1. TIDES Chapter 8

2. 10 1 10 0 10 -1 10 -2

3. A tide has a waveform. - Shallow water wave (large L compared to water depth). -Crest of wave is high tide. -Trough of wave is low tide. -Period is time between successive high (or low) tides. -Wave height of tidal wave is the range crest trough L

4. I.TIDAL RANGES VARY OVER THE DAILY (DIURNAL) PERIOD (vary geographically): 1.Diurnal a)One low one high tide each day. b)Tidal period of 24hrs 25 min. 2.Semidiurnal a)Two high and two low tides each day. b)Tidal period of 12 hrs 25 min. 3.Mixed a)Two high and two low tides of dissimilar range. b)Periods between 12 hrs 25 min & 24 hrs 25 mins.

5. Diurnal and Semidiurnal

6. Real tides are the result Of constructive and destructive wave interference of multiple tidal components, making a “mixed tide”.

9. 1.Spring tides a)The low and high tides each month with highest range. b)Coincides with full and new moons. 2.Neap tides a)The low and high tides each month with lowest range. b)Coincides with first and last quarter moons. TIDAL RANGES ALSO VARY OVER THE MONTLY PERIOD, WITH CYCLE OF MOON

10. Examples of Tidal Records

11. Two Models of Tides 1.Equilibrium model a)(over)-simplified representation. 2.Dynamic model a)Adds more realistic complexity. How to account for the temporal and geographic variations of the tides???

12. Assumptions: –Earth’s surface is covered by seawater of infinite depth. –Wave is progressive. –Wave is in equilibrium with tide generating forces (gravitational and centrifugal). Equilibrium Model of Tides

13. TIDES RESULT FROM TWO FORCES: 1.Gravitational force of moon & sun 2.Centrifugal force

14. Gravitational effect Centrifugal effect Combined gravitation & centrifugal.

15. Equilibrium Tides- Declination

16. Spring and Neap Tides

19. So, to summarize… The equilibrium model of the tides helps explain: 1.Geographic variation of tidal periods (diurnal, semidiurnal, mixed). a.Moon’s declination causes latitudinal variation in sizes of ‘bulges’ seen at a point on the rotating earth. 2.Temporal variation of tidal periods (spring, neap). a)Moon’s phases and alignment with moon & sun.

20. Add more complexity to the simple equilibrium model. To make more realistic we must consider: -shape of ocean basins, land masses -coriolis deflection & geostrophic flow The Dynamic Model of Tides

22. Rotary Tidal Flow in NH P1P1 P2P2 P 1 > P 2 Remember: P = gh N W E S Tidal wave crest

23. Rotary Tidal Flow in NH Remember: P = gh N PG CD W E S

24. Rotary Tidal Flow in NH P2P2 Remember: P = gh N CD W E S PG CD

25. Rotary Tidal Flow in NH P2P2 Remember: P = gh N W E S P2P2 CD X PG

26. Rotary Tidal Flow in NH Remember: P = gh N W E S PG X CD

27. Rotary Tidal Motion Rotary motion of tide is counter-clockwise in NH and clockwise in SH.

28. Amphidromic System

32. Some other terminology… Tidal resonance period of main tidal component equals natural period of basin. Tidal Bore “wall” of water that surges upriver with advancing tide Tidal currents Flood current in advancing tide Ebb currents in retreating tide

33. Electricity can be generated from tidal currents if the tidal range is greater than 5 m in a large bay connected to the ocean by a narrow opening. A dam is constructed across the opening and water is allowed to flow into and out of the bay when sufficient hydraulic head exist to drive turbines and generate power. Power from Tides 8-5