Geoscience Reference
In-Depth Information
Table 10.14
Mean navigable depths over six different bars below Calcutta port during lean
season
Year
Panchpara
Sankrail
Lower Munikhali
Pirsareng
Poojali
Moyapur
1
2
3
4
5
6
7
1974
5.20
4.99
6.11
6.27
5.52
3.91
1975
4.80
4.71
5.18
6.33
5.24
4.24
1976
5.48
5.77
6.72
6.45
6.00
4.18
1977
5.42
6.58
7.61
6.65
5.77
4.02
1982
6.33
8.27
8.64
7.69
6.68
4.38
1987
6.09
8.86
9.43
8.13
7.55
4.20
1992
6.23
8.34
9.09
7.76
5.74
4.39
1996
6.00
8.35
9.52
8.23
6.54
4.57
1997
6.27
8.68
9.32
8.06
6.13
4.55
1998
6.35
9.42
9.24
7.99
7.15
5.24
1999
6.17
9.46
9.17
8.58
6.26
5.07
(v) Poojali
10-44%, and
(vi) Moyapur
8-24%
Before 1975, all the bars below Calcutta up to the Hooghly Point needed reg-
ular, annual maintenance dredging of varying quantity, for movement of ships. In
1976 and 1977, the quantum of dredging was substantially reduced. In 1976, lower
Munikhali, Pirsareng and Poojali bars did not require any dredging; from next year,
Sankrail also needed no dredging. From 1978, all these bars except Moyapur did
not require any dredging. The upland discharge had maximum positive effect on
Sankrail and lower Munikhali bars, but compared to the year 1996, most of the bars
except Sankrail have considerably deteriorated.
The mean navigable depths (MND) over other five bars up to the Hooghly Point
are shown in Table 10.15.
Table 10.15 shows that none of the lower bars, except Roypur has improved much
after upland discharge from Farakka, despite being dredged continuously. Instead,
deterioration of Ninan and Eastern Ghat bars was faster since 1997.
The depth over the bars is utilized for calculation of draughts of ships, navigat-
ing to and from Calcutta by Calcutta Port Trust. Table 10.16 shows the governing
depths, available to Calcutta Port for movement of ships, round the year. Records
from November 1984 are shown in Table 10.16.
The table shows that navigable depths in the port area were falling gradually. The
depth of 3.5-4 m, obtaining for more than 150 days on an average before 1989-
1990 reduced thereafter and from 1995-1996, this depth was not available below
Calcutta, even for a day. The navigable depths gradually reduced to less than 3 m
for a long time since 1994-1995. The normal available depth below Calcutta was
3-3.5 m only.
Tidal bores in the Hooghly, especially in Calcutta Port area before 1975, was
another impediment to smooth navigation. Because of shallowness and restrictions
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