Tides: Tidal Currents, Types and Importance

At a Glance

• Main cause Gravitational pull of the Moon
• Spring tide New and full moon; greatest range
• Neap tide First and last quarter; least range
• Tide interval About 12 hours 26 minutes
• Highest tides Bay of Fundy, Canada
• Exam link UPSC Prelims, Geography: Oceanography

Where Does It Fit?

This topic follows the topic on waves and covers the daily rise and fall of the sea, which is the third movement of ocean water. It includes tidal currents and the tidal bore, and the importance of tides for ports and energy. Previous topic: Waves: Wavelength, Amplitude, Frequency. Next topic: Major Ocean Currents of the World and Their Effects.

What Causes Tides?

In short: the Moon pulls the water on the near side of the Earth, and the spinning of the Earth-Moon pair throws up a bulge on the far side.

Tides are the periodic rise and fall of the sea level, usually twice a day. Their principal cause is the gravitational force of the Moon, which is strongest on the side of the Earth facing the Moon, and weakest on the far side. The Earth and the Moon revolve about a common centre of mass, located inside the Earth, and this motion produces a centrifugal force that is the same everywhere but is balanced against gravity only at the centre. As a result, there are two tidal bulges: one toward the Moon, where gravity exceeds the centrifugal force, and one on the opposite side, where the centrifugal force exceeds the Moon’s pull. The Earth rotates under these bulges, so each place has two high tides and two low tides in a lunar day of about 24 hours 50 minutes. Since the Moon moves about 50 minutes later each day, tides come about 50 minutes later each day, and the interval between two high tides is about 12 hours 25 minutes.

The Sun also produces tides, but its effect is only about 46 per cent of that of the Moon, because it is much farther away, although it is far larger. Tidal force depends on the cube of the distance, and so the Moon, which is nearer, dominates. The tides are therefore a result of the geometry of the Earth, Moon and Sun, as in Solar and Lunar Eclipses.

What Are Spring and Neap Tides?

The term spring has nothing to do with the season. It means a tide that springs up. The tides are also affected by the distance of the Moon. When the Moon is at perigee, its closest point to the Earth, about 357,000 km away, the tidal range is larger by about 20 per cent than normal, giving a perigean tide, and at apogee, the farthest point at about 406,000 km, the tides are weaker. When a new or full moon coincides with perigee, the exceptional perigean spring tides appear. The Earth is also closest to the Sun, at perihelion, in early January, and farthest at aphelion, in early July, and the Sun’s effect varies slightly with it. A very high tide combined with a storm surge is the most dangerous, as in Tropical Cyclones: Formation Conditions, Paths, Naming Conventions, Early Warning Systems.

What Are the Types of Tides by Frequency?

• Semi-diurnal tide: two high tides and two low tides each day, of nearly equal height; the most common type, found along the Atlantic coasts of Europe and the United States and along the Indian coast.
• Diurnal tide: only one high tide and one low tide each day, of the same height; found in the Gulf of Mexico and parts of Southeast Asia.
• Mixed tide: two high and two low tides each day, with unequal heights; common on the Pacific coast of North America and parts of the Indian coast, such as the east coast.

The tidal range, which is the difference in height between high and low tide, depends on the shape of the coast. Tides are small in the open ocean, about a metre, but are amplified in funnel-shaped bays and estuaries. The Bay of Fundy in Canada has the highest tides in the world, with a range up to about 16 m, and the Gulf of Khambhat and Gulf of Kutch in Gujarat and the Hooghly estuary have among the highest tides in India. Enclosed seas, such as the Mediterranean and the Baltic, have almost no tides. Southampton in England has a double high tide, because of the interplay of the flows in the English Channel and the North Sea. As an analytical point, tides are the only ocean movement caused by forces outside the Earth, which also makes their timing exactly predictable years in advance, in tide tables used by ports and navies.

What Is a Tidal Bore and How Does It Form?

In short: a tidal bore is the incoming tide forced into a narrowing river mouth, so that it rushes upstream as a steep wave.

A tidal bore is a surge of the incoming tide that moves up a river as a wave or a series of waves, with a sharp front, against the river’s current. It is formed when the tidal range is large, usually more than about 6 m, and the estuary or river mouth narrows and shallows like a funnel, which squeezes the advancing tide and raises its height. It is strongest at spring tides, and in the dry season when the river flow is low. After it has passed, the water level rises quickly and the river briefly flows upstream. Bores can be gentle undular waves, or steep breaking walls of water up to several metres high.

Why Are Tides Important?

• Navigation and ports: high tide allows large ships to enter shallow harbours and estuaries, and many ports, such as London, Liverpool and Kolkata, depend on the tide. Tide tables are used to schedule movement. The tide also scours the channels and keeps them from silting, and a strong bore can threaten small vessels.
• Fishing and coastal livelihoods: tides bring fish into the estuaries and expose shellfish beds, and fishermen time their work to the tide. Salt pans along the coast use the high tide to fill the evaporation ponds.
• Ecosystems: the intertidal zone, the area between high and low tide marks, is a rich ecosystem of mudflats, mangroves and salt marshes that nurse fish, filter water and protect the coast. The tidal flushing removes the waste from estuaries and mixes fresh and salt water, as in the Sundarbans.
• Tidal power: the rise and fall of the tide can drive turbines. The first big plant was La Rance in France, built in 1966, with about 240 MW, and the largest is Sihwa Lake in South Korea, with about 254 MW. Tidal power is predictable, unlike wind and solar power, but needs a large tidal range, costs a lot to build and affects the estuarine ecology. India has an estimated potential of several thousand megawatts, concentrated in the Gulf of Khambhat, the Gulf of Kutch and the Sundarbans, but no large tidal plant is working, because of the cost, as in Resources from the Ocean: Mineral Resources on the Deep Sea Floor, Energy Resources, Biotic Resources.
• Science and defence: naval operations, amphibious landings and the laying of cables depend on the tide.

How Do Tides Add to Coastal Risk?

High tide flooding, sometimes called sunny-day flooding, happens when a spring tide or a perigean tide pushes water above the street level in low-lying cities even without a storm, and it is increasing because sea level is rising. In a cyclone, the storm surge on top of a high spring tide is the greatest danger, as in Tropical Cyclones: Formation Conditions, Paths, Naming Conventions, Early Warning Systems, which is why warnings give the time of landfall in relation to the tide. Mumbai’s monsoon floods become worse when heavy rain coincides with a high tide, which closes the outfalls of the drains. As an analytical point, the tide is the best-predicted natural force, so the damage from tidal flooding is a matter of planning, drainage and land use, and not of surprise.

Why Is This Topic Important?

• Concept Spring and neap asked
• Prediction Port and naval use
• Hazard Storm surge on top of spring tide

What Came Next?

The next topic describes the major ocean currents of the world and their effects. Next topic: Major Ocean Currents of the World and Their Effects.

Key People and Terms

• Tide Periodic rise and fall of the sea
• Tidal range Difference between high and low tide
• Syzygy Sun, Moon and Earth in line
• Perigee Moon closest to Earth
• Apogee Moon farthest from Earth
• Semi-diurnal Two tides per day

Exam Corner

Mains Angle

Questions ask for causes and types. These points add depth.

• Cause Gravity and centrifugal force
• Pattern Spring, neap, perigean
• Local Range depends on coast shape