CHAPTER 8 Waves and Water Dynamics

1. CHAPTER 8Waves and Water Dynamics

2. Waves are visual proof of the transmission of energy across the ocean

3. Origin of waves • Most waves are wind-driven • Moving energy along ocean/air interface • Wind main disturbing force • Boundary between and within fluids with different densities • Air/ocean interface (ocean waves) • Air/air interface (atmospheric waves) • Water/water interface (internal waves) – movement of water of different densities Atmospheric Kelvin-Helmholtz waves are caused when a certain type of cloud moving horizontally one way interacts with a stream of air moving horizontally at a different speed. Eddies develop, making beautiful, unusual, curling waves of cloud. http://www.siskiyous.edu/shasta/map/mp/bswav.jpg

4. Internal waves • Associated with pycnocline • Larger than surface waves – up to 100 m • Caused by tides, turbidity currents, winds, ships • Possible hazard for submarines Fig. 8.1a Internal waves (wavelength about 2 km) which seem to move from theAtlantic ocean to the Mediterranean Sea, at the east of Gibraltar and Ceuta http://envisat.esa.int/instruments/images/gibraltar_int_wave.gif

5. Other types of waves • Splash wave • Coastal landslides, calving icebergs • Seismic sea wave or tsunami • Sea floor movement • Tides • Gravitational attraction among Moon, Sun, and Earth • Wake • Ships

6. Wave motion • Waves transmit energy by oscillating particles • Cyclic motion of particles in ocean • Particles may move • Up and down • Back and forth • Around and around • Particles in ocean waves move in orbital paths

7. Progressive waves • Waves that travel without breaking • Types • Longitudinal– push/pull waves in direction of energy transmission • sound • Transverse– back and forth motion • Only in solids • Orbital • Combination of longitudinal and transverse • around and around motion at interface of two fluids

8. Orbital or interface waves • Waves on ocean surface at water/air interface • Crest, trough, wave height (H) • Wavelength (L)

9. Orbital waves • Wave characteristics • Wave steepness = ratio of wave height to wave length H/L • If wave steepness > 1/7, wave breaks • Wave period(T) = time for one wavelength to pass fixed point • Wave frequency= # of wave crests passing fixed location per unit of time, inverse of period or 1/T

10. Circular orbital motion • Water particles move in circle • Movement up and down and • Back and forth

11. Orbital motion • Diameter of orbital motion decreases with depth of water • Wave base= ½ L • Hardly anymotion below wave base due to wave activity

12. Types of Waves dependent on interaction with bottom

13. Deep-water waves • No interference with ocean bottom • Water depth is greater than wave base ( > 1/2L) • Wave speed (celerity) proportional to wavelength • Longer the wave, the faster it travels

14. Shallow-water wave • Water depth is < 1/20L • Wave “feels” bottom, because water is shallower than wave base • Orbits are compressed  elliptical • Celerity proportional to depth of water • The deeper the water, the faster the wave travels

15. Transitional waves • Characteristics of both deep and shallow-water waves • Celerity depends on both water depth and wavelength

16. Wave development • Most ocean waves wind-generated • Capillary waves(ripples) formed first • Rounded crests, very small wavelengths • Provide “grip” for the wind • Increasing energy results in gravity waves • Symmetrical waves with longer wavelengths

17. Wave energy • Factors that control wave energy • Wind speed • Wind duration • Fetch– distance of uninterrupted winds

18. Maximum wave height caused by wind that is known: • Reliable measurement • Measured on US Navy tanker caught in typhoon • Wave height 34 m or 112 ft Fig. 8.10

19. Wave energy • Fully developed sea • Maximum wave height, wavelength for particular fetch, speed, and duration of winds at equilibrium conditions • Swell • Uniform, symmetrical waves that travel outward from storm area • Long, rounded crests • Transport energy long distances http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10

20. Swell • Longer wavelength waves travel faster and outdistance other waves • Wave train = group of waves with similar characteristics • Sorting of waves by their wavelengths is wave dispersion • Wave train speed is ½ speed of individual wave

21. Wave interference patterns • Different swells coming together • Constructive interference • In-phase wave trains with about the same wavelengths • Add to wave height • Rogue waves– unusually large waves • Rare but can happen and be unusually large

22. http://www.ethnomusic.ucla.edu/courses/ESM172a Wave interference patterns • Destructive interference • Out-of-phase wave trains with about the same wavelengths • At least partially cancel out waves • Mixed interference • Two swells with different wavelengths and different wave heights

