Environmental Engineering Reference
In-Depth Information
two low tide levels. The wavelength is equal to
half the circumference of the earth.
a different approach for understanding. Tide
waves become discontinuous due to the separa-
tion of the ocean by the continents except in the
southern ocean around Antarctica. The tide wave
has a long wavelength as compared to the
ocean
Discrepancies in Equilibrium Theory
Any physical theory is always provisional, in the
sense that it is only a hypothesis when it cannot
be proved directly. When a new experiment or
observation agrees with the predictions, the the-
ory gets the life, but if it disagrees, the theory has
to be abandoned or modi
s depth; therefore, tides behave as shallow
water wave. Laplace developed the dynamic
theory of tide. In this theory, the tidal waves are
considered instead of a bulge. The tidal waves
are produced by tide-generating forces as con-
sidered in
'
ed. A comparison of
tidal observations at different parts of the earth
with equilibrium theory indicates a lot of dis-
crepancies. They are
(i) Tidal high water occurs at a time not in
conformity with the prediction as per equi-
librium theory.
(ii) Tidal ranges are different in most cases than
predicted by equilibrium theory.
(iii) Diurnal inequality often bears little resem-
blance to the theory.
(iv) The bulges as per equilibrium theory would
be separated by half the earth
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equilibrium theory
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of tide. In contrast
to
of
tides recognizes that water covers only three-
quarters of our planet and is con
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equilibrium theory
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, the
'
dynamic theory
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ned to seas and
ocean basins that are
xed on a rotating earth and
experiences forces of constantly changing mag-
nitude and direction (the earth
s crust is also
affected by these forces, creating small but
detectable
'
).
The horizontal force
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earth tide
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elds that act on the
water masses rotating with the earth are not at all
random like winds blowing across the surface of
a lake. Instead, they repeat denite patterns with
a kind of
s circumfer-
ence (20,000 km) and would be moving at a
velocity of more than 1,600 km/h. Tidal
wave is a shallow water wave, and a depth
of ocean of 22 km is necessary for the wave
to attain this velocity.
The equilibrium theory, however, predicts the
periodicity or nearly so and therefore cannot be
abandoned but must be modi
'
rhythm. Ocean
waters respond with a wave-like motion charac-
terized by de
'
push me, pull me
'
nite period and tidal periods,
identical to the ones derived from equilibrium
theory. However, water cannot remain con
ned
to basin engaged in wave motion at all like a
'
ed to account for
several complicating factors. These complicating
factors are as follows:
(i) Irregular shape of the ocean.
(ii) Varying depth of the ocean.
(iii) World is not entirely covered with water
(iv) Rotation of the earth.
(v)
that supposedly sweep around the
globe as depicted in equilibrium theory, without
encountering a solid boundary that results in
re
tidal bulge
'
ection of the wave from that then travel back
the way it came. When a re
fl
ected wave meets a
new waveform that does not go anywhere, but it
merely oscillates up and down like a sea-saw.
The shallow water tide wave moves in theory
at about 200 m/s, but at equator, the earth moves
eastward under the tide wave at more than twice
the speed at which the tide can travel freely. This
eastward displacement due to friction continues
until the portion of the moon
fl
Inertia (in equilibrium theory, inertia was
not considered, and therefore, water was
supposed to respond immediately).
Dynamic Theory
The equilibrium theory of tides allows us to
understand the basic concepts associated with the
oceans
s attractive force
that pulls the tide wave eastward balances the
friction force. This balance between these two
forces holds the tide crest in a position to the east
of the moon rather than directly under it.
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tide, but the actual tidal pattern that
occurs in the discontinuous ocean basin requires
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