Environmental Engineering Reference
In-Depth Information
Table 1.7
Tidal ranges at different coast
Region
tide here is exaggerated. The actual bulge is in the
order of 1 m. Equilibrium tide is theoretical tide.
The effect of sun and moon
Tidal range (m)
s gravity and the
rotation of the earth on tides are most easily
explained by studying equilibrium tides on a
smooth water-covered sphere. Let us consider the
earth and the moon as a single unit, the earth
'
Hugli Estuary
5
Sagar Island
4
Diamond Harbour
4.5
Gulf of Kutch
7.25
-
moon system that orbits the sun. The moon orbits
the earth, held by the earth
Gulf of California
9
End of Manas basin
15.6
s gravitational force
acting on the moon (B) which must be balanced by
a force called centrifugal force (B
'
Chignecto bay
14
Bay of Foundy
15.6
) acting to pull
the moon away from the earth and send it spinning
out into space, and this balance maintains the
moon in its orbit. Similarly, the moon
Magnitude components are described in terms
of
tidal range and tidal prism,
and the time
s gravita-
tional force acting on the earth (C) must be bal-
anced by the centrifugal
'
component is described by
.
Majority of the coastal processes operate
within the tidal range, and so if the values of
HTL and LTL are known, then on the basis of
these limits, one can demarcate the zones with
different magnitude of tidal
tidal period
). These
centrifugal forces are caused by the earth
force (C
moon
system rotating about an axis at the centre of the
system
-
'
s mass, which is 4,640 km away from the
uence such as
subtidal, lower intertidal and upper intertidal.
These zones are important from geomorphologic
and ecological point of views, because each zone
is associated with its own set of physical and
biological characteristics.
Tidal prism is usually computed to
in
fl
earth
'
s centre. Again, the earth
moon system is
-
'
held by the sun
s gravitational attraction (A).
A centrifugal force again acts to pull the earth
-
moon system away from the sun (
Á
). To remain in
this orbit, the earth
moon system requires that the
gravitational forces equal the centrifugal forces.
The moon
-
nd out
the volume of sediment load that is carried into
the estuary during the
s gravitational force is stronger on
particles on the side of the earth closest to the
moon, while the centrifugal force is stronger on
particles on the side of the earth furthest from the
centre of mass of the earth
'
ood and the volume
carried out during ebb tide. If one multiply the
suspended sediment concentration value by the
tidal prism, the total volume can be calculated
which can give an idea about positive or
negative balance of sediment in the estuary or
creek.
The semi-diurnal tide in the sea has a period
averaging about 12 h 25 min, and the mean time
of water rise roughly equals to that of the
waterfall. However, inside bays or estuaries,
these two time limits do not follow symmetric
pattern, which goes a long way in interpreting the
complexities of
fl
moon system. The
distribution of forces tends to pull surface parti-
cles away from the centre of the earth and creates
the tide-generating force
-
eld on the earth.
The stronger gravitational force of the moon
acting on a unit mass of water at the earth
s
surface closest to the moon is proportional to
M
'
r/R
3
, where
·
'
M
'
is the mass of the moon,
'
r
'
is
the radius of the earth and
is the distance
between the centres of the earth and the moon,
because the water covering the earth is liquid and
deformable, and the moon
'
R
'
ow and sediment movement.
The understanding of tides is presently based
on two important theories as discussed here.
fl
s gravity moves water
towards a point under the moon, producing a
bulge in the water covering. At the same time,
the centrifugal force acting on the surface
opposite the moon creates a similar bulge of
water. The earth model develops two bulges with
two depressions in between the bulges or two
crests and two trough or two high tide levels and
'
Equilibrium Theory
According to equilibrium theory, the tidal bulge is
caused by gravitational attraction and moon is the
major component regulating the ocean tide. The
