Silting In Navigational Channels Of The Cochin Harbour Area

Abstract

Suspended material in the Cochin Backwater varies considerably with the state of tides and seasons. The water column has maximum suspended material during the monsoon period, but its quantity declines progressively during the post-monsoon and pre-monsoon months. Short term changes induced by the tidal currents are quite large and during the pre- and post-monsoon months the total material transported from the sea into the backwater amounts to approximately 900 tonnes/day. The total amount of material dredged annually from the navigational channels is about 2 X10* m'. Of this, nearly 1.2 x 10° m'/year is dredged from the approach channel to the harbour. At one point, where much of the silting takes place during the monsoon months, the bed of the approach channel rises 1.2-1.8 m every year. In the adjoining backwater, where silting occurs during the pre- and post-monsoon months, most of the silted material gets resuspended during the monsoon months, and with the strong ebb currents, gets transported into the sea. Thus the two processes—transportation of the material from the sea into the backwater during the post- and pre-monsoon months and its retransportation from the backwater into the sea during the monsoon months—seem to alternate with each other. The peculiar pattern of circulation in the estuary influences the transportation of material into one of the inner channels where heavy silting occurs.