Table of Contents
Causes and Importance of Tides:
A visit to the seaside shows that the seawater rises up from the beach at a certain time and then recedes or ebbs with a fall in the level of seawater. This alternate and periodic rise and fall in sea level occurs twice a day and is called tide. When the level of seawater rises, it is a high tide. When the water level falls it is a low tide or ebb tide. Tides are noticed near the shores and not in the open seas. In a day we have two flow tides and two ebb tides.
Causes of Tides:
The gravitational pull of the moon and the sun causes the ocean waters to rise. The effect of the moon is more pronounced because it is closer to the Earth. Water bulges on the side of the earth closest to the moon. In addition, an equally large tidal bulge is produced on the side of the earth directly opposite the moon due to the centrifugal force produced by the rotation of the earth. The tide-generating force is the difference between the gravitational attraction of the moon and the centrifugal force. These bulges are high tides. Between these two high tide zones, low tides occur. In this way, the ocean is always rising in two areas of the world and lowering in another two.
Funnel-shaped bays greatly change tidal magnitudes. When the tide is channeled between bays and estuaries it is called a tidal current. Tides vary in their frequency, direction, and movement from place to place and time to time. When the sun and the moon are aligned near the time of the new and full moon, their forces are aligned and added. This produces spring tide. The water leaps and jumps to a very great height and this occurs twice a month. Spring also produces a large tidal range, that is, the largest variation between high tide and low tide.
During the first and third quarters of the moon, the sun and the moon are at right angles to each other. The pull of the Sun neutralizes the pull of the moon. As a result, the height produced by the moon’s attraction is reduced by the sun’s attraction on the waters in the low tide regions. Sunc a tide is called a neap tide.
The periodicity of tides has been referred to earlier when it was mentioned that there are two high tides and two low tides in a twenty-four-hour period. The time difference between two high tides is twelve hours and twenty-six minutes. This occurs because the revolution of the moon around the earth occurs in the same direction as the earth’s rotation from west to east. The tide water rises gradually for a little over six hours till it reaches high tide. Then for the next six hours, there is a fall in the sea level owing to low tide. The cycle then begins once again.
Importance of Tides:
Tideas are useful to us in several ways.
(1) They provide easy navigation for ocean-going vessels. The tides help in shipping and trade. Small ships and fishing boats use the incoming tide to enter or exit the otherwise shallow waters of the deltas and the estuaries at high tide. The popularity of Diamond Harbor near Kolkata on the River Hugli and of London port on the River Thames can be attributed to the high tide waters that enable entry to these inland river ports.
(2) Tides can be predicted because of the known position of the sun, the moon, and the earth. So the fishermen can plan their activities accurately. Incoming tide also brings food for breeding prawns, crabs, etc. Anadromous fish move upstream to breed and tides facilitate their catches.
(3) High tides flood the salt pans with sea water which evaporates to leave behind salt deposits.
(4) The tidal water clears away a lot of mud, silt, and garbage, brought by the rivers. Tides wash away these deposits into the depths of the sea.
(5) The tides are an unending source of energy for generating electricity. This has been done in Canada, the UK, France, Japan, and China. In India, there is a potential for generating tidal power in the Gulf of Khambat and the Gulf of Kuchch on the Gujarat coast.
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