1. CHAPTER 9 Tides

2. Overview Rhythmic rise and fall of sea level
Very long and regular shallow-water waves
Caused by gravitational attraction of Sun, Moon, and Earth

3. Tide-generating forces
Tide-generating forces
Barycenter between Moon and Earth – center of gravity of “Earth-Moon System”
Mutual orbit due to gravity and motion
Fig. 9.1

4. Gravitational forces Every particle attracts every other particle
Gravitational force proportional to product of masses, toward center of gravity of Moon
Inversely proportional to square of separation distance – decrease the farther away from Moon
Fig. 9.2

5. Centripetal force Center-seeking force – force equal at all points
Tethers Earth and Moon to each other
Fig. 9.3

6. Tide-producing forces
Resultant forces =
differences between centripetal and gravitational forces (blue arrows)
Near moon, gravitational forces are greater
Away from moon, centripetal forces are greater
Tide-generating forces are horizontal components resulting in “bulges”
Fig. 9.4

7. Blue arrows are formed from the tip of the red arrows (centripedal forces) to the tip of the black arrows (gravitational forces).
Fig. 9.4

8. Tidal bulges (lunar)
Small horizontal forces push seawater into two bulges on opposite sides of Earth
One bulge faces Moon
Other bulge opposite side Earth
Fig. 9.6

9.
Tidal bulges (lunar)
Moon closer to Earth so lunar tide-producing force greater than that of Sun
Ideal Earth covered by ocean
Two tidal bulges
Two high tides, 12 hours apart
High tide, flood tide, seawater moves on shore
Low tide, ebb tide, seawater moves offshore

10. Lunar Day Moon orbits Earth
24 hours 50 minutes for observer to see subsequent Moons directly overhead
High tides are 12 hours and 25 minutes apart
Fig. 9.7

11. Tidal bulges (solar)
Similar to lunar bulges but much smaller because Moon is much closer to Earth
New/full moon
tidal range greatest – spring tide
lunar and solar tides in line – constructive interference
Time between spring tides about two weeks
Quarter moons
tidal range least – neap tide
lunar and solar tides at 90O – destructive interference

12. Earth-Moon-Sun positions and spring and neap tides
Fig. 9.9

14. Other complicating factors: declination
Angular distance Moon or Sun above or below Earth’s equator
Sun to Earth: 23.5o N or S of equator
Moon to Earth: 28.5o N or S of equator
Shifts lunar and solar bulges from equator  unequal tides at given latitude
Fig. 9.11

15. Declination and tides
Unequal tides (unequal tidal ranges at given latitude)
Fig. 9.13

16. Other complicating factors: elliptical orbits
Tidal range greatest at perihelion (January) and perigee
Tidal range least at aphelion (July) and apogee
Moon’s perigee and apogee cycle 27.5 days
Omit
Fig. 9.12

17. Idealized tide prediction
Two high tides/two low tides per lunar day
Six lunar hours between high and low tides

18. Real tides affected by many factors
Earth not covered completely by ocean
Continents and friction with seafloor modify tidal bulges
Tides are shallow water waves with speed determined by depth of water
Tidal bulges cannot form (too slow)
Tidal cells “slosh” around amphidromic point

19. Tidal cells in world ocean
Cotidal lines
Tide wave rotates once in 12 hours
Counterclockwise in Northern Hemisphere
Omit

20. Omit
Fig. 9.14

21. Tidal patterns Semidiurnal Two high tides/two low tides per day
Tidal range about same
Diurnal
One high tide/one low tide per day
Rarest
Mixed
Tidal range different
Most common

23. Global distribution of tides
tides/media/

24. Tides in coastal waters
Standing waves
Tide waves reflected by coast
Amplification of tidal range
Example, Bay of Fundy maximum tidal range 17 m (56 ft)
Video

25. Bay of Fundy http://academics.sru.edu/GGE/pictures/Newfoundland/Images
~dutting/photos.htm

26. Tides in coastal waters
Tidal bore in low-gradient rivers
Video
Fig. 9A

27. Coastal tidal currents
Reversing current
Flood current or tide – approaching high tide (crest moves into harbor)
Ebb current or tide – approaching low tide (trough moves into harbor)
Slack tide – period of little current at high or low tide
High velocity flow in restricted channels
Fig. 9.18

28. Peak flood and ebb currents in middle of tidal cycle
~1/2 tidal flow moves thru within middle two hours of cycles
Example for semidiurnal tide - “rule of 12ths”
1 – 1/12 of volume  slack tide
2 – 2/12 of volume
3 – 3/12 of volume  fastest current
4 – 3/12 of volume  fastest current
5 – 2/12 of volume
6 – 1/12 of volume  slack tide

29. Tidal range is not a good predictor of tidal currents; altered by:
Shape and volume of basin
Restriction of flow at basin mouth
Winds
Currents stronger at channel center than at edges

30. Coastal tidal currents
Whirlpool
Rapidly spinning seawater
Restricted channel connecting two basins with different tidal cycles
Fig. 9.19

31. Tides and marine life Tide pools and life Grunion spawning Fig. 9C
Tides and marine life
Tide pools and life
Grunion spawning
Fig. 9C

32. Tide-generated power
Renewable resource – mostly hypothetical at this point
Does not produce power on demand
Possible harmful environmental effects

33. End of CHAPTER 9 Tides
Fig. 9.21