Sub-tidal water-level oscillations in the Mandovi estuary, west coast of India

Abstract Using water-level data collected at six locations during March–April 2003 in the main channel of the Mandovi estuary, one of the 50 odd estuaries on the west coast of India, we describe the nature of variability of water level in the estuary at periods longer than two days. We then examine the causes behind the variability using data on wind and current measured near the mouth of the estuary. The main channel of the estuary is 50 km long and March–April is a period when local winds are weak. Our analysis shows that an important factor that generates sea-level variability at the mouth of the estuary is the northward propagating coastal-trapped waves, which are frequently observed along the coast and have sea-level variations that are in approximate geostrophic equilibrium. We also raise the possibility that local winds generate currents and associated sea-level variations. The relationship between the wind, the current and the sea level in this case, which is expected to be ageostrophic, needs further study. Once generated, a sea-level oscillation at the mouth propagates into the estuary undamped. Geometric amplification arising from convergence of the estuarine channel from mouth to head balances frictional dissipation to sustain the undamped propagation.