Vulnerability of Cochin Backwaters to meteorological disturbances with special reference to tidal propagation

Abstract

Features of water level oscillation in the Cochin backwaters are presented in relation to its vulnerability to meteorological disturbances. A steep surge formed after a sharp atmospheric depression (8 mb) followed by storm (=20 m/s) and incessant rains (664 mm spanning over a fortnight) lead to a significant rise in estuarine water levels (-50 cm). Delay of =3 days between the depression and flooding is attributed to the run off time from catchment areas. Sensitiveness of this estuary to meteorological disturbances has significant implications on its ecological and environmental health. Spectral analysis of tides revealed some of the special features of a complex shallow estuary. As the tide propagates into the interior estuary, it undergoes progressive attenuation (-27%) and weakening of semi-diurnal character. But, the fortnightly tidal constituent (MJ unusually gets amplified with its crest (trough) coinciding with spring (neap) tides. Influence of M constituent increases =10-fold towards the upper reaches. Non-existence of the S sub(t) constituent, manifested throughout the estuary, is a peculiar feature. Progressive weakening of the tides' semi-diurnal character is due to the presence of OO, constituent, which, unlike other diurnal constituents, amplify with increasing distance from the entrance. Influence of OO, constituent increases =3-fold towards the upper reaches. Propagation delay of low (high) tide phase at the farthest location from the inlet is approximately 5.6 (4.6) h. Dominance of M sub(sf) constituent in the upper reaches is expected to render a fortnightly character to the flushing process, and in turn, the biological productivity of this region.