Only a Barrage Can Save! - The Ganga: Water Use in the Indian Subcontinent - page 86

in 1865 by the British Raj in Kolkata. Analysis of water at Palta showed increase
in salinity; in 1900, it was about 50 ppm, in 1935, it rose to 350 ppm, in 1957 to
1,500 ppm and in 1966 to about 3,000 ppm, the potable limit of salinity being about
250 ppm. As salinity intrusion in the Hooghly, or any other river, is countered by
increase of sweet water at the end of freshets, increasing salinity intrusion from 1900
to 1966 must have been due to gradual fall in sweet water supply from upland and
also on loss of volume of water following shrinkage of sections. Increase in salin-
ity posed problems to the railways and industries along the banks of the Hooghly;
boilers and other machineries were scaled and corroded. Farm lands were affected,
leading to low yield of crops etc.
Tube wells were sunk to supplement the city's water supply but their water was
also brackish. Besides being almost equally high in salinity, tube well water was
generally hard (total hardness varies from 700 to 900 parts and permanent hardness
from 300 to 600 parts per million). The World Health Organization consultants who
examined the water-supply problems of greater Kolkata observed that tube well
water had high iron content too.
Salinity also depended on the relative strength of upland supplies. Together with
the capacity of the river channel, this determined the building up of a sweet-water
reservoir before the cessation of freshets. Because of the ever-varying tides and
irregular upland supplies, salinity used to rise every day. To counter it, the only
solution seemed to be an assured supply of controlled upland discharge to increase
the channel capacity and build up a fresh-water reservoir. The report of the WHO
consultant team in January 1960 said:
If measures are not taken to increase the flow of fresh water down the Hooghly and so
prevent the intrusion of salt water into the reach in which the Palta intake is situated, raw
water becomes so saline that it will be unusable over a period of weeks, or even months.
The seriousness of this threat should not be minimized since all the indications are that
the salinity will increase and that this situation may in consequence arise within a few
years' time. Moving intake upstream above Palta would only temporarily defer the threat,
if supplementation of the Hooghly does not occur.
Hydraulic studies by Calcutta Port indicated average loss of river water volume
in the reach above Kolkata up to Bansberia in Hooghly between 1944 and 1963 as
2.37 million cubic metre, per year for a stretch of 46 km, i.e., about 0.05 million
cubic metre, per km per year. The average ebb tide excursion was of the order of
24 km, which meant a loss of about 1.20 million cubic metre per year. In 30 years,
this storage would entail a loss in ebb discharge of 46,350 cusecs (1,312 cumecs).
Thus, to revert to and maintain 1936 condition of the Hooghly, a minimum discharge
of 46,350 cusecs (1,312 cumecs) would be necessary and to revert to and maintain
the 1924 condition, it would require 64,890 cusecs (1,838 cumecs).
Prof. Hensen also stressed the importance of headwater discharge, as sustained
discharge in a tidal river increases the duration and strength of the ebb tide and
decreases those of the flow-tide. It also raises the water-level, i.e., increases depth of
water. As bore tide depends on the depth, it influences them also. Calcutta Port Trust
studied the probable rise of low water that would accrue from headwater discharge
of 40,000 cusecs (1,132 cumecs) after commissioning of the Farakka Barrage. As