1. The Tides

2. Tides
Tides are waves with very long wavelengths. In this lecture we will learn about:
Tidal characteristics
The origin of the tides
Tides in small and elongated basins
Tidal currents
Power from the tides

3. The Tides (low tide in Brittany)

4. High tide in Brittany

5. Tidal Characteristics
Tides are single waves that stretch across ocean basins.
They are also shallow-water waves because their wavelengths greatly exceed the depth of the ocean.
They occur due to complex interactions of the sun and moon and Centrifugal force

6. Tidal Characteristics
Key wave features of tides:
High tide : Wave crest
Low tide : Wave trough
Tidal range : Wave height
Tidal periods are either 12 hours, 25 minutes
or 24 hours, 50 minutes depending on location.
Tidal Range – The difference in water level between the high and low tides

7. The Bay of Fundy, Nova Scotia has the greatest range in the world – 53 feet.

8. Tidal range is the difference between high and low tide volume.

9. Tides are Shallow water waves Deep water Waves Interactive Waves
Tsunamis

10. Tides occur because of the interactions of the
Sun, Gravity and the Moon
Moon, Gravity and Centrifugal force
Sun, Moon, Centrifugal force
Sun and Moon only

11. Tidal Characteristics
Tides can be classified based upon their monthly record. The tidal range varies slightly each day in a systematic pattern.
Spring tide: phase when tidal range is maximal.
Neap tide: phase when tidal range is minimal.
There are two spring and two neap tides each month.

12. Springs & Neaps

13. Origin of the Tides Gravitational attraction Centrifugal force
Unlike wind-driven surface waves, tides are cause by two principal factors:
Gravitational attraction
Centrifugal force

14. Origin of the Tides
The gravitational effect of the moon creates a tidal bulge.

15. Centrifugal forces
Centrifugal forces arise as the Earth and Moon revolve around one another.
Because of the unequal masses of the Earth and Moon, the center of rotation lies beneath the Earth’s surface.
The water of the ocean shifts away from the center of rotation creating a second tidal bulge.

16. Centrifugal forces cause a second tidal bulge.

17. Gravitational attraction and centrifugal force create two bulges

18. Other Influences on Tides
Tides are extremely long waves caused by the gravitational attraction of the Moon and the Sun on Earth. They are also affected by the:
1. shape and depth of the ocean floor
2. latitude (greater pull at equator)
3. volume of water
NOAA
NASA

19. Sun and Moon
The sun exerts a gravitational attraction on the earth and its ocean – 46% of the Moon’s effect.
The moon because of its proximity to the Earth has a greater gravitational effect on the Earth than the Sun, despite the fact that the Sun is much more massive than the Moon.

20. If we include the effect of the Sun, we can account for neap and spring tides.
The Sun creates two smaller tidal bulges.
If we take into account the alignment of the Sun, Moon, and Earth we can explain neap and spring tides.

21. Spring Tides
When the Earth, Moon and Sun are orbiting in a linear arrangement, the Moon is in either the new or full stage.
With the Sun and Moon pulling together on the water, tides
are increased - called a spring tide.

22. Spring tides occur when the Earth, Sun, and Moon are aligned.
New Moon and Full Moon phases
Now you have these large tidal ranges
They are called spring tides because they seem to “spring up” with a surge of water.
They occur twice a month (full and new moons)

23. Neap Tides
At quarter phases, the Moon pulls opposite and equal to the Sun - resulting in lower tides called neap tide.

24. Neap tides occur when the Sun and Moon are aligned at right angles to one another.
Quarter Moon phases
Small tidal ranges
Also occur twice a month when the moon is in its first and third quarters (half moons)

25. Springs & Neaps

26. This shows the different views of the Moon from Earth during a lunar month’s revolution of 28 days.
SUN
Day 1
Day 7
Day 21
Day 14

27. Tidal Periods
and Patterns

28. The Earth and Moon revolve around each other and rotate on their axes.
Earth’s rotation = 24 hours (one day)
Moon’s rotation = 24 hours and 50 minutes (lunar day)
Tides occur 50 minutes later every day because of the rotation time difference.
Moon
At high tide (flow), the Moon pulls water out toward it. Centripetal force pulls water out on the opposite side - causing a tidal bulge.
high tide
low tide
As the Earth turns, locations move out of the center of gravity and low tides (ebb) result.

