Cyclone and Storm Surge - Iczmpwb.org

2.20
Another interesting feature of tides lies in the fact that the tidal range at any location fluctuates
steadily and gradually from day to day in accordance with the lunar phase. The tidal range at the full
and new moons is found to the maximum. This phase is called the "Spring Tide". After the
occurrence of the spring tide at Full moon or New Moon the tidal range starts diminishing till the
occurrence of the Neap Tide at the middle of moon cycle when the tidal range is the minimum.
The tidal curve at the sea face of a tidal estuary in Bay of Bengal is found to have a symmetrical
harmonic shape. The periods of both flow and Ebb tide being practical1y equal. As it goes upwards,
the period of Ebb tide increases more than the low tide. Thus the f low velocities are more than the
subsequent Ebb velocities along the tidal channel as one proceeds upwards. There is another
interesting point in this connection: the tidal range starts increasing steadily and gradually upwards
from the sea face, attains maximum value at the certain points and starts diminishing thereafter till at
the terminal point of tidal propagation it becomes practically nil.
The water levels at the confluences of the major rivers with the sea are constantly changing due to
the tides. These changes propagate into the Sundarban not only along their own courses but also
along the many bifurcations, changing with varying time-lags the water levels at both ends of each
river - or creek section, initiating a pattern of flows in the various waterways.
Due to erosion and sedimentation the average depth of many sections of the system of waterways
change; some sections will become deeper others less deep. A change of the size and the shape of
any individual section do not only change the flow through this section, it will also alter the pattern of
water movements during the process of filling and emptying the live storage of the Sundarbans. The
following case studies have been taken from Kanjilal (2006).
An example of this can be seen presently in the Bidya river. There is a strong sedimentation in the
section of this river from its confluence with the Gomar river till the confluence with the rivers Kartel
and Durgadoani. Because of this the propagation velocity in this section is reduced and dur-ing rising
tide the water level near the latter confluence will rise slower than it did before. Due to this a larger
difference in water level is created not only in this section of the Bidya river but also in the Kartel and
the Durgadoani river. The slope in all three sections is increased.
As a result of the reduced wetted cross-section of the discussed part of the Bidya river, this extra
slope will not lead to a larger discharge; the discharges of the two other rivers increase however. Due
to this the erosion in those rivers became stronger and is endangering the stability of the river banks.
This is significant near the market place at Basanti and at many other places along both rivers.