Decline of Bhagirathi-Hooghly Channel - The Ganga: Water Use in the Indian Subcontinent - page 72

300

3000

Discharge Variation

200

2000

Time Span Variation

100

1000

Discharge
Time Span

0 0

1915 1925 1935 1945 1955 1965

YEAR

Fig. 6.3 Discharge and time span reduction at offtake point of Bhagirathi

shows the maximum discharge and the number of days for which it lasted at the
offtake point near Jangipur from 1915 to 1972.
The water-level at this point depended on the maximum and minimum levels of
the Ganga; therefore, fluctuations were not much. Minimum discharge depended
on the condition of the site while the maximum discharge and the time-span came
down substantially over the years with ups and downs in between. This shows that
the inflow into the Bhagirathi gradually reduced in quantity and duration. This is
clear from Fig. 6.3 where the average discharge and time-span variations have been
given. It is seen that the two lines are almost parallel, which means that the uniform
fall in the time-span depended on the reduction in the maximum discharge intensity
at the offtake point (Fig. 6.3).
To ascertain the nature of deterioration of the river's non-tidal reach, the changes
of low-water stages at selected places at Jangipur, Beharampore and Katwa may be
examined. Before 1915, they had a rising trend which was seen more at the upper
reach at Jangipur gauge than at Beharampore. By 1915, the mean trend of Jangipur
and Beharampore gauge became almost flat while Katwa gauge showed a steeply
falling trend. The bed gradient continued to be steep with rising sand deposits in the
lower reach. Owing to decrease in discharge at the offtake point, the river practically
remained stagnant in most part of the year. In the Hooghly tidal reach, the cubic
capacity of channel between Nabadweep and Cossipore (140 km) was measured in
different years from 1924 to 1963 and analysed in three segments. The capacity of
the channel gradually deteriorated, as shown in Table 6.3.
It is seen that the cubic capacity came down as the river flowed downstream,
because the movement of sand and silt from the sea along with flood-tide did not get
sufficient force to return to seaward again during ebb-tide and deposited in the river-
bed increasingly. Huge quantity of sediment moved up and down along with tidal
water in an oscillating way and deposited gradually downstream in conducive situ-
ations. Between Cossipore and Hospital Point, the cubic capacity, taking together,
or separately, the pre- and post-freshet season capacities, fell, indicating loss in the
channel section. As the two banks were more or less stable because of industries,