Cyclone and Storm Surge - Iczmpwb.org

3.9
ebb out as that flowed in, it follows that the average velocity of ebb is less than that of flow. Now the
sediment carrying capacity of flow is a direct function of velocity. Hence it follows that the ebb tide is
generally unable to carry back fully the sediment which has been carried up the tidal river during flow
tide. Consequently sediment will get deposited on the channel bed. Even a slight deposition of
sediment will go on accumulating, since the tides occur twice a day, throughout the year without a
break. As a result, the channel bed will begin to rise and its carrying capacity will deteriorate. The
deterioration will impede the propagation of tidal wave which would cause further deterioration and
the vicious circle would continue till the channel is completely dead.
To maintain the life of a tidal channel it is necessary to provide additional supply of water not
saturated with sediment, which has reserve capacity to pick up more sediment, to supplement the
tidal flow during the ebb so as to scour out fully that has entered the channel during the flow tide.
This can be done by (1) supply of upland water, (2) local drainage or (3) throwing open additional
spill areas. After the shifting of the main flow of the Ganga from the Bhagirathi to the Padma, upland
supply of sediment free flow got severely reduced to the Bhagirathi and the other distributaries of the
vicinity. The reduction was so severe that before Farakka Barrage diverted some flow into it, the
Bhagirathy remained completely cut off from the Ganga for full nine months in a year. The second
option is there; but local drainage is small in volume and seasonal in nature and so of very little
consequence. And it is very difficult to implement the third option. Anthropogenic interventions like
erection of marginal embankments for premature land reclamation and erection of fisheries and brick
fields have actually drastically reduced natural spill areas. Hence, it is extremely important to monitor
the condition of these tidal channels in and around Kolkata and to ensure their sustenance.
3.8. Deteriorating state of health of river Bidyadhari
The recommendations (1924) of the Calcutta Improvement Trust were made with the supposition that
the Bidyadhari river was capable of carrying off the entire volume of sewage and drainage effluent
from the city. But unfortunatery; the river by this time had already shown signs of distrees. When the
Central Lake Channel of the Bidyadhari river was selected as the city's outfall, the river was in fine
shape. It was 1500ft. wide and 60ft. deep at low water. Bidhyadhari used to get a considerable
supply of upland water including a portion of the dry weather flow of the Ganga (Bhagirathy channel),
which carried the major part of the flow of theGanga till the end of the 15th Century or early in the
16th Century. It also received the upland flow of the local channels like the Sunti, Nowi and Nona
Gang. Even when the main flow of the Ganga was diverted, it seems probable that it still continued to