Decline of Bhagirathi-Hooghly Channel - The Ganga: Water Use in the Indian Subcontinent - page 69

than the ebb-tide, 5 and 7 h approximately; therefore, the intensity of flow-flux in the
Hooghly is more than that of the ebb-flux. During flow-flux, huge quantity of sand
and silt enters the Hooghly up to the tidal limit owing to very high velocity, forming
a high wall of water. But because of longer duration of the ebb-flow, its velocity
is reduced, causing partial deposition of sediments on the bed. Moreover, there is
a slack period between the end of flow-tide and the beginning of ebb-tide and vice
versa. Maximum deposits occur during the slack periods, the ebb-flow alone cannot
scour the entire bed, resulting in gradual rise of the bed-level. Submerged bars also
rise gradually owing to such deposition. The river capacity is thus decreased grad-
ually, blocking the navigation channel. This, in turn, accelerates siltation, affecting
the overall performance of the river. If the upland flow is maintained, the siltation
cannot affect the river-bed much, as the ebb-flow is supplemented by upland dis-
charge. Deposition is scoured and will keep the river-bed free and the navigation
channel is also not affected. But if the upland flow is reduced, or stopped, silta-
tion accelerates. Another adverse effect is that tides gradually push upward and the
salinity of water increases. In this way, the rise in the bed level of the Hooghly has
synchronised with that of the Bhagirathi and the entire Bhagirathi-Hooghly river
system has been affected.
As stated, the Ganga carries huge silt-load during three monsoon months; except
wash-load, it is nominal in nine dry months but increasing from June, near Farakka,
the maximum silt-load being in August and September. It decreases thereafter
and by the end of November, the flow becomes relatively free from silt-load.
The total silt-load, carried by the Ganga at Farakka, from June to November, is
about 180-200 million cubic metres. During monsoon, the discharge through the
Bhagirathi from the offtake with the Ganga is unrestricted. The discharge carries
more or less the same intensity of load from the Ganga. The total silt-load passed
through the offtake of the Bhagirathi between June and November will be about
8-10 million cubic metres. The tributaries of the Bhagirathi-Hooghly also contribute
sufficient silt-load during those monsoon months. A rough assessment of silt-load
is given in Table 6.1.
The silt-load in the Bhagirathi has reduced at its offtake point after the con-
struction of Farakka Barrage and the entry of discharge was controlled but owing
to increased bank sloughing at various places in the river, as described before,
about 8.5 million cubic metre soil is added to the flow, making no difference

Table 6.1 Annual silt load of the Bhagirathi-Hooghly tributaries

Sl. No. Name of the tributaries Silt load in million M3

1 Bhairab-Jalangi 10.0

2 Mathabhanga-Churni 2.0

3 Damodar 2.0

4 Rupnarayan 6.0

5 Haldi 4.0

6 Rasalpur 4.0

7 Others 2.0

Tot a l 30 . 0