Tides: why the sea rises and falls

Detail

What a tide is

• A tide is the regular rise and fall of sea level. It happens once or twice a day.
• Main cause: the Moon's gravitational pull (gravity is the force by which one body pulls another).
• Smaller cause: the Sun's pull. The Sun is far bigger than the Moon, but it is much farther away, so its effect is weaker.
• What is NOT a tide:
• Strong winds and changes in air pressure can also raise the sea for a while. These are called surges. A storm surge is one example: a cyclone's winds push sea water onto the coast.
• Surges do not come at regular times, so they are not tides. Tides are regular and can be predicted.

The tide-generating force: how the bulges form

• Centrifugal force is the outward push felt by a body moving in a circle. The Earth and the Moon move around a shared centre, so the Earth feels this outward force. It balances gravity.
• Tide-generating force is the difference between the Moon's pull and the centrifugal force. This difference is what makes tides.
• Near side (facing the Moon):
• This side is closer to the Moon, so the Moon's pull is stronger than the centrifugal force.

• Water is pulled towards the Moon → first bulge, towards the Moon.

• Far side (away from the Moon):

• This side is farther from the Moon, so the Moon's pull is weaker.

• Centrifugal force is now stronger → water is pushed outwards → second bulge, away from the Moon.

• Horizontal vs vertical part: the force has an up-down (vertical) part and a sideways (horizontal) part, along the Earth's surface. The horizontal part matters more. It slides water sideways and piles it up into the bulges (NCERT Figure 13.2).

Tidal bulges and the daily rhythm

• Tidal bulge: a broad hump of ocean water raised by the tide-generating force. There is always one on the Moon side and one on the far side.
• Cause → process → result:
• The Earth rotates on its axis.
• Each place on the coast passes under both bulges, and through the two low "troughs" between them.

• So most coasts get two high tides and two low tides.

• Why tides come about 50 minutes later each day:

• The Moon moves the same way the Earth turns. So after one full turn, the Earth has to turn a little more to "catch up" with the Moon.
• So a lunar day (the time for a place to face the Moon again) is 24 hours 50 minutes, not 24 hours [5].
• Two high tides come 12 hours 25 minutes apart [5].
• So high tide arrives about 50 minutes later each day.

Types of tides by frequency

Type	Pattern in one lunar day	Example area
Semidiurnal	2 highs + 2 lows, about equal in height	Much of the east coast of North America [6]
Mixed semidiurnal	2 highs + 2 lows, unequal in height (higher high water, lower high water)	Much of the west coast of North America [6]
Diurnal	1 high + 1 low	Many areas of the Gulf of Mexico [6]

Spring and neap tides: how the Sun changes tide height

• Spring tide (nothing to do with the season):
• The Sun, Moon and Earth are in a straight line, at new moon and full moon.

• Their pulls add up, so the tidal range is highest. These tides give the highest and lowest waters of each lunar month [5].

• Neap tide:

• The Sun and Moon are at 90° to each other as seen from the Earth, about midway between new and full moon.
• Their pulls partly cancel. The range is usually 10–30% less than the average range [5].

Tidal range: why local shape decides height

• Tidal range: the difference in height between high water and low water.
• Where the range is large:
• Wide continental shelves: water piles up over the shallow sea floor, so bulges are higher.

• Bays and estuaries, especially funnel-shaped ones. An estuary is the wide mouth of a river where it meets the sea. The incoming water is squeezed into a narrower and narrower space, so it rises higher.

• Where the range is small: mid-ocean islands. The deep open ocean around them gives no shelf or funnel to pile the water up.

• Tidal current: the flow of tide water when it is squeezed between islands or into bays and estuaries.

Bay of Fundy: the world's highest tides

• Location: between Nova Scotia and New Brunswick, Canada, on the North Atlantic coast.
• Bulge: 15–16 m, the highest tides in the world.
• Speed of rise: there are two tides a day (about every 12 hours 25 minutes), so the water rises about 240 cm an hour, roughly 2.4 m every hour.
• Why so high: the bay is long and narrows like a funnel, so the incoming tide is squeezed upwards.

Tidal range along India's coast

• Gulf of Khambhat (Cambay), Gujarat: the largest range on India's coast, around 10–11 m at its head (the narrow inner end of the gulf). It is a funnel-shaped gulf on a wide shelf.
• Gulf of Kachchh (Kutch), Gujarat: the second largest range.
• East coast: ranges are small, except in the Hooghly–Sundarbans delta (West Bengal). There, the river channels act as funnels.
• Order of range (highest to lowest): Gulf of Khambhat > Gulf of Kachchh > Hooghly–Sundarbans > rest of the east coast.

