Tides & tidal waves

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In this presentation, one will find more elaborative and detailed information on the topic of TIDES with full depth knowledge about its various aspects including the forces that act on it, how & when they act on it with illustrative images.
  • 1. Paper 3 Unit 2.3. Name- Kainaat. S, Khateeb… M.Sc Part 2… Roll No- 16…
  • 2. Introduction…  Tides are the pulse of the ocean.  Their effects are felt mostly on the coastal areas, where the periodic rise & fall of the ocean surface, with alternate submersion & exposure of the intertidal zone, affecting plants & animals behavior.  Extremely low tides are occasions for Digging Clams; whereas Extremely high tides combined with storms cause Flooding of low-laying areas & extensive damage to Coastal Habitats.  Tidal Currents, accompanying the rise & fall of the tide, are by far the strongest tidal currents in the coastal ocean.  Even large & modern ships prefer to depart a port with an Outgoing Tide & enter on an Incoming Tide.  Tidal Currents Disperse Pollutants in the Coastal Areas.  Like Waves, Tides are easily observed & studied since ancient times.
  • 3. History…  Pliny the Elder, attributed tides to the effect of the sun & the Moon.  Julius Caesar lost part of his fleet & sustained damage to most of his ships during a night high tide on the coast of England.  Alexander the Great got into similar difficulties at the mouth of the Indus River in the Northern Indian Ocean.  The Venerable Bede wrote of the extensive tidal observations made by medieval British priests & exhibited a clear understanding of the relationship of the tidal phenomena to the moon.  He pointed out that separate tidal predictions must be made for each area that 19 years of observations were required in order to compile accurate tide tables.  Among the oldest records of Oceanographic observations is the “Flood of London Bridge”, a set of tide tables for London Bridge dating from the late Twelfth to the early Thirteenth century.  Table of Tides & Tidal Currents represent one of our most successful efforts at predicting events in the ocean.
  • 4. Tides Generating Forces…  Isaac Newton laid the foundation for understanding the mechanics of the tides.  His equilibrium theory of the tides assumes a static ocean completely covering a non-rotating Earth with no continents.  Gravitational attraction pulls the earth & the moon towards each other while Centrifugal forces acting in the opposite direction, that keeps them apart.  The earth & moon act like twin planets revolving about a common centre , which, in turn, revolves around the sun.  If the earth & moon were of same size, the centre would have been located midway between them.  But the moon is about 1/82 the mass of the earth, hence the centre of revolution of earth-moon system is located nearer to the earth.
  • 5.  The force of Gravity is inversely proportional to the square of the distance separating the two objects; so doubling the separation reduces the force of gravity to one fourth.  Therefore, the moon’s gravitational attraction is greater on the side of earth nearest the moon & least on the opposite side.  However, Centrifugal forces are equal over the earth.  On the side nearest to the moon, the attraction of the moon exceeds the centrifugal force so that the water is attracted towards the moon.  On the opposite side of the earth, the centrifugal forces overbalance the attraction of the moon so that there too, a force acts on the water, effectively dragging it away from the earth.  These unbalanced forces on the earth’s surface, are the Tide-Generating Forces associated with the moon.
  • 6. Equilibrium Forces of Tides…  Tide-Generating forces can be represented by Vectors.  Each vector can be resolved into two components acting at right angles to each other.  One component acts in a vertical direction, perpendicular to the earth’s surface; the other acts in a horizontal direction, parallel to the earth’s surface at that point.  The relative strength of the force acting in a horizontal sense varies over the earth.  Directly beneath the moon & on the opposite side of the earth, Tide-Generating Forces act solely in a Vertical direction.  But these vertical components have little effect, for they are counteracted by gravity, which is about 9 million times stronger.
  • 7.  Horizontal components of the tide-generating force are also weak, but they are comparable in strength to other forces acting on the ocean’s surface.  These horizontal attractive forces cause the Equilibrium Tide, which consists of a tidal bulge on the sides of the earth nearest the moon & on the side opposite the moon.  As a result of these horizontal forces, the water covering the earth forms an egg-shaped water envelope on are imaginary, non-rotating, water covered earth.  The solid earth itself responds to these tide-generating forces & deforms slightly but much less than ocean waters.  The moon changes its position from 28.5O north to the equator to 28.5O south to the equator.  For Eg: Miami-Australia & Northern Chile-Bay of Bengal- South China Sea.
  • 8.  Solar Tide- The sun’s position influences the tides, the sun changes its position from 23.5O north to the equator to 23.5O south to the equator. It is a yearly phenomena unlike the moon’s monthly cycle.  Spring Tide- Occurs every 2 weeks, usually within a few days of the new moon & full moon. During this time, the solar & lunar tide-generating forces act together, causing large tidal bulges & hence greater tidal ranges.  Neap Tide- Occurs during 1st & 3rd quarter of the moon, the tide-generating forces partially counteract each other by acting in different direction, resulting in the lowest tidal ranges.
  • 9. Dynamic Theory of Tides…  Tides in the actual Earth’s oceans behave a bit differently than in our hypothetical ocean-covered Earth due to the placement of landmasses, the shallow depth of water relative to wavelength of tides, the latitudinal variation of the rotational velocity of Earth, and the Coriolis Effect. When these factors are taken into effect, we discover the dynamic theory of tides.  Ocean Depth and Rotational Velocity- Tides are such long wavelength waves, they behave as shallow water waves. This means that all of the water in the oceans are effected by tides – from the water at the surface to the water at the deepest depths. Recall that the speed of a shallow water wave is directly proportional to the water depth – because the seafloor acts to slow down waves.
  • 10.  Continents and the Coriolis Effect- Landmasses on Earths surface prevent the Earth from simply rotating into and out of tidal bulges. When the tidal bulge “hits” the side of a continent some of its energy is dissipated, and some of the energy is reflected back into the ocean basin.  This reflection, coupled with the Coriolis Effect causes water to be rotated around an ocean basin, much in the way water would rotate around a cup if you move the cup back and forth.  This oscillation of water around an ocean basin is called an Amphidromic system and causes the high tide wave crests and low tide wave troughs to move around ocean basins in a clockwise or counterclockwise pattern.
  • 11. Tides as a Source of Power…  Tidal power or Tidal energy is a form of Hydropower that converts the energy obtained from tides into useful forms of power, mainly Electricity.  Although not widely used, Tidal energy has potential for future electricity generation.  Tides are more predictable than the wind & sun.  Among sources of Renewable energy, Tidal energy has traditionally suffered from Relatively High cost & limited availability of sites with sufficiently high tidal ranges, thus constricting its total availability.  However, recent technological developments & improvements, both in design & turbine technology, indicate that the total availability of tidal power may be higher than previously assumed & that economic & environmental costs may be brought down to competitive levels.
  • 12. References…  Oceanography- A view of the Earth : by M. Grant Gross…
  • 13. Thank You…
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