Tides: Spring & Neap Tide, Tidal Bulge, Tidal Bore | UPSC

Tides: Spring & Neap Tide, Tidal Bulge, Tidal Bore

Tides

The periodic short-term rise and fall of the sea level is known as a Tide. Tides are produced due to the gravitational interaction between the earth, sun, and moon. Since the moon is much closer to the earth compared to the sun, it has a more pronounced influence on the tides. The rotation of the earth also plays an important role in aiding the formation of tides.

• The moon’s gravitational pull to a great extent and the sun’s gravitational pull to a lesser extent are the major causes for the occurrence of tides.
• Another important factor is the centrifugal force, which acts in the opposite direction to the gravitational pull of the earth. This centrifugal force is generated due to the rotation of the earth-moon system around their common centre of mass.
• Tides occur due to a balance between all these forces — gravitational attraction and centrifugal force working together to create tidal bulges on opposite sides of the earth.

When the highest part, or crest, of the tidal wave reaches a particular location, high tide occurs. Low tide corresponds to the lowest part of the wave, or its trough. The difference in height between the high tide and the low tide is called the tidal range.

The highest tide in the world occurs in the Bay of Fundy (Canada). The highest tide in India is recorded at Okha, Gujarat. The greatest tidal range in the world is found at the upper end of the Bay of Fundy in eastern Canada, where a 15-meter (50-foot) water-level fluctuation twice a day is not uncommon. A wall of seawater — called a tidal bore — several centimeters to more than a meter in height rushes up the Petitcodiac River in New Brunswick for many kilometers.

The extreme tidal range in the Bay of Fundy is caused by two factors — the funnel-shaped geometry of the bay which forces the water to rise as the bay narrows, and tidal resonance, where the natural oscillation period of the bay (approximately 12.5 hours) closely matches the semi-diurnal tidal cycle, amplifying the tidal heights significantly.

Tide changes proceed via the following stages:

• Sea level rises over several hours, covering the intertidal zone. This is known as the flood tide.
• The water rises to its highest level, reaching high tide.
• Sea level falls over several hours, revealing the intertidal zone. This is known as the ebb tide.
• The water stops falling, reaching low tide.

Oscillating currents produced by tides are known as tidal streams. The moment that the tidal current ceases its flow is called slack water or slack tide. After slack water, the tide reverses direction and is said to be turning. Slack water usually occurs near high water and low water, but there are certain locations where the moments of slack tide differ significantly from those of high and low water.

Tides are commonly semi-diurnal (two high waters and two low waters each day) or diurnal (one tidal cycle per day). The two high waters on a given day are typically not the same height — this difference is called the daily inequality. In tide tables, these are recorded as the higher high water and the lower high water. Similarly, the two low waters each day are classified as the higher low water and the lower low water. The daily inequality is not consistent — it is generally small when the Moon is positioned directly over the equator.

Types of Tides

Tides vary in their frequency, direction, and movement from place to place and also from time to time. They may be grouped into various types based on their frequency of occurrence in one day (24 hours) or based on their height.

Tides based on Frequency

1. Semi-diurnal tide
2. Diurnal tide
3. Mixed tide

Semi-diurnal tide

• This is the most common tidal pattern, featuring two high tides and two low tides each day. (Actually, it varies between 3 tides to 4 tides — 3 tides in rare cases, but 4 is normal.) The successive high or low tides are approximately of the same height.
• Although tides occur twice a day, their interval is not exactly 12 hours. Instead, they occur at regular intervals of 12 hours and 25 minutes.
• This is because the moon revolves around the earth from west to east, and each day it moves a bit further to the east if observed from the same place on earth at the same time on two consecutive days. This causes a time lag of approximately 50 minutes each day.
• This time lag explains the tide interval of 12 hours and 25 minutes, as tides occur twice a day.
• An interesting exception is Southampton in England, which experiences tides 6-8 times a day. This occurs because Southampton receives 2 high tides from the North Sea + 2 high tides from the English Channel + 2 neap tides from the North Sea + 2 neap tides from the English Channel. The North Sea and the English Channel push the water into Southampton at different intervals, creating this unusual tidal pattern.
• Semi-diurnal tides are commonly found along the East Coast of North America, the United Kingdom, and much of Europe.

Diurnal tide

• In this type, there is only one high tide and one low tide during each day.
• The successive high and low tides are approximately of the same height.
• Diurnal tides are less common and are found in areas such as the Gulf of Mexico, parts of Southeast Asia, and along the west coast of Alaska.

