This Ppt helps you to know very well about ocean tides and its theories

1. OCEAN TIDES
Prepared and Presented by,
P.C. Jeba Preethi Jansi

2. INTRODUCTION
 An ocean tide refers to the cyclic rise and fall of seawater.
 Tides are caused by slight variations in gravitational attraction
between the Earth and the moon and the Sun in geometric
relationship with locations on the Earth's surface.
 Tides are periodic primarily because of the cyclical influence of
the Earth's rotation.

3. TIDAL BULGE
 The moon is the primary factor controlling the temporal rhythm and height of tides .
 The moon produces two tidal bulges somewhere on the Earth through the effects of gravitational
attraction.
 The height of these tidal bulges is controlled by the moon's gravitational force and the Earth's gravity
pulling the water back toward the Earth.
 At the location on the Earth closest to the moon, seawater is drawn toward the moon because of the
greater strength of gravitational attraction.
 On the opposite side of the Earth, another tidal bulge is produced away from the moon.
 However, this bulge is due to the fact that at this point on the Earth the force of the moon's gravity is
at its weakest.
 Considering this information, any given point on the Earth's surface should experience two tidal crests
and two tidal troughs during each tidal period.

5. TIDAL PERIOD
 The timing of tidal events is related to the Earth's rotation and the revolution of the moon around the
Earth.
 If the moon was stationary in space, the tidal cycle would be 24 hours long. However, the moon is in
motion revolving around the Earth.
 One revolution takes about 27 days and adds about 50 minutes to the tidal cycle. As a result,
the tidal period is 24 hours and 50 minutes in length.
 The second factor controlling tides on the Earth's surface is the Sun's gravity. The height of the
average solar tide is about 50% the average lunar tide.
 At certain times during the moon's revolution around the Earth, the direction of its gravitational
attraction is aligned with the Sun's .
 During these times the two tide producing bodies act together to create the highest and lowest tides
of the year. These springtides occur every 14-15 days during full and new moons.
 When the gravitational pull of the moon and Sun are at right angles to each other, the daily tidal
variations on the Earth are at their least.
 These events are called neap tides and they occur during the first and last quarter of the moon.

7. HIGH TIDE AND LOW TIDE
 When the gravitational pull is at its highest point, the result is high tide, which is the highest level
of the tide. When the pull is at its lowest point, we see low tide, or the lowest level of the tide.
 The earth itself is also pulled toward the moon but with less strength. This pulls the earth away
from the water on the opposite side of the earth, making the water on that side bulge as well.
 Therefore, high tide occurs on both sides of the planet at the same time. Meanwhile the earth is
rotating. So, we experience tides throughout the day.
LOW TIDE HIGH TIDE

9. TYPES OF TIDES
Based on the number of high and low tides and their relative heights each tidal day, tides are described as semi-
diurnal, mixed, or diurnal.
SEMIDIURNAL AND DIURNAL TIDES
 Now, if the earth were perfectly round with no big land masses, all bodies of water in the world would
experience two nearly equal high tides and two low tides each day.
 This tidal pattern is known as semidiurnal tides.
 However, the continents of earth disrupt water bodies, and so this can produce different tidal patterns. For
example, some bodies of water, such as the Gulf of Mexico, have diurnal tides, which means only one high tide
and one low tide each day.

11. Mixed tides
 Many parts of the world experience mixed tides where successive high-water and low-water stands
differ appreciably In these tides, we have a higher high water and lower high water as well as higher
low water and lower low water

12. SPRING TIDES AND NEAP TIDES
 The earth and moon are constantly in motion around the sun, and all have their own gravitational
pull.
 So, when the alignment between the three bodies changes, it changes the strength of the overall
gravitational pull and therefore the size of the tides.
SPRING TIDES
 Spring Tides are tides that occur when the earth, moon and sun are aligned, and the tidal range
between high and low tide is at its maximum.
 This happens basically twice a month, during the full and new moon phases.
 At these times, the three bodies are in line and their gravitational pulls reinforce each other. When
the spring tide is happening, we see higher than average high tides and lower than average low
tides.

