Agreements & MOUs - The Ganga: Water Use in the Indian Subcontinent - page 209

Table 10.17 Occurrence of Bore Tides in the Hooghly river round the year

January to June - 181 days July to December - 184 days

Year No. of days Percentage occurrence No. of days Percentage occurrence

1 2 3 4 5

1974 88 49 51 28

1975 74 41 70 38

1976 90 50 61 33

1977 79 44 53 29

1982 24 13 27 15

1987 23 13 13 7

1992 33 18 25 14

1996 10 6 7 4

1997 22 12 5 3

1998 11 6 6 3

− −

1999 9 5

The effect of upland discharge in the lower estuary of the Hooghly, i.e., between
the Hooghly Point and Sagar island was minimal, as tides were quite high and the
upland discharge insignificant. millions of cubic metre of water moved up and down
along with huge volume of silt load, lending dynamism to the river. The discharge
of 40,000 cusecs, or less, does not much affect the river morphology in this reach.
The width of the river also varies from 10 km to about 25 km. Therefore, the huge
volume of silt-load moving with the tides oscillates and gets deposited in the bed in
a favourable environment. The upland discharge helps this process, as the silt load
cannot push upland.
As sea-water is saline, flow tides push it inland, over the estuary and the hinter-
land. Fresh water coming downstream interacts with this saline water and enhances
siltation. Sea-water, being heavier than sweet water, moves near the bed and the
mingling of two waters creates some associated problems.
Before the barrage in 1975, there was no upland discharge in the lean season.
The saline water from sea used to intrude up to as far as Naihati, some 50 km north
of Calcutta Port. With induction of upland discharge from 1975 and a perennial
flow even in lean season, saline water got mixed with the sweet water upstream and
shed some salinity below Calcutta to near about Achipur, about 30 km downstream.
Its movement varies from year to year, depending on availability of lean season
flow and monsoon discharges from upstream. The longitudinal variation of maxi-
mum salinity in 1980, 1987, 1992, 1997 and 1999, i.e., after the barrage came up as
against the situation in 1975 (pre-barrage) is shown in Fig. 10.12.
The figure shows that the potable limit of salinity (0.20 ppt) of the Hooghly
water extended to about 50 km upstream of Kolkata in 1975; it came down near
Budge Budge in 1980 and further down to the reach between Achipur and Moyapur
from 1996 to 1999. At the Hooghly Point, the salinity is about 0.5 ppt, at Diamond
Harbour about 1.5 ppt and at Haldia 10 ppt. The drafts in the Hooghly depend on