Agreements & MOUs - The Ganga: Water Use in the Indian Subcontinent - page 160

area and also further down (lower estuary), which resulted in increase of quantum
of dredging over Balari bar for maintaining the navigation channel. Thus, the full
benefits of diversion of water from the Ganga could not be achieved as a result of the
agreement. The ebb tide current did not become sufficiently strong enough during
lean season in order to prevent the flood tide current, transporting sediment upwards.
The total length of the Bhagirathi-Hooghly is divided into five stretches through
a line diagram in Fig. 10.2 to explain the position more clearly. The non-tidal reach
of the Bhagirathi and the tidal reach below Nabadweep are sub-divided into two and
three reaches, respectively. The flow direction and the sand movement are explained
in the figure. Before the barrage came up, the mouth and the bed of the river grad-
ually silted. The river bed which was once at the same level as that of the parent
Ganga rose about 9 m, or 30 feet, in 1960. Plan and cross-section in Fig. 10.3(a, b)
explain the position. Post-Barrage induction of upland discharge from 1978 reduced
siltation in the lean season and pushed down the silt load from the upper reach and
deposited it in the lower (non-tidal) and also in the upper (tidal) reaches. Because
of tides, the silt that was pushed up with tide, moved down below Calcutta dur-
ing ebb tides, but due to reduced upland flow-tide velocity, silt was deposited in
the lower estuary below Diamond Harbour; the most affected reach was at Balari
bar. As ships to Calcutta or Haldia ports came from the Bay of Bengal, they faced
obstructions in the lower bars. Dredging over these bars had to be increased sub-
stantially in the post-Barrage days to keep the navigation channel clear. In spite
of continuous efforts by Calcutta Port, the Balari bar silted up and the navigation
channel from Haldia to Calcutta was completely blocked from 1988. An alternative
navigation route had to be made thorough the Rangafalla channel on the eastern side
of Nayachara island, as shown in Fig. 10.4. At present, ships to Calcutta port off-
load a bulk of the cargo either at Sagar island, or at Haldia, before entering Kolkata
by taking a detour through Rangafalla channel.
Salinity reduced to a large extent in the Hooghly after the barrage came up, com-
pared to that before 1975, when the water supplied to the city and the suburbs for
drinking was quite brackish. Calcutta's drinking water is drawn from the Hooghly
at Palta, about 24 km north of the city. The records of salinity in the dry season,
kept from 1920 to 1967, indicated the condition of the river, as shown in Fig. 10.5.
Salinity of the Hooghly water at Palta rose gradually. As salinity intrusion in the
Hooghly depends on the quantity of sweet water in the river in the lean season, the
volume of water fell fast in the course.
Besides the shortage and contamination of drinking water, boilers and other
machineries of industrial units were heavily damaged for using saline water. The
potable limit of about 0.2 ppt of salinity exceeded even at Serampore, about 50 km
upstream of Calcutta, especially in lean seasons. However, landward migration of
salinity could be arrested in post-Farakka period and the potable limit could be
maintained even near Budge Budge, about 30 km downstream of Howrah Bridge
in lean season, albeit for a short duration. Increase of salinity beyond potable limit
was observed, even near Garden Reach just downstream of Calcutta, in lean seasons
after 1977.