Only a Barrage Can Save! - The Ganga: Water Use in the Indian Subcontinent - page 87

observed hydrographs and levels were known, headwater effect could be isolated
from tidal activity by use of statistical methods, first used in Central Water and
Power Research Station, Pune but with not much success. Calcutta Port's Hydraulic
Study Department developed a method using Fourier analysis for isolating the effect
of 40,000 cusecs for any inlet tide at the mouth of the Hooghly estuary. The analysis
showed that there would be one foot (0.30 m) rise of low-water on an average at
Kolkata after headwater discharge of 40,000 cusecs, round the year, the correction
factor being

±

3in.
With this rise of low water, the incidence of bore tides near Kolkata was observed
in 1961, 1963 and 1964 from March to May, when tides are generally high and
strong and head water supply virtually nil, or negligible. On an average, the number
of bores was 16 in March, 13 in April and 8 in May. If low water rose by a foot after
headwater discharge, the number of bores fell by 70% in March, by 85% in April and
90% in May. As per assessments by Calcutta Port and the CWPRS, Pune, the tidal
apex, i.e., the place where peak energy loss occurs, came down from Chinsurah-
Dumurdaha section, between 26 and 30 nautical miles north of Kolkata to Garden
Reach-Konnagar section, i.e., within 9 nautical miles north of the city in 1955. This
was due to decrease in the volume of tidal compartment, shrinkage of cross-sectional
area and increase in siltation in the river-bed.
Prof. Hensen who examined freshet discharge in Kalna held that it was decreas-
ing. He said, the Bhagirathi being the main source of fresh water from the Ganga,
supply decreased in the past 30 years, or so. In 1938, a peak discharge of 100,000
cusecs was possible but it came down to between 60,000 and 70,000 cusecs in
1957. Prof. Hensen demonstrated the sequence of rise of the river-bed, followed by
encroachment of the tidal apex which moved further down in 1957. He examined
various low-water crossings (bars) and held that to maintain a uniform and regular
depth over these, round the year, a discharge of 80,000 cusecs was necessary but
this much would not be available from the Ganga in the dry season.
Calcutta Port trust studied gradual deterioration of low-water crossings and found
that with progress in shipping industry and invention of steam and oil engines, the
gross tonnage of vehicles visiting the port increased from 0.70 million tonnes in
1862 to 6.8 million tonnes in 1912, to 7.75 million tonnes in 1939. During the
World War-II, it had risen to 9.70 tonnes in 1944, which increased to 10.20 mil-
lion tonnes in 1957. The port expected to handle a gross tonnage of 13.50 million
tonnes by 1955 but the target was not reached mainly because of continuous fall
in depths over the bars and crossings. These diminishing depths restricted the size
and drafts of vessels which could ply on the Hooghly and enter the port. Because
of strong tides, change in bed-levels and orientation of bars, navigation had to be
carried on with utmost care. A large fleet of dredgers were engaged on the bars
and sea-going ships had to be piloted by the Port officers from the Sand heads
to the docks with wireless and depth-finding equipment. The cost of dredging in
1964-1965 was 16.90 million rupees which increased to 38.80 million rupees in
1973-1974 in addition to 12.50 million rupees paid in 2 years - from 1973 to 1975 -
to private Indian and foreign companies for hire of dredgers. Some 1,200 ships vis-
ited the port, each way, every year. The port provided a number of anchorages and