Tsunami, Seiches and Tides

1. Tsunami, Seiches and Tides Chap 11

2. Other Waves • The waves discussed so far, seas, swell and breakers, are all types of progressive waves. • Progressive waves progress: they move from one place to another. • We have three more types of wave that doesn't progress, but just moves up and down in more or less the same place.

3. Other Waves • Internal Waves • Tsunamis • Standing Waves • Seiches

4. 1. Internal Waves • Internal waves are waves that occur not at the surface of the ocean, but within the ocean water. • These generally occur at a pycnocline, where surface waters do not mix with deep waters.

5. Because the density difference between surface water and deep water is not nearly as great as the density difference between wind and surface water, internal waves • are slower than surface waves • can have very large wave heights • have very long wave lengths

6. The large wave heights and slow movements suggest the pycnocline is actually a wavy surface that undulates slowly. • You've probably seen internal wave devices in stores that are sold as gifts for desks. • Periods for the real thing are typically minutes (5-10), whereas periods for surface waves are usually in seconds.

7. Perhaps the most significant internal wave occurs in estuaries, where a river meets the sea. • Fresh water pours into the ocean from the river, and, being less dense, spreads over the surface of the ocean as floating, freshwater. • Internal waves help mix this light, fresh water with the denser salt water beneath. • http://www.youtube.com/watch?v=dECC_L1Tkdk • http://www.youtube.com/watch?v=zHId82eT6uM

8. Rogue Waves • Waves can constructively interfere, and grow even bigger by gaining energy from ocean currents. • The result is a huge rogue wave. (p.244) • Seafarers call these 'freak waves'. • They appear to arise out of nowhere, for the winds that generated them are not nearby. • A ship cannot take a rogue wave broadside, as the wave can flip the ship upside-down (the ship 'rolls over').

10. Normally, ships head into waves, so that the bow (front) of the ship faces the oncoming crests at more-or-less a right angle. • This minimizes roll (roll causes most seasickness) and maximizes 'pitch' of the ship. • But when a rogue wave arrives, the ship facing such a wave head-on will ride to the top of the wave.

11. The middle of the ship (mid-ships) may be supported by the wave while the bow and stern hang out in space, unsupported. • The result: the ship cracks in two. • It sinks rapidly. http://www.youtube.com/watch?v=ZyA_O85X-WQ http://www.youtube.com/watch?v=szk83cONAqM

12. Review the Concepts • What is an internal wave? • Where do the most common internal wave occurs? • What causes a Rouge wave?

13. Many ships have sunk from these waves, probably including many for which we do not know the exact cause. • Note one place where this happens frequently is near the start of the Gulf Stream. • This is the area known as the "Bermuda Triangle".

14. The Bermuda Triangle is in a classic location for not only rogue waves, but giant bubbles of natural gas, called 'gas hydrates', that are released from the sediment below in random bursts. • The gas lowers the water's density, and a ship passing over such a bubble can sink like a stone.

16. There is a comparable area in the Pacific, just south of Japan where the Kuroshio Current begins. • The Japanese call this area the "Dragon's Triangle". • Sorry, sci-fi fans! These are not doors to alien contacts-just natural hells-on-earth. • http://www.youtube.com/watch?v=cwOwfO67oTg • http://www.youtube.com/watch?v=TSpqVpfFTAI

17. Tsunamis • Mistakenly called 'tidal waves', tsunamis are huge waves formed by fault movement in the ocean or underwater landslides. • The first hint we have that a tsunami will occur is when we see an earthquake has occurred within an ocean.

18. A killer tsunami struck Hawaii in April, 1946. • A fault occurred at the subduction zone of the Aleutian Trench off the coast of Alaska in the morning. • Displacement, or movement, of the ocean floor up or down causes the ocean water to slosh up into a wave.

20. This particular tsunami moved at 212 meters per second ([this is celerity] 472 miles per hour), and • Reached Hawaii 5 hours after the earthquake happened.

21. The wavelength is 150 km and the period is 15 minutes. • 1/2 a wavelength of 150 km = 75 km. • How deep is the ocean? • Is a tsunami a shallow-water or a deep-water wave in its behavior?

22. At sea, a tsunami is a small wave, barely noticed by ships. • The tsunami becomes deadly when it moves ashore. • Just as seas and swells reach the shore and wave height increases as water depth decreases, the same thing happens to a tsunami.

23. Only, the energy in a tsunami is much greater than that in a sea or swell • The waves from this tsunami in 1946 reached 35 meters! (115 feet). • More than 150 people were killed. • Japan 2011 • http://www.youtube.com/watch?v=TRDpTEjumdo • http://www.youtube.com/watch?v=4YPOK_3r8Dc

24. The absence of a warning is what places people in peril from tsunamis. • After the April Fool's tsunami of 1946, scientists from all over the world put together a warning system. • This is now linked to satellite information. • When a submarine earthquake occurs in the Pacific, alarms go out all over the Ring of Fire, warning people to get to higher ground.

