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My son, during his childhood, always used to say that
the Bombay boat would not enter the Mandovi estuary
unless he flags her off at Miramar! (The Mandovi and
Zuari estuaries are situated on the west coast of India
in Goa and Miramar is a beach on the Mandovi's mouth).
On couple of instances it had so happened that the Bombay
boat was waiting to enter the channel to anchor at jetty,
only when he waived his little hanky from Miramar beach.
His notion improved when he learnt about the tides in
his textbooks. We live close to the beach. Almost every
day walk on the beach is our weakness. We see lot of
tourists flood in Goa to enjoy on beaches. Many of them
are first-timers and have no practical experience of
the tidal phenomenon. Clean coastal waters in Goa attract
them for bathing. They undress themselves leaving their
garments and other belongings just next to the then
water line. At end of their enjoyable bathing experience,
run in to the difficult situation as the rising tide
either takes away or at least soak their belongings!
Study and knowledge of tides is extremely
important for navigation. Ships line-up outside harbours
for an entry during high tide. Alternately, large investments
are made to manufacture transhippers to transfer the
bulk cargo on the ships, from small vessels, at offshore
locations - without waiting for the high tide and/or
a berth in harbour. Besides, knowledge of the tidal
range at a given location along the waters is equally
important for many other reasons including habitation
along the coastline. Tides not only show rise and fall
along coasts but also in the estuarine channels to a
considerable length where the rivers meet the sea. The
seawater during high and low tide gush in and out of
the channels. Added to this, the river runoff pushes
the freshwater towards to the sea mixing with saline
water. The hydrodynamics of this activity has always
attracted scientists to find out how deep the seawater
goes in to the estuary and also the tidal ranges at
different locations in the channel.
The tides in the oceans refer to the rise and fall of
water level cyclically. The gravitational effects of
the astronomical bodies - mainly moon and the sun -
influence the earth's surface that cause tides (note
that the 'tide' generated due to Tsunami is not due
to any astronomical body's effect). Stay close to the
beach for a day. You would either observe two main cycles
per day (- two high and two lows - semidiurnal tides)
or one cycle per day (- one high and one low - diurnal
tides), or a combination of the two (mixed tides). This
is explained well at
http://www.physicalgeography.net/fundamentals/8r.html
and http://www.nio.org/aroundus/Sagar/tides.jsp.
For studying tides, the effect of tide-producing forces
exerted by the Moon and the Sun, are separated, using
mathematics, into constituents. There are over 128 tidal
constituents used to represent various wavelengths and
frequencies found in nature. The major 5 constituents
are: M2 (Principal lunar semidiurnal constituent),
S2 (Principal solar semidiurnal constituent),
N2 (Larger Lunar elliptic semidiurnal constituent),
K1 (Luni-solar declinational diurnal constituent)
and O1 (Lunar declinational diurnal constituent).
Scientists study these to determine amplitude (the magnitude
of the difference in elevation between low and high
tides at a particular point in a body of water) and
phase lag (the time delay with which one rhythmic activity
follows another of the same frequency). A month long
time series is needed to determine the constituents
and hence to predict the time of high and low tides
at a given place years in advance.
The Mandovi and Zuari estuarine system in Goa is a representative
of all such channels on west coast of India. (Figure
1).
Like all other estuaries on the west
coast, this system has highly seasonal river runoff
- during monsoon (July-September) when the rains pour
over 250cm of water (Figure 2).
The scientists at NIO monitored the variations
in the water level in this system once in every 15 minutes
for a month (March-April), to have clear picture on
the tides without influence of runoff. They also noticed
that the tides occur in these estuaries up to a distance
of about 50kms next to which the elevation of estuarine
channel prevents them propagating beyond this distance.
13 locations (6 in Mandovi, 5 in Zuari and 2 in a canal
connecting them) from mouth to the distance of 50 kms
towards mouth of the estuaries were selected for the
observations.
They observed that the amplitude and phase lag, both,
grow from mouth to head (Figure 3).
Figure 3. Variation in amplitude
(cm) and pahse lag in degree (referenced to IST)
of M2, S2, N2, K1 and O1 in the mail channels of
the mandovi and Zuari estuaries. |
Amplitude of M2 increased by about 20% in
Mandovi from mouth to head and by about 30% in the Zuari.
The increase in K1 was by about 10% in Mandovi
and 15% in Zuari. The phase lag also increased from
mouth to head for both diurnal and semidiurnal constituents,
but variation was not linear with respect to distance
from the mouth.
| Based on the data collected,
the scientists drew seven panels (Figure 4) giving
instantaneous section of the channel topography
and water level along a line drawn through the middle
of the Mandovi channel. Each of the panel is separated
by two hours during spring tide. Two consecutive
panels show how the water level changed during a
period of two hours - all panels together covering
a typical cycle of 12 hours. These panels also help
in understanding how the water level along the channel
changes during a tidal cycle and understand how
the increase in channel elevation causes the tidal
amplitude to drop at the upstream end in an estuarine
channel. Since the magnitude of water level variation
drops sharply from Usgao to Ganjem in the Mandovi,
the tide reaches Ganjem (Panels A, E, F and G) only
when the tide is high enough to overcome the effect
of increased channel elevation. |
Figure 4. each of the 7 panels in
the figure gives an instaneous section of the channel
topography and water level along a line drawn through
middle of the Mandovi channels. Two consecutive
panels are separated by two hours during spring
tide. The water at the upstream end in controlled
by dynamics of the river flow. |
As indicated earlier, since this river system is representative
of all rivers on the west coast of India, this study
has provided us an insight to us how the characteristics
of tidal constituents change from mouth to head in these
shallow (~5m deep), narrow (usually less than few hundred
meters wide, except near the mouth) and converging channels.
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