The importance of tides and tidal energy

Detail

1. Why tides are useful: they can be predicted

• Tides are the regular rise and fall of the sea level, once or twice a day. They are caused mainly by the pull of gravity from the Moon and, to a lesser extent, from the Sun.
• Tides can be predicted well in advance.
• The positions of the Sun, the Moon and the Earth follow fixed, known cycles.
• So the time and height of each high tide and low tide can be worked out months or even years ahead.

• Navigators and fishermen can therefore plan their work around the tide.

• High tide is the highest water level in a tidal cycle. Low tide is the lowest. The tidal range is the difference in height between the two.

2. Navigation: ports on rivers and estuaries

• An estuary is the mouth of a river where fresh river water mixes with salty sea water and the sea level rises and falls with the tide.
• A bar is a ridge of sand or silt that builds up across the entrance of a river or estuary. It makes the water shallow.
• Why tidal height matters most here:
• Deep-draught ships (ships that sit deep in the water) cannot cross a shallow bar at low tide.
• At high tide the water over the bar is deeper, so these ships time their entry and exit for high tide.

• Ports on rivers with bars, such as Kolkata, therefore depend on the tide.

• Indian example: Kolkata.

• Syama Prasad Mookerjee Port (the old Kolkata Port Trust) is a riverine port (a port built on a river, inland from the sea). It lies on the Hooghly, a distributary of the Ganga in West Bengal, which flows into the Bay of Bengal.
• The Hooghly is shallow and silt-laden, and it has sandbars. Ships move up and down it with the tide.

• Haldia lies downstream, closer to the sea. It was built to handle the bigger ships that cannot reach Kolkata.

• The port system therefore has two parts: the Kolkata Dock System upstream and the Haldia Dock Complex downstream.

3. Desilting and cleaning of estuaries

• Desilting means removing the silt (fine mud) that builds up in a channel.
• How the tide does this:
• At high tide, sea water flows into the estuary.
• As the tide falls (the ebb tide), the water flows back out strongly.

• This outgoing flow carries silt and polluted water out to the sea.

• Result: harbour channels stay open longer, and the estuary water stays cleaner.

4. Fishing and ecology

• Fishing: fishermen time when they launch their boats and when they fish by the tide. High tide lets boats cross shallow creeks and bars. Some fish and prawns move in and out with the tidal flow.
• Intertidal zone: the strip of shore that is under water at high tide and exposed at low tide. Crabs, molluscs, mudskippers and other animals here are adapted to this daily cycle.
• Mangroves are salt-tolerant trees and shrubs that grow in this tidal zone along tropical coasts.
• Tides bring them salt water, nutrients and silt.
• Tides also spread their seeds.
• The Sundarbans (Gangetic delta, West Bengal and Bangladesh) are the largest mangrove forest in the world. They depend on the regular tidal flooding of their creeks.

5. Tidal power: how it works

• Tidal energy is electricity made from the rise and fall of sea water. Tides are regular and predictable, so it is a renewable source of energy.
• Barrage method (a barrage is a dam with floodgates built across a narrow inlet, bay or estuary): 1. High tide: the floodgates open, and sea water fills the basin behind the dam. 2. Gates close: the water is trapped at the high-tide level. 3. Falling tide: the sea outside drops, which creates a height difference (a head) between the basin and the sea. 4. Generation: the trapped water is let out through turbines, and the turbines drive generators that make electricity.

• Some plants generate power on both the incoming (flood) tide and the outgoing (ebb) tide [6].

• Condition for a good site: the method needs a large tidal range and a narrow inlet that can be closed by a dam. Few coasts have both, so suitable sites are rare.

6. Tidal power plants in the world

Plant	Location	Capacity	Year	Significance
La Rance	Rance River estuary, Brittany, France (English Channel)	240 MW	Opened 1966; completed 1967 [6]	First large-scale tidal plant; uses both flood and ebb tides [6][8]
Sihwa Lake	Gyeonggi province, South Korea (Yellow Sea)	254 MW [7]	2011	Largest tidal power station in the world [7]
Other plants	Canada, Russia, China (NCERT)	Small	—	NCERT lists Canada, France, Russia and China as countries with tidal plants

• Trap: La Rance was the first large plant, but Sihwa Lake (254 MW) is the largest, ahead of La Rance (240 MW).

7. Tidal energy in India: the ideal sites

• NCERT (Class 10, Minerals and Energy Resources) names three ideal sites:
• Gulf of Khambhat (Cambay), Gujarat: a funnel-shaped inlet of the Arabian Sea between the Saurashtra peninsula and mainland Gujarat. The Narmada, Tapi, Mahi and Sabarmati drain into it.
• Gulf of Kachchh (Kutch), Gujarat: an inlet of the Arabian Sea between Kachchh and Saurashtra, on India's west coast.

• Gangetic delta in the Sundarbans, West Bengal: on the Bay of Bengal, on the east coast.

• Why these sites? Funnel-shaped gulfs squeeze the incoming tide, so the water rises much higher there. This gives the large tidal range a barrage needs.