25. Review the Concepts • What causes the disappearance of ships and airplanes in the Bermuda Triangle? • What kind of wave is a Tsunami? • Why a Tsunami is so destructive?

26. Standing Waves and Seiches •   Standing waves are not progressive, they just sit there and move up and down in the same place. • Standing waves are easy to make in your bathtub: just start sliding back and forth. • The water goes up in the front of the tub, then down. • When it's up in the front of the tub, it's down in the back.

27. So at one point (the front of the tub, for example), you make a wave crest (water goes up), then a wave trough (water goes down). • Once you get the wave going, you do not need to move much to keep it going, just slide a bit back and forth (front to rear and back again). • Where you are moving, the water is not going up or down much at all.

28. That point is the node of the standing wave. • The part that moves up and down the most (at the front and back of the tub) is the antinode. • A standing wave moves just like a seesaw: water doesn't move up or down at the central point (where the seesaw is attached to something), and the ends where you sit move up and down the most.

29. How do standing waves form? • Most standing waves occur in enclosed bodies of water: lakes, for example, • or in a body of water that is nearly enclosed: bays, estuaries & harbors

30. When wind blows over such a body of water, it can pile the water up at one end, then slosh back to the other. • The wave will persist for a short while and eventually die out. • http://www.youtube.com/watch?v=5J2_oM0etiY • http://www.youtube.com/watch?v=NpEevfOU4Z8

31. Tides • Tides in the eastern Bay of Fundy on the Atlantic coast of Canada. (f.11.31) • Tidal range is 15 meters (50 feet). • Water rises 1 meter (3.3 ft.) in 23 minutes.

33. The tides are the regular rising and falling of sea level. • They are caused by the gravitational attraction of the moon and the sun. • These celestial bodies pull on the Earth. • The solid Earth moves only a tad toward the moon and sun, but the liquid ocean moves more.

34. Let's look at the effect of the moon, first. • It has a much stronger effect on the tides because it is so much closer: • The pull of the moon causes the ocean to 'bulge' out away from Earth and toward the moon:

35. The bulge is less than 1 meter high. • If some water bulges up toward the moon, some water must go down elsewhere: note the water layer is thinner over the poles in the above picture, and much thicker in the bulge toward the moon.

36. The water level is also low along the equator in the direction toward you and away from you. • Where the bulge occurs is high tide; where the low points occur (toward you and away from you) is low tide. • The Earth rotates every 24 hours, so points on Earth rotate into the high tide, then into the low tide.

38. Now here comes one of the many bizarre aspects of the tides: there is also a tidal bulge on the opposite side of the planet: • How is this possible? Centrifugal force. • Swing an object attached to a string in a circle. • Your hand is pulling the object toward you, but another force must be pushing the object away from you! That force is centrifugal force (centrifugal means 'fleeing the center').

39. The tide is a standing wave: it has two crests (high tide), and two troughs (low tide). (f.11.24) • The Earth rotates once every 24 hours. • http://www.youtube.com/watch?v=zOUi-m8UyFk • http://www.youtube.com/watch?v=u3LtEF9WPt4

40. If you were to plot sea level over this 24 hour period, it would look like, f.11.25.

41. Such a tide is called semidiurnal, because there are 2 high tides and 2 low tides every day. • The difference in height between high tide and low tide is called the tidal range.

42. How high the high tide gets, and how low the low tide gets, depends on the other celestial body we have ignored up until now: the sun. • The effects of the sun are to raise high tides to a maximum, when the sun's gravity pulls in the same direction as the moon's, and to lower high tides to a minimum, when the sun's gravity pulls in opposition to the moon's.

44. Three types of tides: • Semidiurnal: two high tides and two low tides each day, both about the same height • Mixed: two high tides and two low tides each day, but one high tide is higher than the other • Diurnal: one high tide and one low tide each day.

46. Tidal Bore • In macrotidal areas, the high tide may come in as a visible wave called the tidal bore. • A tide crest encounter a narrow river passage, and it rushes forward to cause the bore (f.11.32) • http://www.youtube.com/watch?v=LWumonz87rA • http://www.youtube.com/watch?v=hOG_9W0Obu0

48. Tidal Currents • When a point on Earth passes through the tidal crest, high tide is coming, and water moves onshore. • It may move up rivers as a visible bore, or it may just rise almost imperceptibly. • Either way, the water moves onshore and this is called the flood tide or flood current.

49. At the center of the crest, water stops moving onshore and stands still for a little while. • This is a stage of the tide called slack water or slack tide. • Now the point rotates into the trough of the tide, and low tide is coming. • Water changes direction now and moves offshore, back out to sea.

50. This is called the ebb tide or ebb current. • Once the center of the trough is reached, there is no more water movement offshore and the water lingers again. • This is also called slack water or slack tide.