13. SPRING TIDE

14. NEAP TIDES
 A few weeks after the spring tides, we see the neap tides.
 These are tides that occur when the moon and sun are at right angles to the earth's orbit, and the
tidal range between high and low tide is at its minimum.
 The neap tides occur when the moon is in its first and last quarter phase.
 Because of the position of the moon and sun, their gravitational pulls on the waters of earth partially
cancel each other out, resulting in smaller differences between the high and low tides.

15. NEAP TIDE

16. TIDAL BORE
 In shallow steep funnel shapes rivers the tides enhances as a single wall of water.
 This is called tidal bore.
 These tides move with a speed of 25km.
 They move up to height of 8 m.
 These tidal bores are commonly seen in Quintang river, Amazon, Bay of Fundy and many English and French
rivers.

18. The Tide-Generating Forces
 As the earth revolves around the gravitational centre of the sun/earth system, the orientation of the
earth´s axis in space remains the same. This is called revolution without rotation .
 The tide generating force is the sum of gravitational and centrifugal forces. In revolution without
rotation the centrifugal force is the same for every point on the earth´s surface, but the gravitational
force varies .
 It follows that the tide generating force varies in intensity and direction over the earth's surface. Its
vertical component is negligibly small against gravity; its effect on the ocean can be disregarded. Its
horizontal component produces the tidal currents, which result in sea level variations.
 The gravitational force exerted by a celestial body (moon, sun or star) is proportional to its mass but
inversely proportional to the square of the distance.
 The Sun's mass is equivalent to some 332,000 Earth masses, while the mass of the Moon corresponds
to only 1.2 percent of the mass of the Earth.

19.  The mean distance Sun -Earth is 149.5 million km, the mean distance Earth - Moon only 384,000 km.
If the gravitational force of the Sun and Moon are compared, it is found that the Sun's enormous mass
easily makes up for its larger distance to Earth, to the extent that the gravitational force of the Sun
felt on Earth is about 178 times that of the Moon.
 As a result the Earth's orbit around the Sun is not seriously distorted by the Moon's movement around
the Earth.
 However, as is evident, tides are not produced by the absolute pull of gravity exerted by the Sun and
the Moon but by the differences in the gravitional fields produced by the two bodies across the
Earth's surface.
 Because the Moon is so much closer to the Earth than the Sun, its gravitational force field varies
much more strongly over the surface of the Earth than the gravitational force field of the Sun.
 Quantitative analysis shows that the differences of the gravitational forces across the Earth's surface
are proportional to the cube of the distances Sun - Earth and Earth - Moon.
 As a result the Sun's tide-generating force is only about 46% of that from the Moon. Other celestial
bodies do not exert a significant tidal force.

20. Tidal theories
EQUILIBRIUM THEORY
 The first theory which attempted to explain the tides in the ocean was the equilibrium theory developed by
Newton (1687).
 Newton showed that the attractive forces of the moon on the side of the earth nearest to the moon would be
greater than the average forces on the opposite side of the earth and therefore the water would pulled be less
than the average forces and therefore the water would move away from the moon.
 In an ocean completely covering the earth and of equal depth; the horizontal component of the tide generating
forces pulls the water towards the points nearest and farthest away from the moon, thus causing elevation of
the water at these points, with a corresponding depression in the water level halfway between these two points
on the earth’s great circle.
 The sun also would tend to give similar effects related to itself.
 According to Newton, this process can continue only till the resultant horizontal pressure differences in the
ocean tend to return the water to its former position, so that the free surface of the water would be in
equilibrium.
 Equilibrium theory can explain certain characteristics of ocean tides, especially the occurrence of
semidiurnal tides.