Tidal energy: using the range for power

• How it works: a barrage (a dam across a bay or estuary) traps water at high tide. The water is let out through turbines at low tide. This needs a large range.
• Minimum range: to get enough power, high tide must be at least 5 m (16 feet) higher than low tide. In India, only the Gulf of Cambay and the Gulf of Kutch have such sites (PIB, 2012–13) [3][4].
• Estimated potential, about 8,000 MW (Ministry of Power, PIB, 2017) [2]:

Location	State	Potential
Gulf of Khambhat	Gujarat	~7,000 MW
Gulf of Kachchh	Gujarat	~1,200 MW
Gangetic delta, Sundarbans	West Bengal	~100 MW

• Later estimate: the total identified tidal potential is about 12,455 MW. Sites were found in the Khambhat and Kutch regions and in large backwaters where barrage technology could be used (PIB, 2019) [7].
• Policy: in 2019, the Power Minister approved declaring ocean energy (including tidal energy) as Renewable Energy [7].
• Problem: high capital cost, about ₹30–60 crore per MW (PIB, 2012–13), so tidal power is not yet commercial in India [3][4].

Tidal bores

• Tidal bore: a wall of water that rushes upriver against the river's flow.
• Cause → process → result:
• A strong incoming tide (usually a spring tide) enters a funnel-shaped, narrowing estuary.
• The water is squeezed and cannot spread out, so the front of the tide steepens.
• The result is a moving wave-wall travelling upstream.

Bore	River / country	Note
Qiantang	Qiantang River, China	The largest in the world
Pororoca	Amazon, Brazil	Local name means "great roar"
Severn bore	Severn, United Kingdom	Best known in Britain
Hooghly bore	Hooghly, near Kolkata, India	A danger to boats and to Kolkata Port shipping

Prelims Hooks

• The tide-generating force is the difference between the Moon's pull and centrifugal force. Its horizontal part, not the vertical part, mainly builds the bulges.
• The bulge on the far side of the Earth forms because centrifugal force is stronger than the Moon's pull there. Trap: it is NOT caused by the Sun's pull.
• Storm surges and wind-driven rises are irregular, so they are not tides.
• A lunar day is 24 h 50 min. High tides come 12 h 25 min apart, and each day's tide is about 50 minutes later [5].
• Spring tide happens at new and full moon (Sun, Moon and Earth in a line). Neap tide happens when the Sun and Moon are at 90° [5].
• Bay of Fundy (Nova Scotia–New Brunswick, Canada) has the world's highest tides, 15–16 m.
• Ranking of tidal range in India: Gulf of Khambhat > Gulf of Kachchh > Hooghly–Sundarbans.
• Tidal power potential (2017): Khambhat ~7,000 MW, Kachchh ~1,200 MW, Sundarbans ~100 MW, total ~8,000 MW. A later estimate (2019) is ~12,455 MW [2][7].
• Bore–river matches: Qiantang – China (largest), pororoca – Amazon, Severn – UK, Hooghly – India.
• Tidal range is small at mid-ocean islands and large over wide shelves and in funnel-shaped bays.

Mains Points

• What makes the tidal range differ from place to place: the astronomical force (Moon > Sun) only sets the basic rhythm. Coastal shape decides the height. Wide shelves and funnel-shaped gulfs (Fundy, Khambhat, Kachchh) magnify the tide. Open-ocean islands and the narrow-shelf east coast of India have small ranges. (GS-I: distribution of key physical features.)
• Tidal energy in India (GS-III): Gujarat's two gulfs hold almost all the potential (~8,000–12,455 MW). Using it is held back by a cost of ₹30–60 crore per MW, harm to mangroves, mudflats and fisheries in the gulfs, and silt. Declaring ocean energy as renewable (2019) helps these projects count towards renewable purchase obligations [2][3][7].
• Tides and navigation: tidal currents and bores in the Hooghly make sailing and dredging difficult at Kolkata Port. Ships must use tide tables, so tide prediction matters for ports, fishing and coastal infrastructure.
• Tides and disasters: a storm surge that arrives at high spring tide floods much more land, as in the Sundarbans and Odisha during cyclones. This makes tide data important for coastal early warning and disaster management (GS-III).