NCERT Class 11 - Fundamentals of Physical Geography, Chapter 14

Tides, Waves and Coral Reefs

Tides are the periodic rise and fall of sea level caused by the gravitational pull of the moon and the sun together with the earth's own rotation. They are entirely predictable, which distinguishes them from waves and from tsunamis.

How tides are caused

• The moon's gravitational pull is the main cause. Although the sun is far more massive, the moon is much nearer, and its tide raising effect is more than twice as strong.
• Water on the side facing the moon is pulled towards it, producing a bulge and a high tide; a second bulge forms on the opposite side because of the centrifugal effect of the earth-moon system.
• Between the two bulges the water level falls, giving low tide.
• As the earth rotates, each place passes through the bulges, so most coasts get two high and two low tides in a little over twenty-four hours.
• Successive high tides at a place are about twelve hours and twenty-five minutes apart, which is why the time of high tide shifts a little each day.

Spring and neap tides

• Tides are classified by frequency as semi-diurnal, two highs and two lows of about equal height each day, diurnal, one high and one low, and mixed, with unequal heights.
• A tidal bore is a wall of water that rushes up a narrowing river mouth on the incoming tide.

Importance of tides

• Tides scour river mouths and harbours, carrying away silt and keeping ports navigable without constant dredging.
• Ships enter and leave shallow harbours with the high tide, so tide tables are essential for navigation.
• Tidal energy can be harnessed where the tidal range is large, and the Gulf of Khambhat and the Gulf of Kachchh in India have among the highest potential.

Waves and coral reefs

• In an ordinary wave the energy moves forward but the water does not - individual water particles move in circles and return nearly to their starting point.
• Wave size depends on wind speed, the length of time the wind blows, and the fetch, which is the distance of open water over which it blows.
• A tsunami is not a tidal wave at all - it is caused by a sudden displacement of the sea floor, usually by an undersea earthquake, and it travels very fast with a low height in the open ocean, rising to great height only in shallow coastal water.
• Coral reefs are built by tiny marine animals called polyps, which need warm water above about 20 degrees, shallow and clear water that lets sunlight through, and salt water free of sediment - which is why reefs are absent near large river mouths.
• The three types are the fringing reef attached to the shore, the barrier reef separated from the shore by a lagoon, and the atoll, a ring of coral enclosing a lagoon.