21.  Thus this theory can explain the occurrence of semi-diurnal tides.
 This theory can also explain the formation of spring and neap tides.
 When the sun and the moon are in line with respect to the earth(full moon and new moon) their tide generating
forces are added, producing highest high tides known as spring tides.
 When the sun and the moon are perpendicular to one another with respect to the earth, their effects tend to
cancel one another, producing the lowest low tides known as neap tides.
Dynamic Theory of Tides
 Nearly a century after Newton put forward his equilibrium theory. Laplace(1775) formulated the
dynamic theory of tides.
 According to this theory, the tide producing forces produce tide waves in the ocean whose period corresponds to
that of the generating forces.
 The dynamical theory attempted to understand tides by considering the effect of the depth and width of the o
The Coriosis force (deflection due to earth’s rotation) is zero at the equator and increases with latitude towards
the poles. It varies with the speed of the tidal current

22.  Thus, the earth’s rotation has a major role in the formation of tidal currents and on the general nature ofocean
tides.
 Friction also influence the tides, since the force of friction reduces the speed of the tidal currents.
 In the simplest case so far, we have considered the moon to be directly above the equator.
 But actually the moon has a declination which changes up to a maximum of 28.5° north and south.
 Due to this reason the tidal forces will not be symmetrical with reference to the equator. Therefore, the
problem of wave motion in latitudinal channels would become more complicated.
 In spite of the complexities of the dynamic theory, it has one important advantage, in that it enables to predict
the course of the tides for any given location along a coast.

23. Modern Tidal Theory
 Our knowledge on tides has not yet reached the final stage, so as to explain the tides satisfactorily and
quantitatively without referring to observed values, attempts are being made.
 The following three fundamental ideas have provided the basis from which the investigation is proceeding
successfully (Macmillan, 1966).
 1. The theory of standing oscillations
 2. The theory of resonance between the natural oscillation periods of water masses in bays, gulfs, seas and
oceans and cyclic rhythms of the astronomical disturbing forces which can be predicted accurately.
 3. The application of gyroscopic principles to determine the effect of the earth’s rotation upon these water
masses, which are set in motion horizontally by hydraulic factors, due to the differences in level of the sea
surface induced by the atractive forces.

24. Tides Affect Coastal Regions
 Tides affect coastal regions in different ways.
 High tides push large amounts of water far up onto beaches and leave the sand and sediment mixed with the
water behind when the tide goes out. Therefore, tides transport sand and sediment and shape shorelines.
 Tides feed estuaries. Estuaries are coastal areas where freshwater mixes with ocean water that is delivered by
the tides.
 High tides bring nourishing sediment and sea life into estuaries. Estuaries are home to biologically diverse and
unique plant and animal communities because their waters contain a mix of freshwater and salty ocean water.
 Tides can present challenges to coastal regions, especially during storms. Storms that form out in the ocean
intensify waves. If the storm waves come ashore at high tide, tides can worsen damage and erosion caused by
storm waves.

25. TIDES AFFECT MARINE LIFE
 Organisms living in intertidal zones must be able to live both above and below water, depending on the tide.
 Other organisms that depend on tides for survival include coral reefs, which depend on tides to deliver food to
them.
 The distances and positions of the sun, moon and Earth all affect the size and strength of the Earth’s tides. The
magnitude of tides are also strongly influenced by the shape of the shoreline.
 The height of tides is greatest when tides hit a wide portion of the continent. On the other hand, mid-oceanic
islands usually experience very small tides because they are positioned further away from the continent.
 Tides carry nutrients, moderate temperatures and influence other conditions.
 High tide refers to water at its highest level, and low tide is water at its lowest level. Ebb, or falling, tide
occurs when the water seems to flow back out between a high tide and a low tide.
 When the water flows back in between low tide and high tide, this period of time is known as flow, flood or
rising tide.
 Each of the Earth's two tidal bulges travel at a speed of approximately 24 hours, which means there are two high
tides and two low tides each day, with high tides occurring 12 hours and 24 minutes apart.