep water, a wave breaks when its height exceeds one-seventh of its wavelength – H:L ratio exceeds 1:7 (depth is 1.3 times the height). • Due to drag from the seafloor, the bottom of the wave slows so that the top of the wave is traveling faster than the bottom. This, and exceeding the 1:7 ratio, makes the wave break, toppling the upper part of the wave forward.

23. 10.6: Deep-Water Waves Change to Shallow-Water Waves As They Approach Shore (1) The swell “feels” bottom when the water is shallower than half the wavelength. (2) The wave crests become peaked because the wave’s energy is packed into less water depth. (3) Constraint of circular wave motion by interaction with the ocean floor slows the wave, while waves behind it maintain their original rate. (4) The wave approaches the critical 1:7 ratio of a wave height to wavelength. (5) The wave breaks when the ratio of wave height to water depth is about 3:4. The movement of water particles is shown in red. Note the transition from a deep-water wave to a shallow-water wave.

24. 10.5-10.6 Concept Questions • Explain destructive interference. • Explain constructive interference. • Explain how a rogue wave is created. • Explain the process of how a wave breaks. Be sure to include the critical ratio.

25. 10.9: Destructive Waves - Storm Surges • Storm Surges: • An abrupt bulge of water driven on shore by a tropical cyclone or a frontal storm. • Short-lived. • Consist of only a crest, so they cannot be assigned a period or wavelength, and cannot be called a wave. • Storm surge + normal tide = storm tides.

26. 10.9: Storm Surges Form beneath Strong Cyclonic Storms A storm surge. (a) The low pressure and high winds generated within a hurricane can produce a storm surge up to 9 meters (30 feet) high.

27. 10.9: Storm Surge –Hurricane Katrina, Alabama, 2005

28. 10.9 Concept Questions • What is storm surge and where does it occur? • Explain the concept of storm tide. • How is storm surge created?

29. “The Science Behind Superstorm Sandy’s Crippling Storm Surge” • Atlantic City, NJ • Fort Lauderdale, FL

30. “The Science Behind Superstorm Sandy’s Crippling Storm Surge” • Possible solutions? • Thames River, London • Tidal barriers • Read article and answer all 11 questions in complete sentences using evidence from the article. • Due by the end of class

31. 10.11: Destructive Waves - Tsunamis • Tsunami: • Long-wavelength, shallow-water, progressive waves • Caused by the rapid displacement of ocean water. • Earthquakes that generate seismic waves • Landslides • Icebergs falling from glaciers • Volcanic eruptions • Asteroid impacts • Other direct • displacements • of the water • surface

32. 10.11: Tsunamis Are Always Shallow-Water Waves • Tsunamis are always shallow-water waves. • Their wavelength is about 120 miles, there is no ocean deep enough to make a tsunami behave as a deep-water wave. • Tsunamis have very long periods and they are so huge that they are nearly undetectable as they travel. • Boats at sea may rise and fall several meters as a tsunami passes, but cannot detect it had passed under them.

33. Figure 10.29 A computer simulation of the movement of a 1960 tsunami that originated in western Chile and sent destructive waves to Japan. The images represent the successive positions of the waves. (Source: Research by Philip L. F. Liu, Seung Nam Seo, and Sung Bum Yoon, and Civil and Environmental Engineer-ing, Cornell University. Visualization by Cornell Center for Advanced Computing. Used by permission.)

34. Figure 10.29 A computer simulation of the movement of a 1960 tsunami that originated in western Chile and sent destructive waves to Japan. The images represent the successive positions of the waves. (Source: Research by Philip L. F. Liu, Seung Nam Seo, and Sung Bum Yoon, and Civil and Environmental Engineer-ing, Cornell University. Visualization by Cornell Center for Advanced Computing. Used by permission.)

35. Figure 10.29 A computer simulation of the movement of a 1960 tsunami that originated in western Chile and sent destructive waves to Japan. The images represent the successive positions of the waves. (Source: Research by Philip L. F. Liu, Seung Nam Seo, and Sung Bum Yoon, and Civil and Environmental Engineer-ing, Cornell University. Visualization by Cornell Center for Advanced Computing. Used by permission.)

