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... and the tide rises,

The Tides. Fig. 8-2b. ... and the tide rises,. and the tide falls. et cetera, et cetera, et cetera. And the Tides are. Slow, up and down movement s of sea level Once or Twice a day. And the Tides are not … ocean waves, “tsunamis” or rip tides. Topics for Today.

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... and the tide rises,

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  1. The Tides Fig. 8-2b ... and the tide rises, and the tide falls ... et cetera, et cetera, et cetera

  2. And the Tides are ... Slow, up and down movements of sea level Once or Twice a day And the Tides are not … oceanwaves,“tsunamis”or rip tides

  3. Topics for Today • Tides are caused by the pull of the sun and the moon • Two highs and two lows a “day” in most places • Open ocean: tides are simple and single waves that stretch across the entire ocean • Near coastline: tides are greatly altered by bottom topography • Predictions are computed for particular sites along coast

  4. Tidal Characteristics • Tidal Range - vertical distance between high and low tides (crest-trough) • Wave period - time between high tides • Tides are waves of very long period and a tremendous amount of energy • Measured – onshore using tidal pen recorders – offshore pressure sensors • Are tides deep water waves or shallow water waves?

  5. Tidal Periods • Diurnal- about once a day • 24 hours and 50 minutes • Semidiurnal- about twice a day • 12 hours and 25 minutes (equal magnitude) • Mixed- twice a day, but with unequal highs and lows • Springandneaptides following Moon’s phases

  6. Tide Records __________ Twice a day _____ Twice a day with variations _______ Once a day

  7. Why do the Tides Occur ? • Balance of forces as themoon orbits theearthand they Both go around thesun. • What Forces ? • Gravity, Pulls Objects Together • Centrifugal force Separates Objects

  8. Earth Sun Moon Earth-Moon and Earth-Sun Systems Gravitational Attraction andCentrifugal Force fromsun and mooncause the Tides

  9. Tide Generating Forces • Tides produced bygravitationalandcentrifugal forceof both Earth-Moon and Earth-Sun systems. • Despite the fact that the sun is 107 x more massive than the moon • The moon still dominates Tides Why? Moon is much closer to Earth (384,835 km vs. 149,758,000 km)

  10. So, Consider First Just the Earth-Moon System As Moon orbits the Earth they both rotate around the centre of mass of the earth-moon system, the‘balance point’

  11. Barycenter Centrifugal Force Gravitational Attraction Earth Moon The Earth - Moon System The Barycenter is locatednearthe earth, but not at the center.

  12. Equilibrium Model of Tides Assumptions: • Earth is 100% covered by ocean of infinite depth • No bottom and no land masses • Tides areassumed to be progressive waves • Always inequilibrium with • Gravitational attraction of Moon • Centrifugal force • Neglect Effect of thesun (for now !)

  13. Equilibrium Model • Moon’sgravitypulls on the earth, the ocean and you. • Ocean water flowstowardsthe Moon, accumulating and bulging up under it

  14. Equilibrium Model • Earth-Moon also rotate about a commoncentre of gravitycausingcentrifugalforces • Resulting in bulgeaway fromMoon

  15. Thus, we have Two Bulges As Earth rotates on its axis, the point you stand on passes beneath two bulges each 24 Hr creatingtwo tidal bulgeseach day.

  16. But Wait, There’s More Ever notice that high tide is about 50 minutesaheadeach day? Why is that? Because thelunar “day”is longer than thesolarday by about 50 minutes

  17. Moon moves 1/30 way around earth each hour 24 h / 30 = 0.8 h or about 50 min Lunar half-day is12 hours 25 min This produces thefirstHigh Tide The Lunar Day: 24h 50 min

  18. … and one more thing - • Earth’s axis is tilted28.5°to the plane of moon’s orbit (declination). • Thus, the bulges that cause the tides are also at 28.5°. • Leads tolatitudinal variationof tides: • diurnal • mixed • semi-diurnal

  19. Types of Tides

  20. Earth and Moon Ocean: all over and infinitely deep Bulges in balance with: Gravity & centrifugal forces and tilt of axis Explains: diurnal semidiurnal mixed Equilibrium Model Summary and Questions

  21. Here Comes the Problem • Similar Effects: two morefactors • In24 hours • Net tidal force of Sun ishalfthat of the Moon, thus: • Lower tidal amplitude for solar component • Amplitudes for Moon and Sun are: • different • Not alwaysin sync

  22. Why are the Solar Tidal Forces Less ? Gravitational pullprop. to: (m1m2) / r3 (Dist. Between bodies more important for Tides) Sun is 107 times moremassive but390 timesfurther away Thus, Sun’s Tidal Force is: 27,000,000 / (390)3 = 0.46 or abouthalfthat of the Moon

