Only a Barrage Can Save! - The Ganga: Water Use in the Indian Subcontinent - page 83

saving Calcutta port and set put their findings in the memorandum of February 1969
and submitted it to the government of India. It suggested the following methods for
assessing requirement of water:

• Examination of low-water crossings,

• Loss of ebb flow owing to siltation,

• Requirement of flow following intrusion of salinity,

• System analysis in high-speed computer,

• Analysis with electric analogue model and harmonic analysis,

•

Hydraulic model studies,

•

Physical model, based on observed and prototype data.

A study of upper Hooghly above Kolkata, made on the basis of these data
between 1924 and 1963, confirmed steady deterioration. The condition of river
below Kolkata was also studied. It was seen that the cubic capacity in the section
between Kashipur and Damodar outfall had decreased by 11 million cubic metres in
1917, by 39.26 million cubic metres in 1954 and by 64.17 million cubic metres in
1962. The rate of deterioration in 1954 was about 0.9 million cubic metres per year,
as calculated by Prof. Hensen but it increased to about 3.4 million cubic metres per
year, as calculated by Calcutta Port Commission from the data up to 1962. From
1963 to 1965, the government of West Bengal dredged about 4 million cubic metres
of Ghusari sand near Kashipur for Salt Lake reclamation and dumped it on the
marshes where a township, Salt Lake City came up with the spoils of dredging.
This improved the reach between Kashipur and Damodar outfall, temporarily, but
its condition worsened again in 1966. Thus, the progressive decline in the volume
of water led to progressive loss of depths.
Prof. Hensen examined dredging data in the lower Hooghly between 1925 and
1956. Average dredging per year in this period was 3,614 h but from 1925 to 1965,
it went up to 4,093 h and from 1956 to 1965 to 5,888 h. Annual dredging volume
increased from 8 million tons in 1956 to about 10 million tons in 1965. Though
freshets do a bit of scouring in dry season because of strong flow tide, the resultant
re-distribution not only nullifies it but shrinks the volume even more. About 80% of
the siltation occurs from February to May when the upland discharge is minimum.
At the instance of Prof. Hensen, mathematical studies were made by the depart-
ment to assess the effects of deterioration on the tidal zone in the Hooghly. The
navigational channel was excellent once upon a time and bore tides were few.
Increase in tidal range and fall in depths of the navigable approaches had greatly
increased the intensity and frequency of bore tides owing to shallow water. In shal-
low reaches, the velocity of tidal waves depend on elevation so that the troughs travel
slower than the crests; the front of the wave rises steep, which causes the bores in
the form of rushing high waves, breaking on the shallow part of the section. Up
to 1950, 4 months - from November to February - were practically free from bore
tides but afterward, these were a common feature throughout the non-freshet period.
Bore tides damage river-side berths, jetties and moorings, push up their maintenance
cost and can even immobilize them. As bores occur mostly with spring tides, they