Mixed tide

• Tides having variations in height are known as mixed tides. In this pattern, there are two high and two low tides per day, but they differ significantly in their heights.
• These tides generally occur along the west coast of North America and on many islands of the Pacific Ocean.

Tides based on the Sun, Moon and the Earth Positions

The height of rising water (high tide) varies appreciably depending upon the position of the sun and moon with respect to the earth. Spring tides and neap tides come under this category.

Spring tides

• The position of both the sun and the moon in relation to the earth has a direct bearing on tide height.
• When the sun, the moon, and the earth are in a straight line (a configuration known as syzygy), the height of the tide will be higher than normal. This is because the gravitational forces of the sun and moon act together in the same direction, reinforcing each other.
• These are called spring tides and they occur twice a month — once during the full moon period and once during the new moon period.
• During spring tides, the high tides are unusually high and the low tides are unusually low, resulting in the greatest tidal range.
• The word “spring” in spring tides does not refer to the season. It comes from the concept of the water “springing up” to a higher level.

Neap tides

• When the Moon is at the first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the Earth. In this configuration, the solar tidal force partially cancels the Moon’s tidal force.
• The Moon’s gravitational attraction, though more than twice as strong as the sun’s, is diminished by the counteracting force of the sun’s gravitational pull acting at a right angle.
• At these points in the lunar cycle, the tide’s range is at its minimum. This is called the neap tide, or neaps.
• Neap is an Anglo-Saxon word meaning “without the power”, as in forđganges nip (forthgoing without-the-power).
• Normally, there is a seven-day interval between the spring tides and neap tides.
• Like spring tides, neap tides also occur twice a month.
• During neap tides, the high tides are lower than average and the low tides are higher than average, producing the smallest tidal range.

The magnitude of tides based on Perigee and apogee of moon

• The moon does not orbit the earth in a perfect circle — its orbit is elliptical. This means the distance between the moon and the earth keeps changing throughout the month.
• Once in a month, when the moon’s orbit brings it closest to the earth, this point is called perigee. At perigee, the moon’s gravitational pull is at its strongest, causing unusually high and unusually low tides. During this time, the tidal range is greater than normal.
• Two weeks later, when the moon is at the point farthest from the earth, this position is called apogee. At apogee, the moon’s gravitational force is limited, and the tidal ranges are less than their average heights.
• When a perigee coincides with a spring tide (known as a perigean spring tide), the tidal range becomes exceptionally large, producing the highest high tides and lowest low tides of the month.

Magnitude of tides based on Perigee and Apogee of earth

• Just like the moon’s orbit is elliptical, the earth’s orbit around the sun is also elliptical. This means the earth-sun distance varies throughout the year.
• When the earth is closest to the sun, a position known as perihelion, which occurs around 3rd January each year, tidal ranges are much greater. During this period, unusually high and unusually low tides are observed.
• When the earth is farthest from the sun, a position known as aphelion, which occurs around 4th July each year, tidal ranges are much less than average.

Apogee is the point in an orbit about the earth that is furthest from the earth.

Aphelion is the point in the elliptical orbit of a planet, comet, etc., where it is farthest from the sun.

Importance of Tides

Since tides are caused by the earth-moon-sun positions, which are known accurately, the tides can be predicted well in advance. This is extremely useful for navigators, fishermen, and coastal planners in organizing their activities.

Navigation

• Tidal heights are very important, especially for harbours near rivers and within estuaries that have shallow ‘bars’ (Marine Landforms) at the entrance. These bars can prevent ships and boats from entering the harbour during low tide.
• High tides help in navigation. They raise the water level close to the shores, which helps ships to arrive at the harbour more easily and safely.
• Tides generally help in making some of the rivers navigable for ocean-going vessels. London and Calcutta (Tidal Ports) have become important ports owing to the tidal nature of the mouths of the Thames and Hooghly rivers respectively.
• Without tidal assistance, many large cargo ships would not be able to enter or exit these port cities, making tides essential for international maritime trade.

Fishing

• The high tides also help in fishing. Many more fish come closer to the shore during the high tide, carried in by the stronger currents.
• This enables fishermen to get a plentiful catch without having to venture far out into the open sea.
• Coastal and estuarine fishing communities around the world plan their fishing schedules according to the tidal cycle.

Desilting

• Tides are also helpful in desilting the sediments and in removing polluted water from river estuaries.
• The incoming and outgoing movement of tidal water flushes out accumulated silt and pollutants, keeping the river mouths and estuaries clean and navigable.

