Oceanography: Ocean Tides

Basics

• What is it?: The periodic rise and fall in the sea level is known as Tide.
• Caused due to
  • The gravitational force exerted on Earth by the Sun.
  • The gravitational force exerted on Earth by the Moon.
  • Rotation of the Earth
• Tidal Range: The difference in height between the high tide and the low tide is called the tidal range.

• Oscillating currents produced by tides are known as tidal streams. 
• The moment that the tidal current ceases is called slack water or slack tide.
• Tides are commonly semi-diurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day).

Stages of Tides

• Sea level rises over several hours, covering the intertidal zone; flood tide.
• The water rises to its highest level, reaching high tide.
• Sea level falls over several hours, revealing the intertidal zone; ebb tide.
• The water stops falling, reaching low tide.

Types of Tides

• Based on Frequency
  • Semi-Diurnal Tide
    • An area has a semidiurnal tidal cycle if it experiences two high and two low tides of approximately equal size every lunar day.

• • Diurnal Tide
    • An area has a diurnal tidal cycle if it experiences one high and one low tide every lunar day.

• • Mixed Tide
    • The mixed tidal cycle, or simply mixed tide, is formed by a tidal cycle with two unequal high and low tides.

• Tides Based on the Sun, Moon and the Earth Positions
  • Spring Tides
    • Spring tides occur when Moon, Earth, and Sun fall in a straight line, which is called as syzygy (siz-eh-gee), the greatest difference between high and low tide water levels can be observed.
    • These spring tides occur twice each month, during the full and new Moon.
    • If the Moon is at perigee, the closest it approaches Earth in its orbit, the tides are especially high and low.
  • Neap Tides
    • When the Sun and Moon form a right angle, as when a half moon can be seen, their gravitational pulls fight each other and one can notice a smaller difference between high and low tides. These are called neap tides.

Significance of Tides

1. Navigation
   • Tidal heights are very important, especially in harbours near rivers and within estuaries having shallow ‘bars’ [Marine Landforms] at the entrance that prevent ships and boats from entering into the harbour.
   • High tides help in navigation. They raise the water level close to the shores. This helps the ships to arrive at the harbour more easily.
   • Tides generally help in making some of the rivers navigable for ocean-going vessels. London and Calcutta [Tidal Ports] have become important ports owing to the tidal nature of the mouths of the Thames and Hooghly respectively.
2. Fishing
   • The high tides also help in fishing. Many more fishes come closer to the shore during the high tide. This enables fishermen to get a plentiful catch.
3. Desilting
   • Tides are also helpful in desilting the sediments and in removing polluted water from river estuaries.