36. Figure 10.29 A computer simulation of the movement of a 1960 tsunami that originated in western Chile and sent destructive waves to Japan. The images represent the successive positions of the waves. (Source: Research by Philip L. F. Liu, Seung Nam Seo, and Sung Bum Yoon, and Civil and Environmental Engineer-ing, Cornell University. Visualization by Cornell Center for Advanced Computing. Used by permission.)

37. Figure 10.29 A computer simulation of the movement of a 1960 tsunami that originated in western Chile and sent destructive waves to Japan. The images represent the successive positions of the waves. (Source: Research by Philip L. F. Liu, Seung Nam Seo, and Sung Bum Yoon, and Civil and Environmental Engineer-ing, Cornell University. Visualization by Cornell Center for Advanced Computing. Used by permission.)

38. 10.11: Tsunami Have a Long and Destructive History Eleven destructive tsunami have claimed more than 180,000 lives from 1990-2004.

39. 10.11: Tsunami Move at High Speed • The great Indian Ocean tsunami of 26 December 2004 began when a rupture along a plate junction lifted the sea surface above. • The wave moved outward at speeds of 212 meters per second (472 miles per hour). • At this speed, it took only about 15 minutes to reach the nearest Sumatran coast and 28 minutes to travel to the city of Banda Ache.

40. Figure 10.30 Each concentric circle in this figure represents a travel time of 30 minutes for the 2004 Indian Ocean tsunami. The scale at the right indicates the arrival times in hours.

41. Figure 10.32 The regional Indonesian capital of Banda Aceh before (a) and after (b) the 26 December 2004 Indian Ocean tsunami. Waves 12 meters (40 feet) high overwashed the peninsula, moved the coastline, and killed thousands of people in moments.

42. Figure 10.33 Maximum calculated open-ocean wave height for the 2004 Indian Ocean tsunami. The scale at the right indicates height in centimeters. Remember that the open-ocean height of a tsunami is much less than the near- or onshore height of the waves. Note that waves were sensed on the Pacific and Atlantic coasts of North and South America.

43. Figure 10.36 A pressure sensor on the seabed detects subtle pressure changes of a rise and fall in sea level from the passage of a tsunami. The sensor transmits a signal to this fl oating buoy that relays the warning by satellite. The information is analyzed at the Pacific Tsunami Warning Center. Similar systems are now deployed in the Indian Ocean and central Atlantic.

44. 10.11 Concept Questions • Explain the underlying cause of a tsunami. • Explain at least 3 possible events that can cause what you explained in the first question. • Explain why a tsunami is a shallow-water wave and why it will always be classified as one. • How are they improving tsunami detection in order to save lives?

45. 11.1: The Cause of Tides • Tides are daily variations in the ocean’s level. • They result from the gravitational pull of the • Moon • And to a lesser degree, the sun. • Besides lunar and solar gravity, the imperfect sphere of the Earth, the season, the shape of the ocean basin and the Coriolis effect, all influence the tides.

46. 11.1: The Cause of Tides • The sun and moon pull ocean water into a huge wave with a wavelength the size of the ocean basin. • In principle, the sun and moon create two bulges on opposite sides of the Earth. • As the positions of the sun and moon change slowly, so the bulge rotates around the Earth. • As a coastline rotates into the bulge, the tide rises. • As it rotates out, the tide falls.

47. 11.2: The Sun, Moon, and Types of Tide • The influence of the moon on tides is about twice the influence of the sun. • When there’s a new moon (no moon visible), both the sun and moon are aligned on the same side of the Earth. • During a full moon, the sun and moon are aligned on opposite sides of the Earth. • Both positions create the highest and lowest tides, called spring tides.

48. 11.2: The Sun, Moon, and Types of Tide • When the moon is in a quarter moon phase, it and the sun are at a right angle to the Earth. • The sun’s gravitation pulls to the side of the moon’s tidal bulge. • This tends to raise the low tide and lower the high tide. • These weaker tides are called neap tides.

49. 11.2: The Sun, Moon, and Types of Tide