23. Wave height is extremely variable • ~50% of all waves are less than 2 m (6-7 ft) • 10-15% are greater than 6 m • Up to 15 m in Atlantic and Indian oceans • Up to 34 m in Pacific - long fetch (speed at 102 km/hr)

25. Largestrogue wave can sink largest vessels • Largest = 34 m (120 ft) high (above theoretical max) • 1:1200 over 3x average height; • 1:300000 over 4x height • Waves hitting current may double height suddenly and break • Most common near strong currents, long fetches, storms Rogue waves that rise as high as 10-story buildings and can sink large ships are far more common than previously thought, imagery from European Space Agency satellites has shown. A rogue wave is seen in this rare 1980 photo taken aboard a supertanker during a storm near Durban, South Africa. (Reuters) http://www.allhatnocattle.net

26. Storm surges • Large wave moving with a storm (not just hurricanes) • Low pressure above water  water level rises at center • Up to 3-4 m higher than normal • Preceded by low sea-level in front of storm • Added to increased wind waves + high tide  most damage

27. Hurricane Katrina – 2005Record storm surge in Pass Christian, MS - ~27.8 ft

28. Waves approach shore • Deep-water swell waves shoal  • Transitional waves  • Become shallow-water waves (< L/2) • Wave base “touches” sea bottom

29. Waves approach shore • During transition to shallow-water waves • Wave speed and wavelength decreases • Wave height and steepness increases • Waves break • Period remains constant

30. Breakers in surf zone • Different types of breakers associated with different slope of sea floor • Spilling • Plunging • Surging http:// www.mikeladle.com

31. Spilling breaker • Water slides down front slope of wave • Gently sloping sea floor • Wind “onshore” • Wave energy expended over longer distance Wind Onshore http://www.winona.edu/geology/oceanography

32. Plunging breaker • Curling crest • Moderately steep sea floor • Wind “offshore” • Wave energy expended over shorter distance • Best for surfers Wind Offshore http://www.seagrant.umn.edu/seiche/2002

33. Surging breaker • Breakers on shore • Steepest sea floor • Energy spread over shortest distance • Challenging for surfers http://www.bbc.co.uk/wales/surfing/images/ecards/400_232/hawaii/sandy_beach_bridgey.jpg

34. Wave 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

35. Sets – series from relative calm to largest waves • Interference in wave train cancel some, adds to others • Destructive interference  lull “between sets”

36. Rip currents are wave energy escaping shoreline • Stream of water returning out to sea through surf zone • Flows up to a few hundred meters offshore then dissipates http://www.ocean.udel.edu/mas/wcarey http://www.ripcurrents.noaa.gov/overview.shtml

37. Wave energy distribution at shoreline • Energy focused on headland • Headland eroded • Energy dissipated in bay • Bay filled up with sediment Fig. 8.17b

38. Tsunami or seismic sea wave • Sudden changes in sea floor caused by • Earthquakes, submarine landslides, volcanic eruptions • Long wavelengths ( > 200 km or 125 m) • Shallow-water wave characteristics (<L/2)

39. Speed proportional to water depth so very fast in open ocean • Not steep when generated (low H/L ratio) • Crest of only 1-2 ft over 16 min period • Move very fast -- up to 212 m/sec (470 mile/hr)

40. As crest arrives on shore, slows but grows in height quickly • Sea level can rise up to 40 m (131 ft) when tsunami reaches shore • Fast, onrushing flood of water rather than a huge breaker • Series of waves • Warning initial rushing out of water from shore

41. Tsunami or seismic sea wave • Most occur in Pacific Ocean (more earthquakes and volcanic eruptions) • Damaging to coastal areas • Loss of human lives • Krakatau eruption (1883) in Indonesia created tsunami that killed more than 36,000 people • Aura, Japan (1703) tsunami killed 100,000 people • Indonesia (Dec. 26, 2004) tsunami killed over 229,000 around Indian Ocean

42. Speed of tsunami Undersea earth-quake at 6:59 AM

43. Scale of tsunami damage on Sumatran coast in Aceh province Landsat image before tsunami: 13-Dec. 2004 ** Note sediment covered area impacted by tsunami 1-5 km inshore Landsat image after tsunami: 29-Dec. 2004 www.jpl.nasa.gov/news

44. Tsunami watches and warnings • Pacific Tsunami Warning Center • Seismic waves forecast possible tsunami • Issues tsunami watches and warnings • Increasing damage to property as more infrastructure constructed near shore • Evacuate people from coastal areas and send ships from harbors • Water “sucked” out before first http://www.drgeorgepc.com/tsuStationsTravelChart.jpg

45. 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 • Offshore power plants?

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