29. The reason why tidal periods are not 24 hours or 12 hours in length is that the Moon moves forward in it’s orbit each day. In fact it takes 50 additional minutes for a spot on the Earth’s surface to regain it’s position relative to the Moon.
For example- you see the moon at a certain spot in the night sky tonight at 9:00pm the moon will be at that same spot tomorrow night at 9:50pm
The Moon has an elliptical course, and the Earth’s distance from the sun also varies

30. Tides are also greater at times during the month when the moon and earth are closer together.

31. There are some exceptions but basically there are 3 types of Tidal Patterns:
Semi-Diurnal = 2 equal high and low tides per day each 6 hours and 12 1/2 minutes apart
ex: east coast of U.S.
2. Diurnal = 1 high and 1 low tide per day each 12 hours and minutes apart. Usually Unequal but differences usually not great.
ex: Gulf of Mexico, S.E. Asia
3. Mixed = 2 high and 2 lows of unequal range at 6 hrs. and 12 1/2 minutes apart
ex: west coast of U.S.

33. Charting
Tidal
Patterns

34. Other Tidal Facts

35. Tides in Small and Elongated Basins
The shape of basins greatly influences the tidal pattern.
In broad basins the pattern resembles the open ocean.
In narrow basins the wave cannot rotate. Instead the wave (tide) moves in and out such that tidal currents reverse flow

36. Bay tides The dimensions of a bay can affect tidal range
the tide can be enhanced (Bay of Fundy, 15 m tide)

37. And this is not even
the lowest tide…...

38. Low Tide - Bay of Fundy
High Tide

39. Tidal Bore
The Qiantang River
estuary in China

40. A tidal bore is a wall of water that moves up certain low-lying rivers due to an incoming tide. Tidal bores form when an incoming tide rushes up a river, developing a steep forward slope due to resistance to the tide's advance by the river, which is flowing in the opposite direction.
Thus we have the phenomenon of the river changing its flow before your very eyes, flowing in OVER the outgoing river water.
The height of the tidal bore increases with the range of the tide and may very in height from just a ripple to several feet.

41. The Qiantang River estuary in China

42. They can be quite powerful– churning up the mud and water

43. Fishing for eels is
done on the incoming
tidal current, but only
after the tremendous
turbulence of the
bore has passed.

44. Surfing a Tidal Bore

45. Power from the Tides
The rise and fall of the tides can be used to generate electrical power.
There must be a substantial tidal range (> 5 m).
Water must flow through a restricted inlet into a large basin. At the mouth a dam can be constructed such that water flow can be controlled and used to turn giant turbines.

46. The tidal power
station on the
La Rance estuary
in France

47. Special turbines
at the La Rance
power station
allow water to
flow in both
directions to
generate power

48. Summary Tides are long wavelength shallow-water waves
They are generated by the gravitational attraction of the Moon and the Sun as well as Centrifugal Force of the spinning Earth.
Tidal Periods are either 12hrs 25 Min or 24 Hrs 50 Min.

49. Summary Three Types of Tidal Patterns are
Mixed - 2 high and 2 lows of unequal range at 6 hrs. and 12
1/2 minutes apart
Diurnal - 1 high and 1 low tide per day each 12 hours and
25 minutes apart
Semi-Diurnal- 2 equal high and low tides per day each 6 hours and 12 1/2 minutes apart
Tidal bore - is a wall of water that moves up certain
low-lying rivers due to an incoming tide.
Electrical power can be generated by having flood and ebb tide currents flow over specially designed turbines.