Propagation of tides in the Mandovi-Zuari estuarine network

Located in Goa on the west coast of India and joining the Arabian Sea, the Mandovi and the Zuari are two estuaries, each about 50 km long, connected by a narrow canal. A number of small rivers join the two estuaries, forming a network of channels, whose cross-sectional area decreases rapidly in the upstream direction. They receive large freshwater influx during the southwest monsoon and little during the rest of the year. During April (dry season) and August (wet season) 1993, the water level and salinity at 15 locations in the network were monitored for 3 days to determine characteristics of tidal propagation in the network. Analysis of the data shows that the speed of propagation of both the diurnal and the semi-diurnal tide through the main channels of the network is approximately 6 m/s. Amplitudes of these tides in the channels remain unchanged over a distance of about 40 km from the mouth and then decay rapidly upstream over the next 10 km. The undamped propagation is a consequence of the balance between geometric amplification, due to decrease in the cross-sectional area in the upstream direction, and frictional dissipation. The rapid decay near the upstream end of the channels appears to result primarily from freshwater influx.