1. Unit 8 - Tides

2. What are Tides?
Tides are the periodic raising and lowering of ocean sea level
Tides are very long shallow water waves
Caused by combination of gravity and motion between Earth, Moon, and Sun
Isaac Newton’s gravitational laws explain the tides

3. Gravitational Forces
Gravitational force is greatest at zenith – closest to moon
Gravitational force is lowest at nadir – furthest from moon and opposite zenith

4. Centripetal Forces
Centripetal force tethers the Earth and Moon to each other (keeps them in orbit)
Centripetal force is equal on all parts of the earth

5. Resultant Forces produce tides
The resultant force is the difference between gravitational and centripetal forces
It is the relatively small force that produces the tides

6. Resultant Forces
The resultant force pushes water into two simultaneous bulges
One toward Moon
One away from Moon

7. Tidal Bulges – Moon’s Effect

8. Tidal Periods Tidal period – time between high tides
Lunar day - Time between two successive overhead moons equal to 24 hours, 50 minutes
High tides are 12 hours and 25 minutes apart (2 per lunar day)

9. Tidal Bulges – Sun’s Effect
Similar to lunar bulges but much smaller (only 46% of the lunar bulge)
The moons effect on tides is greater because it is so much closer than the sun

10. Monthly Tidal Cycle Spring tides Neap tides
New or full moons when sun and moon are in alignment
Tidal range is the greatest
Syzygy – when celestial bodies are in alignment
Neap tides
Quarter moons when sun and moon are at right angles relative to the Earth.
Tidal range is the lowest
Quadrature – when celestial bodies are at right angles

11. Spring and Neap Tides

12. Complicating Factors
Declination – Angular distance of the Moon or Sun above or below Earth’s equator
Sun to Earth: degrees north or south of equator
Moon to Earth: 28.5 degrees north or south of equator
Lunar and solar bulges shift from equator
This produces unequal (mixed) tides at different spots around the world

13. Declination and Tidal Bulges

14. Effects of declination on tides

15. Elliptical Orbits add Complication
Earth around Sun:
Tidal range greatest at perihelion (January)
Tidal range least at aphelion (July)
Moon around Earth:
Tidal range greatest at perigee (Moon closest to Earth)
Tidal range least at apogee (Moon furthest from Earth)
Perigee–apogee cycle is 27.5 days

16. Relative sizes of the moon

17. Effects of Elliptical Orbits

18. Actual tides are more complex
Continents and friction with seafloor modify tidal bulges
Tides are shallow-water waves with speed determined by depth of water
Idealized tidal bulges cannot form because they cannot keep up with Earth’s rotation

19. Where do the tides start?
Tides start in the centers of the world’s oceans.
Tides act like water swirling in a bowl.
There will be a node in the center where the water level never rises or falls.

20. Amphidromic Points
Cotidal map shows tides rotate around amphidromic points.
There are 140 amphidromic points in the world’s oceans.
Amphidromic Points
Figure 9-14

21. Cotidal Lines
Cotidal lines show where high tides occur at the same time

22. “if you don’t understand amphidromic points I’m going to bite you!”
Spidey Says…
“if you don’t understand amphidromic points I’m going to bite you!”

23. Tidal circulation
Tides progress around basins, clockwise in S hemisphere and counterclockwise in N hemisphere

24. Animation of Tidal Elevations in the Pacific

25. Tidal Patterns Diurnal One high tide and one low tide per day
Semidiurnal Two equal high and low tides per day
Mixed Two unequal high and low tides per day (most common type)

26. Tidal Patterns in the U.S.

27. Tidal Graphs
Tidal graphs show all the tidal patterns for a location over time.
Spring and neap tides
Diuranal, mixed, or semidiuranal tides.

28. Flood and Ebb Currents Flood current is when the tide is coming in.
Ebb current is when the tide is going out

29. Tides in Coastal Waters
Tide waves are reflected by coastlines producing complicated effects
This can amplify the tidal range
Cook Inlet Alaska and the Bay of Fundy in Nova Scotia are examples of extreme tides caused by coastal effects

30. Cook Inlet Tides

31. The Bay of Fundy
Tidal Range is the difference between high tide and low tide. The Bay of Fundy in Canada has the greatest tidal range on the planet -56 ft.
Nova Scotia bends when the tide comes in!
As 14 billion tons of sea water flow into Minas Basin twice daily, the Nova Scotia countryside actually tilts lightly under the immense load !
Bay of Fundy
tidal bore

32. Bay of Fundy – World’s Largest Tidal Range

33. Tidal Bores
The wave on the incoming (flood) tide in certain rivers is known as a Tidal Bore.
Tidal bores occur in about 100 rivers throughout the world.
The Qiantang Bore in China reaches heights of 15 ft. and travels 15 mph.
The Pororoca, in the Amazon River, forms waves 12 ft. high and can reach speeds of 20 mph.
The Severn Bore in England is a popular one to surf as shown below.
Qiantang Bore
Surfing the Severn Bore

34. Whirlpools! Rapidly spinning seawater
Occur in restricted channels connecting two basins with different tidal cycles
The Maelstrom in Norway is the world’s largest whirlpool

35. Tide-Generated Power Renewable energy source but not yet widely used.
Possible harmful environmental effects from blocking tidal currents
Oldest was built in France in the 1960s
Largest was finished in South Korea this year (2011)

36. Power Plant at La Rance, France