October 25, 2013

Tides: Causes, Nature, Periodicity, Types & Importance of Tides - Geography for Competitive Exams

The rise and fall of sea water due to gravitational forces of the sun and the moon are called tides. Tidal currents affect the whole water mass from the sea surface to the bottom. The sea waves produced by tides are called tidal waves. The rise of sea water and its movement towards the coast is called tide and the resultant high water level is known as high tide water. The fall of sea water and its movement towards the sea is called ebb and the resultant low water level is called low  tide water. The difference between high tide water and low tide water is called tide range.

FACTORS THAT CAUSE TIDES: The tides have their origin in the gravitational forces of the sun and the moon. The earth rotates from west to east and revolves around the sun following an elliptical orbit. Similarly, the moon, rotates from west to east and revolves around, the earth along an elliptical orbit so that the distance between the moon and the earth changes during different times every month. The period of the farthest distance between the moon and the earth is called apogee while the period of the nearest distance is called perigee.
The surface of the earth facing the moon experiences maximum gravitational force of the moon, while it will be minimum at the opposite side of the earth. Consequently, the water of the earth's surface facing the moon is attracted and pulled up and thus, the high tide occurs. High tide is also formed at the opposite side of the earth simultaneously, because of the reactionary force (centrifugal) of the gravitational (centripetal) force of the moon causing outward bulge of the water

NATURE OF THE TIDES: Thus, two tides and ebbs are experienced twice at every place on the earth's water surface in 24 hours. When the sun, the earth and the moon are in the same line (at the time of full moon and new moon) their gravitational forces work together and high tides are formed.

On the other hand, when the sun and the moon are at the position of right angle with reference to the earth the gravitational forces of the sun and the moon work against each other and hence low tides are formed. This situation occurs during the 8th day of each fortnight of a month.

PERIODICITY OF TIDES: On an average, every place experiences tides twice a day. Since the earth completes its rotation in roughly 24 hours, every place should experience tide after 12 hours but this never happens. Each day tide is delayed by 26 minutes because the moon also rotates on its axis (west to east) while revolving around the earth. Since the earth rotates from west to east, the tide centre shifts westward. When the tide centre completes one round, the moon's position is ahead of the tide centre by that time. The moon also revolves around the earth, with the result, the tide centre takes another 52 minutes to come under the moon. Thus, a particular tide centre takes 24 hours 52 minutes to come under the moon but by that time there is another tide at the opposite side of the referred tide centre and this happens after 12 hours 26 minutes.

TYPES OF TIDES: The oceanic tides arc caused due to tide producing forces of the sun and the moon. There is a lot of temporal and spatial variation in the tide producing forces because of different positions of the sun and the moon with the earth. Because of variations in the intensity of tide producing forces several types of tides are caused.
Two important types of tides are given below.
 (i)        Spring Tide: Very high tide is caused when the sun, the moon and the earth are almost in the same line. Such high tides are called Spring Tide situations. The gravitational forces of the sun and the moon work together with combined force and thus a high tide is caused. The height of such spring tides is 20 per cent more than that of the normal tides. Such tides take place twice every month (during the full moon and the new moon) and their timing is fixed.
(ii)        Neap Tides: The sun. the earth and the moon come in the position of quadrature (i.e.. form right angle) on the seventh or eighth day of every fortnight of a month. The tide producing forces of the sun and the moon work in opposite direction, with the result, a low tide occurs. Such a tide, which is lower in height than that of the normal tide, is called Neap Tide. The height of neap tides is generally 20 per cent lower than that of the normal tides.

Importance of Tides:
  1. Commercial and recreational Navigation through coastal waterways, and within estuaries, bays, and harbours.
  2. The establishment of chart datums for Hydrography, which are then used for demarcation of a base line or "coastline" for fixing offshore territorial limits, both on the sea surface and on the submerged lands of the Continental Shelf.
  3. For the furnishing of data useful to fishing, recreational boating, surfing, and a considerable variety of related water sport activities and tourism activities.
  4. Work on harbour engineering projects, such as the construction of bridges, docks, breakwaters.
  5. Tidal flows are of importance in navigation, and significant errors in position will occur if they are not taken into account. 
  6. Tidal heights are also important; for example many rivers and harbors have a shallow "bar" at the entrance which will prevent boats with significant draft from entering at certain states of the tide. 

What is a seiche and when does it occur?
A seiche is a standing wave that occurs in an enclosed or semi-enclosed water body and is usually caused by strong winds and/or changes in atmospheric pressure. The seiche can continue, in a pendulum fashion, even after the cessation of the originating force.

What are Tidal Bores?
A tidal bore is the leading edge of the rising tide as it enters a river. It is a wave like phenomenon that moves up the mouth of rivers which are subjected to exaggerated tides. Tidal bores are most pronounced where river channels narrow.

What are "Reversing Falls"?
Reversing Falls are phenomena resulting from tidal action. At low tide, the inland waters empty into the sea over a rocky shelf in a waterfall. As the tide rises above the falls, the seawater forces its way against the river flow. The resulting turbulence, in the form of whirlpools, eddies and rapids, makes the falls appear to have actually reversed.

What is a Tide Rip?
A tide rip, is readily apparent at the surface of the ocean. A rip is often a stretch of turbulent water at sea or in a bay or strait caused by conflicting tidal currents, or a tidal current moving over a rough bottom. Tide rips can appear as stretches of slightly choppy water running alongside glassy-calm water, or they might resemble white water rapids amid otherwise calm seas.

What is a Rip Tide?
A strong, sub surface tidal current that conflicts with another current or currents causing a violent underwater disturbance, usually in a direction contrary to that of the surface water is called a rip tide. Although rip tides may appear as dark or calm paths running through breakers, they can exist where there is no apparent surface commotion.

Where are the largest tides in the world? What causes them to occur there?

The largest tidal ranges in the world occur in the Bay of Fundy (more exactly Minas Basin) and in Ungava Bay (more exactly Leaf Basin) on the East Coast of Canada, where you can observe a 16 metre (53 foot) tide range. Tidal range varies during the month according to phase of the moon. They are largest at the new and full moons, and smallest at the quarter phases. Tidal ranges in the Bay of Fundy and Ungava Bay are the highest in the world because of an unusual combination of resonance (or seiche) and the shape of the bay.


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