Hydraulics
Prof. B.S. Thandaveswara
37.5 Tides
The periodic rise and fall of the planetary ocean level in response to the gravitational
interaction of the earth, moon, and sun. The periodic rise and fall of sea level that
results from the gravitational attraction exerted on the earth by the moon, and to a
lesser extent by the gravitational pull on the sun, is the tide. Tides are complex because
of:
(1) the movements of the moon in relation to the earth's equator, (2) changes in
positions of the moon and sun with respect to the earth, (3) uneven distribution of the
water on the earth's surface, and (4) irregularities in the configuration of ocean basins.
Because of these variables, tides vary. For example, in the Bay of Fundy, between New
Brunswich and Nova scotia, they are over 12 meter high, at Gulf of Camby (Gujarat,
India, it is of the order of 8 m and in chennai it is above 1 m) but along the coast of the
Mediterranean Sea, they are virtually unnoticeable.
Measurements of tides can be made by using an ordinary graduated pole anchored to
the ocean bottom. Periodically at, say, one-hour intervals, reading of the water level on
the pole are recorded. The height of the water at given interval will determine the
construction of the tidal curve.
Indian Institute of Technology Madras
Hydraulics
Prof. B.S. Thandaveswara
September
1
3
5
6
4
2
0
E
A
N
E
7
9
11
13
15
17
12
10
8
6
4
2
0
6
4
2
0
21
23
25
27
29
MHWS
MHW
New York
10
8
6
4
2
0
19
N
P
3
S
MLW
MLWS
MHWS
MHW
Port Adelaide
Total
datum
MHHW
MHW
MLW
MLLW*
Seattle
MHHW
MHW
MLW
MLLW*
Los Angeles
Tidal curves for selected coastal cities (Source: Nathaniel Bowditch,
American Practical Navigation, rev. ed., H.O. Publ. 9, Washington, D.C.)
Tides are at their maximum when the moon and the sun are in the same plane as the
earth. These spring tides occur every 14 days, at new and full moons. When the moon
and the sun are at right angles to each other, low tides occur. These neap tides occur
every 14 days, always at half moon. See figure
Indian Institute of Technology Madras
Hydraulics
Prof. B.S. Thandaveswara
Last quarter
(neap tide)
Moon
Earth
Sun
New Moon
(Spring tide)
Earth
Moon
Moon
Moon
Earth
Full Moon
(Spring tide)
Earth
First quarter
(neap tide)
Spring tides occur with greatest amplitude when the Sun and Moon are
lined up at the same time of new and full moon. Neap tides occur with
lowest amplitude when the moon is at first and last quarter
Diurnal and semidiurnal tides occur because of the gravitational attractions of the earth,
the moon, and the sun. The moon, because of its closeness to earth, is the strongest
influence on tides. The tide producing force of the moon is twice as strong as that of the
sun. The moon takes 29.53 earth days to complete one revolution around the earth.
During the course of this revolution, the earth and the moon are gravitationally attracted.
But this gravitational attraction is balanced by centrifugal force, which orginates from
their orbital motion about each another (figure). Interaction between gravitational and
centrifugal forces is the primary source of tides.
Indian Institute of Technology Madras
Hydraulics
Prof. B.S. Thandaveswara
N
Water surface
(tidal bulge)
Moon
Earth
Fc
(Centrifugal
force)
Fg
(gravitational
force)
S
The primary cause of tides is the gravitational pull of the moon.
Although its pull is only 1/300,000 of the earth's gravitational force,
it is sufficient to raise the sea level on the moon-facing portion of the earth;
Centrifugal force balances the opposite side
Tidal Bores: A steep-nosed tide crest rushing (along with a high tide) upstream. Often
under favourable conditons in some tidal rivers, incoming high tides reverse their flows
as they move upstream in breaking waves called tidal bores.
Tidal bores have been reported to rush between 25 and 30 kilometers per hour and
have heights up to 10 metres. Notable examples of tidal bores include the Tsientang
Kiang River at Hangschou Bay in China (which has a 3.5 meter height and moves at
about 1 meter per second) and the Amazon river (which has a 5-meter height and
moves at more than 0.5 meter per second).
Summary
1. Waves are formed in response to prevailing winds. Waves are described in terms of
height, wavelength and period.
2. The Mechanism of wave formation in complex and is not yet fully understood.
3. Major wind generated waves include seas, swells, and surfs.
4. Wave refraction is a phenomenon that waves display as they approach shore.
Indian Institute of Technology Madras
Hydraulics
Prof. B.S. Thandaveswara
5. Longshore currents and rip currents are important types of wave currents.
6. Catastrophic waves include Tsunamis, which are produced either by volcanic or by
earthquake activity.
7. Storm waves or suges produced by abnormal meteorlogical and oceanic conditons.
8. Internal waves are found below surface waters. When they emerge at the surface,
they are identifiable as slicks and carry fine sediments and plankton.
9. Tides are a daily phenomenon produced by the gravitational force of the moon, and
to lesser extent, by the sun.
10. Tides are harnessedto manufacture electricity, for example, in France. There are
160 locations throughout the world under consideration to obtain energy from tides.
World's Potential Tidal Power Plant Sites (Selected Sites only)
1. Cook inlet (Alaska);
2. Baja Calfornia;
3. Passamaquoddy (Maine);
4. Bay of Fundy;
5. Frobisher Bay (Canada);
6. Maranho (Venezuela);
7. San Jose Gulf (Argentina);
8. Severn River (Great Britian);
9. Rance River (France);
10. Kislaya (USSR);
11. Cambay River (India);
12. Seoul River (South Korea);
13. Abidjan (Africa);
14. Darwin (Australia);
15. Kimberleys (Australia);
Indian Institute of Technology Madras
Hydraulics
Prof. B.S. Thandaveswara
Most tidal curves record two high tides and two low tides per tidal day, occuring every
24 hours and 50 minutes (Figure 8-14). The time 12 hours and 50 minutes, between
high or low tides is called tidal period. The combination of two high and low tides is
called semidaily or semidiurnal tides. When there is only one high tide and one low tide
daily, they are called daily or diurnal tides. Often the tidal curves record two high tides
and two low tides per tidal day, but if there are differences between two high tides and
between two low tides, mixed tides result. Mixed tides are abbreviated HHW (higher
high water) and LHW (lower high water). Similarly, low and lower marks are expressed
as LLW (lower low water) and (higher low water). Tide predicting machines are used at
many locations.
Indian Institute of Technology Madras