  23. To Sun Combined Effects of Sun and Moon are additive Spring Tide To Sun Neap Tide

  24. So, Moon & Sun effects are additive BUT • Sun’s effects will pass in and out ofphase with Moon’s effect • New and Full Moons: forcesadditive ,spring tides • First and Last Quarter Moons: forces aresubtractive:neap tides

  25. Spring-Neap Tide Cycle

  26. Spring-Neap Tidal Range

  27. When would you get the Smallest and Highest Tides ? Orbits are ellipses, not circles (29 days for moon, 365 days for sun) Depends on Earth and Moon Orbits SpringTides occur when Bodies areclose together Answer: A spring tide with moon at Perigee and sun at Perihelion Two ‘king tides’ per year - one during summer and one during winter.

  28. Summary • Spring and Neap Tides • Tilt of Earth’s axis • Declination (celestial latitude) • Inequality in bulges at any given spot • Diurnal tides at high latitudes • Mixed at mid-latitudes • Semidiurnal at low latitudes • Unequal tidal heights within a given day • What if the Moon didn’t exist?

  29. Dynamic Model of Tides • Water confined to bodies of finite depth • Tidal bulge is squashed against basin’swesternedge, flowsdownslope (pressure gradient) and to theright (Coriolis)in Northern Hemisphere • Rotary waves moveanticlockwisein Northern Hemisphere

  30. Dynamic Model of the Tides Water confined to finite basins High and Low Tides onopposite sides of basin Rotate Counter- clockwise in N.H. (due to Coriolis)

  31. Time = x Time = x + 2Hr Amphidromic Point Rotary Tidal Motions in Amphidromic(rotation about a node) Systems Cotidallines (high tide same time) vs. Corangelines (equal tidal range) Rotary Wave – has attributes of both progressive and standing wave

  32. Dynamic Model • Broad basins: • Rotary wave about amphidromic point • clock-like spokes of co-tidal lines • progressive and standing • Narrow basins: • tidal bore,

  33. Tides in Basins Gulf of St. Lawrence versus Bay of Fundy

  34. Global Amphidromic Systems Bending of the cotidal line reflects wave refraction (2 = tide 2 hours later, 6 = tide 6 hours later etc.,)

  35. Tides Near Amphidromic Point • Tides are zero at the ‘node (amphidromic point) and increase to a maximum at antinodes (located at the edge of the basin)

  36. Tides Across the Globe

  37. Tidal Resonance Like sloshing in your bathtub If the natural resonance of the embayment and the tide are in phase -greatly amplified tidalrange Most often used example is the Bay of Fundy or Severn Estuary

  38. Severn Estuary :

  39. Tidal Bore - Wall of water surging up-river Large tidal range + tapering basin + decreasing depth produces the wave Hardly noticeable 20 cm H, to 5-m in Amazon River, (20 km/h) to 7-8 m in Fu-Ch’un River, (25 km/h)

  40. Tidal Currents Sea’s rise and fall means water must move from place to place Flood currents move water landward Ebb currents move water seaward Strong near the coast, bays and inlets Rotary pattern in open ocean due to Coriolis force

  41. Tidal Currents in the Chesapeake Bay

  42. Prediction of Tides Tabled forrecordingstations Predictedfor other localities Newspapers Television and radio Marinas, bait shops Tables and calendars Web sites and programs Government and commercial

  43. Tidal Predictions - Measurement of tidal component curves, a harmonic analysis typically using 37+ cosine terms Lunar and solar components :complex astronomical tide predictions

  44. Tide Predictions and Real-Time Datahttp://www.opsd.nos.noaa.gov/

  45. Atmospheric Conditions Astronomical tide predictions versus Atmospheric Conditions Wind set-up (ordinary wind shear) Storm surge (extra-ordinary) Wind shear Low Atmospheric pressure Ekman Transport (coriolis) 1999 Storm Surge

  46. Tidal Rhythms and the Ecology of the Tides • Rocky intertidal communities and zones • Sandflats, mudflats and salt marshes • Feeding and activity rhythms of fiddler crabs are attuned to the tides... • Grunion spawning as well • Horseshoe crab spawning and egg-laying

  47. Energy from Tides • Differences intidal heightdrive generator turbines • Although some 150 sites world-wide are suitable... • Relatively few have been constructed • http://www.darvill.clara.net/altenerg/tidal.htm • http://www.energy.org.uk/EFTidal.htm

  48. Locations With Large Tidal Range Is Tidal Powerfeasibleand economic At all these Locations?

  49. French Tidal Power Station La Rance River Tidal Power Plant at St. Malo

  50. La Rance River Tidal Power Plant

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