Only a Barrage Can Save! - The Ganga: Water Use in the Indian Subcontinent - page 90

A. C. Mitra in his report observed that under present conditions, headwater dis-
charge of 40,000 cusecs will be necessary during the non-freshet season to neutralize
the landward drift of sediment in the tidal portion. Less discharge, say of 30,000-
35,000 cusecs, would reduce the total volume of accretion and hence the rate of
deterioration but the zone of most significant accretion which was above Kolkata
then would shift seaward and be most pronounced between Garden Reach and
Babughat where the major docks, jetties and moorings were located. If deteriora-
tion in the non-freshet season can be arrested by sustained headwater discharge of
40,000 cusecs in this season, improvement that would follow from freshet flows,
every year, would gradually improve the river condition. If and when improvement
could be substantial and reverted the river's regime to that from 1930 to 1935, the
requirement of headwater discharge to prevent deterioration of the regime in non-
freshet seasons would be somewhat less. This reversion could be expedited with a
discharge of 45,000 cusecs in the first few years of operation of the Feeder Canal.

View of Dr. J. J. Dronkers

Dr. J. J. Dronkers visited Kolkata in November 1968 at the invitation of Calcutta
Port Commissioners, and in his report, based on prototype data, observed:

To balance the sand movement of Garden Reach, the ratio of the flood and ebb flow must be
unity. Superimposing the corresponding ebb velocity needed in the period, together with the
change in the period of flood and the ebb, it is seen that a discharge of the order of 50,000 cfs
(1430 m 3 /sec) need to be maintained. Taking a conventional allowance of ± 10% on such
computation, it would appear that for arresting the upland sand movement above Calcutta
for a range of 15 feet inlet tide at the Sagar sea-face, the lowest discharge of the order of
45,000 cfs (1280 m 3 /sec) would be necessary. The ebb sediment movement with upland
discharge would be present in the lower ranges of the tide but the most significant landward
movement need to be arrested, when the ranges of tides are between 9 feet and 18 feet;.
Therefore, taking the mean significant tide for sediment movement as 14 feet;, the order of
discharge on the lower limit would be 40606 cusecs (1150m 3 /sec).

Prof. Hensen was again consulted in November 1971 by a team of officers of
Calcutta Port, who carried with them all up-to-date data and analysis to get him
ascertain the scope of reduction of the discharge, if at all. After due examination,
Prof. Hensen in his report in the same month said:

A supply of the order of somewhat higher than 40,000 cusecs is needed, throughout the year,
to reverse the process of sanding up the ship route to Calcutta harbour . Ship routes are

ebb-tide oriented during freshets; they change to flood-side between November and May. If
headwater is not maintained at 40,000 cfs in these seven months, the ship route in crossing
would quickly move from ebb to flood and would stop passage of ships.

...

Dr. K. L. Rao's contention that the minimum requirement of the Hooghly to save
Calcutta port was 40,000 cusecs, was confirmed by S. S. Dhawan, then Governor
of West Bengal in a letter to Indira Gandhi on 27th August 1970. Later on, Dr. Rao
changed his mind and advocated supply of head water into the Hooghly in a phased
manner, varying from 40,000 to 20,000 cusecs in dry season.