Tides

• Tide is a periodic short term rise and fall in the sea level.
• These are produced due to the gravitational interaction of earth, sun, and moon.
• Tides are caused by the combined effects of gravitational forces exerted on Earth by the Sun and the Moon, and rotation of the Earth. It occur due to a balance between all these forces.
• Energy of tides is much more than the rogue waves or storm breakers.
• When the highest part or crest of the 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 Funday (Canada), and 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.
• Oscillating currents produced by tides are known as tidal streams. The moment that the tidal current ceases is called slack water or slack tide, which usually occurs near high water 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 (the daily inequality); these are the higher high water and the lower high water in tide tables, similarly for the two low waters.
• The daily inequality is not consistent and is generally small when the Moon is over the equator.

• Tide changes proceed via the following stages:
  • Sea level rises over several hours, covering the intertidal zone; flood tide.
  • The water rises to its highest level, reaching high tide.
  • Sea level falls over several hours, revealing the intertidal zone; ebb tide.
  • The water stops falling, reaching low tide.

Types of Tides

• Tides vary in their frequency, direction and movement from place to place and also from time to time.
• Tides may be grouped into various types based on their frequency of occurrence in one day or 24 hours or based on their height.
• Tides based on Frequency:
  • Semi-diurnal tide
  • Diurnal tide
  • Mixed tide
• 
• Semi-diurnal tide:
  • The most common tidal pattern is two high tides and two low tides each day.
  • Successive high or low tides are approximately the same height.
  • Although tides occur twice a day, their interval is not exactly 12 hours. They occur at regular intervals of 12 hours and 25 minutes due to the moon’s movement. The moon moves a bit to the east each day, causing the time lag between tides.
  • Eg: A place in England, Southampton, experiences 6-8 tides per day (2 high tides from North Sea + 2 high tides from English Channel + 2 neap tides from the North Sea + 2 neap tides from the English Channel). These are caused by the North Sea and English Channel pushing water at different intervals.
  • 
• Diurnal Tide:
  • Only one high tide and one low tide during each day occurs. The successive high and low tides are approximately of the same height.
  • 
• Mixed Tide:
  • Tides having variations in height are known as mixed tides. 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:
  • Sun and moon’s position in relation to the earth affects tide height.
  • A straight line configuration of sun, moon, and earth, known as syzygy, results in higher tide heights.
  • Tides with higher heights during syzygy are called spring tides.
  • Spring tides occur twice a month, during full moon and new moon periods.
  • 
• Neap Tides:
  • When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the Earth.
  • The solar tidal force partially cancels the Moon’s gravitational attraction during this time.
  • The Moon’s attraction is more than twice as strong as the Sun’s but is diminished by the counteracting force of the Sun’s gravitational pull.
  • The tide’s range is at its minimum during this time and is called the neap tide or neaps.
  • Neap is an Anglo-Saxon word meaning “without the power”.
  • There is usually a seven-day interval between spring tides and neap tides.
  • Neap tides also occur twice a month like spring tides.

The magnitude of tides based on Perigee and apogee of moon

• Once a month, when the moon’s orbit is closest to Earth, high and low tides are unusually high or low.
• This period is called perigee.
• During perigee, the tidal range is greater than normal.
• Two weeks later, when the moon is farthest from Earth, the tidal ranges are less than their average heights.
• This period is called apogee.
• 

Magnitude of tides based on Perigee and Apogee of earth

• Tidal ranges are affected by the position of the Earth in its orbit around the sun
• When the Earth is closest to the sun (perihelion) around 3rd January, tidal ranges are greater with unusually high and low tides
• When the Earth is farthest from the sun (aphelion) around 4th July, tidal ranges are less than average.

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 helps the navigators and fishermen plan their activities:
• Navigation:
  • Tidal heights are crucial for harbors near rivers and estuaries with shallow bars at the entrance.
  • High tides are helpful in navigation as they raise the water level close to the shores, making it easier for ships to arrive at the harbor.
  • Tides aid in making some rivers navigable for ocean-going vessels.
  • London and Calcutta are significant ports due to the tidal nature of the mouths of the Thames and Hooghly, respectively.
• Fishing:
  • Fishes come closer during high tide. This enables fishermen to get a plentiful catch.
• Desilting:
  • Tides are also helpful in desilting the sediments and in removing polluted water from river estuaries.
• Green Energy:
  • Tides are used to generate electrical power (in Canada, France, Russia, and China).
  • A 3 MW tidal power project was constructed at Durgaduani in Sunderbans of West Bengal.

Characteristics of Tides

• Tidal bulges are higher on wide continental shelves.
• Tidal currents in the open ocean are weak.
• Tidal bulges become low when hitting mid-oceanic islands.
• The shape of bays and estuaries can magnify tides’ intensity.
• Funnel-shaped bays greatly increase tidal magnitudes (e.g. Bay of Fundy with the highest tidal range).
• Large continents block the westward passage of tidal bulges during Earth’s rotation.
• Tidal patterns vary greatly from ocean to ocean and location to location.

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.
• Tides also occur in gulfs. The gulfs with wide fronts and narrow rears experience higher tides.
• 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 owing to the piling up of water of the river against the tidal wave and the friction of the river bed.
• The steep-nosed tide crest looks like a vertical wall of water rushing upstream and is known as a tidal bore.
• Tidal bore develops due to the strength of the incoming tidal wave, the slim and depth of the channel, and the river flow.
• The Amazon River is the largest river in the world, and it still has a strong tidal bore despite not having a narrow mouth due to shallow water and many low-lying islands and sandbars.
• Tidal bores are common in the Hooghly river in India and the most powerful tidal bores occur in the Qiantang River in China.
• Tidal bores occur in areas with a large tidal range, usually 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.

Impact of Tidal Bore

• Tides are stable and predictable, but tidal bores are less predictable and can be dangerous.
• Tidal bores can adversely affect shipping and navigation in estuaries.
• Boats and ships can capsize due to strong tidal bores.
• Tidal bores disrupt fishing zones in estuaries and gulfs.
• Tidal bores have an adverse impact on the ecology of the river mouth.
• Tidal-bore affected estuaries are rich feeding zones and breeding grounds for several forms of wildlife.
• Animals hit by the leading edge of a tidal wave can be buried in the silty water.
• Carnivores and scavengers are common behind tidal bores.