Vertical Oscillation of Sea Surface is known as tide. At most places it occurs twice daily. The tide rises until it reaches a maximum height called the High Water and then falls to a minimum level called the low water.
The interval between the High Water & Low Water times is called Duration of Tide. The difference in their respective heights is called the Range of Tide.
The period at High water & low water during which there is no sensible change of level is called “Stand” or “Slack Water”.
Equilibrium Theory of tides : Newton proposed the first explanation for semidiurnal tides with his equilibrium model according to which:
- Earth & Moon exist in isolation
- Earth is a non rotating sphere
- A single ocean encircles the globe
- Ocean is static i.e. has no currents
- The only forces acting on ocean result from the movement of Earth & Moon about their common center of mass.
Shortcomings of the Equilibrium Model :
- It predicts semidiurnal tides at all locations – not observed
- It predicts that high tides should occur when moon passes overhead or 12 hours 25 minutes later – rarely observed
- Calculations suggest that tidal ranges should be 20-50 cm– observed tidal ranges are often much larger.
- It predicts values for daily inequities that are rarely observed.
Dynamical Theory of Tides :
- Begins where equilibrium model ends, i.e. with two bulges created by gravitational interactions of Earth & Moon.
- Explains real tides by envisioning tidal bulges as tidal wave
- Tidal wave has small wave height-about 50 cm in open ocean
- Tidal Wave has a very long wavelength(L) about one half of earth’s circumference = 20,000 kms.
- Water Depth of oceans = 4 km <<<<< L/2
- Tidal Wave is a shallow water Wave everywhere, i.e. in interacts continually with the ocean bottom
- As a shallow water wave, tidal wave feels the bottom, slows, steepens and sometimes breaks
- Tidal Wave reflects, refracts & interferes with other waves or with reflections of itself
- Depending upon the shape of a basin, the net result may be a standing wave that corresponds to a diurnal tide, a semidiurnal tide, or a mixed tide.