Other

• Tides are used to generate electrical power (tidal energy) in countries like Canada, France, Russia, and China. Tidal power plants harness the kinetic energy of tidal currents and the potential energy of tidal height differences to produce clean, renewable electricity.
• A 3 MW tidal power project was constructed at Durgaduani in the Sunderbans of West Bengal, India.
• The La Rance Tidal Power Station in France was the world’s first large-scale tidal power station. South Korea’s Sihwa Lake Tidal Power Station is currently the world’s largest tidal power installation with a capacity of 254 MW.

Characteristics of Tides

• The tidal bulges on wide continental shelves have greater heights compared to those in the open ocean.
• In the open ocean, tidal currents are relatively weak and tidal ranges are small, typically around 1 meter or less.
• When tidal bulges hit mid-oceanic islands, they become low because there is no continental shelf to amplify the tidal height.
• The shape of bays and estuaries along a coastline can also magnify the intensity of tides significantly.
• Funnel-shaped bays greatly change tidal magnitudes by forcing the water into a progressively narrower space, pushing the water level higher. Example: the Bay of Fundy — which has the highest tidal range in the world.
• The large continents on the planet block the westward passage of the tidal bulges as the Earth rotates on its axis.
• Tidal patterns differ greatly from ocean to ocean and from location to location, depending on the local geography, depth of the ocean basin, and the configuration of the surrounding landmass.

Tidal bore

• A tidal bore is a large wave or bore caused by the constriction of the spring tide as it enters a long, narrow, shallow inlet. These waves result from the forces and turbulence in the water which cause a rumbling roar.
• Tides also occur in gulfs. Gulfs with wide fronts and narrow rears experience higher tides because the incoming water is compressed into a smaller area.
• The in-and-out movement of water into a gulf through channels is called a tidal current.
• When a tide enters the narrow and shallow estuary of a river, the front of the tidal wave appears to be vertical. This happens because the water of the river piles up against the incoming tidal wave, and the friction of the river bed slows the base of the wave.
• The steep-nosed tide crest looks like a vertical wall of water rushing upstream and is known as a tidal bore.
• The favorable conditions for tidal bore include the strength of the incoming tidal wave, the slim shape and depth of the channel, and the volume of river flow.
• There are exceptions — the Amazon River is the largest river in the world. It empties into the Atlantic Ocean. The mouth of the Amazon is not narrow, but the river still has a strong tidal bore. A tidal bore develops here because the mouth of the river is shallow and dotted by many low-lying islands and sand bars, which constrict and redirect the incoming tidal wave.
• In India, tidal bores are common in the Hooghly River. The most powerful tidal bores in the world occur in the Qiantang River in China, where the bore can reach heights of up to 9 meters and travel at speeds of up to 40 km/h.
• The name ‘bore’ comes from the sound the tidal current makes when it travels through narrow channels.
• Bores occur in relatively few locations worldwide, usually in areas with a large tidal range, typically more than 6 meters (20 ft) between high and low water.
• A tidal bore takes place during the flood tide and never during the ebb tide. Tidal bores almost never occur during neap tides, because neap tides happen during quarter moons when tides are at their weakest.
• Other notable tidal bores around the world include the Severn Bore (England), the Mascaret (France), and the Pororoca (Amazon, Brazil).

Impact of Tidal Bore

• Tides are stable and can be predicted with high accuracy. However, tidal bores are less predictable and hence can be dangerous for people and vessels near river estuaries.
• The tidal bores adversely affect shipping and navigation in the estuarine zone. Ships and boats can be caught off guard by the sudden arrival of a tidal bore.
• Tidal bores of considerable magnitude can capsize boats and ships of considerable size, making them a serious hazard for maritime operations.
• Strong tidal bores disrupt fishing zones in estuaries and gulfs, scattering fish and damaging fishing equipment.
• Tidal bores have an adverse impact on the ecology of the river mouth. The tidal-bore-affected estuaries are often rich feeding zones and breeding grounds of several forms of wildlife, including fish, crabs, and various bird species.
• Animals slammed by the leading edge of a tidal wave can be buried in the silty water. For this reason, carnivores and scavengers are common sights behind tidal bores, as they feed on the animals and organisms displaced or killed by the incoming wall of water.
• Despite the dangers, tidal bores also have positive ecological effects — they bring nutrient-rich ocean water into river estuaries, which supports marine biodiversity and helps sustain the estuarine food chain.