Hydrographic characteristics and tidal prism at the Cochin Harbour mouth

Abstract

Diurnal and seasonal variations (May 1975 to April 1976) in hydrographic characteristics at the inlet of the Cochin Harbour, Kerala, India are presented. During the SW monsoon season under the predominant influence of fresh water influx into the estuary no appreciable variations in the temperature and salinity values of the thin surface layer (< 4 m) are observed in relation to tide; whereas the subsurface water (4 m to bottom) shows close relation with the tide. Significant vertical gradients in temperature and salinity are noticeable between surface and bottom layers during this season. prominent saline wedge is formed with the flood current in this season and during the ebb current homogeneous water of low salinity (< 10 %) pervades the entire depth of the harbour mouth. Mean vertical salinity profiles for the monsoon, post and premonsoon seasons indicate that the highly stratified conditions that exist during the monsoon season gradually change to partially mixed and homogeneous conditions of the post and premonsoon periods apparently due to decreasing river flows and increasing tidal influence. Maximum velocity of the current observed is 174 cm/sec which occurs during the ebb current of the spring tide of the postmonsoon season. Tidal prism in the premonsoon season during which time the fresh water addition to backwaters is least is found to be 31.5 x 10⁶ m³. During the other seasons, the values of tidal prism vary between 9.5 x 10⁶ m³ and 132 x 10⁶ m³. Factors effecting the stability of the inlet are discussed.