Tides

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  • Periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon, is called a tide.
  • Movement of water caused by meteorological effects (winds and atmospheric pressure changes) are called surges.
  • The ‘tide-generating’ force is the difference between these two forces; i.e. the gravitational attraction of the moon and the centrifugal force.
  • Tidal bulges on wide continental shelves, have greater height. mid-oceanic islands they become low.
  • The tide is channeled between islands or into bays and estuaries they are called tidal currents.
Types of Tides
Tides based on Frequency
  • Semi-diurnal tide - Two high tides and two low tides each day. high or low tides are approximately of the same height.
  • Diurnal tide - only one high tide and one low tide during each day. high and low tides are approximately of the same height.
  • Mixed tide - Tides having variations in height are known as mixed tides (west coast of North America & islands of the Pacific Ocean).
Tides based on the Sun, Moon and the Earth Positions
  • Spring tides: position of both the sun and the moon in relation to the earth has direct bearing on tide height. Height of the tide will be higher. Occur twice a month.
  • Neap tides: Seven-day interval between the spring tides and neap tides. sun and moon are at right angles to each other.
Other Tide Action
  • Perigee – high and low tides occur. Tidal range is greater than normal.
  • Apogee - The moon’s gravitational force is limited and the tidal ranges are less than their average heights.
  • Perihelion (3rd January), tidal ranges are also much greater, with unusually high and unusually low tides.
  • Aphelion (4th July), tidal ranges are much less than average.

Ebb - time between the high tide and low tide, when the water level is falling.
flow or flood - time between the low tide and high tide, when the tide is rising.
Importance of Tides
  1. Navigation.
  2. harbours near rivers and within estuaries having shallow ‘bars’ at the entrance, which prevent ships and boats from entering into the harbor.
  3. Desilting the sediments and in removing polluted water from river estuaries.
  4. Generate electrical power (Canada, France, Russia & China) - 3 MW tidal power project at Durgaduani in Sunderbans of West Bengal is under way.

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