8. India's tidal potential (with year)

• Total identified potential: about 12,455 MW (MNRE, 2019). The main sites are the Khambhat and Kachchh regions, plus large backwaters where barrage technology could be used [3]. (NCERT-based figure: about 12,000 MW.)
• Estimated potential by site: about 8,000 MW (MNRE, 2017), from a study by IIT Madras with CRISIL Risk and Infrastructure Solutions [2]:

Site	State	Estimated potential
Gulf of Khambhat	Gujarat	~7,000 MW [2]
Gulf of Kachchh	Gujarat	~1,200 MW [2]
Gangetic delta, Sundarbans	West Bengal	~100 MW [2]

• Ranking: Khambhat > Kachchh > Sundarbans. Gujarat holds more than 95% of the site-wise estimate.
• Policy: in 2019 the Ministry of Power declared ocean energy (tidal, wave and ocean thermal energy) to be renewable energy. Power from it can now count towards the non-solar Renewable Purchase Obligation (RPO), a rule that requires power distribution companies to buy a set share of renewable power [3].

9. Durgaduani tidal power project (Sundarbans)

• What it was: a demonstration (pilot) tidal power plant at Durgaduani creek in the Sundarbans, South 24 Parganas, West Bengal.
• Capacity: 3.75 MW. MNRE sanctioned it to the West Bengal Renewable Energy Development Agency (WBREDA) in 2008 [4]. (NCERT: a 3 MW project "under way".)
• Status: the Government of West Bengal dropped the project because its cost was very high [4]. It was never commissioned.
• Other Indian proposals that did not go ahead:
• Mandvi, Gulf of Kachchh: a barrage plant was proposed.
• Kalpasar, Gulf of Khambhat: a large dam across the gulf for fresh water storage, a road link and tidal power.

• Neither has been built.

• India today has no working commercial tidal power plant.

10. Why tidal power has not taken off

• High capital cost: barrages need a lot of heavy civil construction, so the power is costly and its tariff is high [3][4].
• Silting:
• A barrage slows the water behind it.
• The silt carried by the water then settles in the basin.

• The basin becomes shallower, and the plant's output falls.

• Damage to ecology:

• A barrage changes the natural tidal flooding.
• This harms mudflats (muddy shores exposed at low tide, which feed fish and migratory birds) and mangroves.

• It also affects fish movement and local fishing livelihoods.

• Few suitable sites: a site needs both a large tidal range and a narrow inlet. In India these occur mainly in Gujarat, far from many demand centres.

• Generation is not constant: a plant produces power only for some hours of each tidal cycle, and the timing shifts every day with the Moon.
• Other ocean energy (wave energy, such as the Vizhinjam pilot, Kerala, 1991, and OTEC, ocean thermal energy conversion) is covered in energy-resources-india.

Prelims Hooks

• Tides are predictable because the Sun–Moon–Earth positions are known in advance. This is the basis of tide tables used by navigators and fishermen.
• Port–river pair: Syama Prasad Mookerjee Port (Kolkata) is on the Hooghly, and ships use high tide to cross its shallow bars. Haldia lies downstream for larger ships.
• Order of India's tidal potential (IIT Madras–CRISIL study, MNRE 2017): Gulf of Khambhat (~7,000 MW) > Gulf of Kachchh (~1,200 MW) > Sundarbans (~100 MW). Total identified potential is about 12,455 MW (MNRE 2019).
• Durgaduani creek is in the Sundarbans, West Bengal (not Gujarat). It was a 3.75 MW demonstration plant, dropped because of very high cost.
• Trap: La Rance (France, 240 MW, 1966) was the first large tidal plant, but Sihwa Lake (South Korea, 254 MW, 2011) is the largest.
• Barrage working: water is trapped at high tide and released through turbines as the tide falls. It needs a large tidal range and a narrow inlet.
• The Kalpasar project is proposed for the Gulf of Khambhat. Mandvi is in the Gulf of Kachchh.
• Ocean energy was declared renewable energy in 2019, so tidal power now counts towards the non-solar RPO.
• Both Indian gulfs with high tidal potential (Khambhat and Kachchh) are in Gujarat, on the Arabian Sea coast.

Mains Points

• Economic value of tides (GS-I): tides make river ports like Kolkata usable, because ships cross bars at high tide. They flush silt and pollution out of estuaries, and fishing schedules depend on them. But Kolkata's port still needs costly dredging and a downstream port at Haldia, which shows the limits of a riverine port.
• Tidal energy potential vs reality (GS-III): India has an identified potential of about 12,455 MW, concentrated in the funnel-shaped Gulfs of Khambhat and Kachchh. Yet it has zero working capacity, because Durgaduani, Mandvi and Kalpasar all stalled over high capital cost and high tariffs.
• Environmental trade-off (GS-III): barrages trap sediment and change tidal flooding, which harms mudflats, mangroves (such as the Sundarbans) and fisheries. This clashes with biodiversity and livelihood goals, so in-stream tidal turbines (which need no dam) are a possible alternative.
• Policy response (GS-III): declaring ocean energy as renewable energy (2019) and making it eligible for the RPO creates a market. Further help could include viability gap funding, pilot projects and site studies, and linking tidal power with other uses (as Kalpasar plans) to share costs.