Physics

Prof. Satish R. Shetye
Former Director, NIO, Goa and Former V.C., Goa University

Abstract  :

An estuary is a partially enclosed coastal body of brackish water with one or more freshwater carrying rivers or streams flowing into it, and with a free connection to saline waters of open sea where tides push in and pull out water from the coastal body. About a hundred such bodies occur along the coast of India, each usually having on its coast an industrial or commercial urban area. Of these about 50 are on the west coast. Most receive freshwater from the rivers that originate in the Western Ghats. They are relatively small in length, compared to the estuaries on the east coast, and receive high runoff on during the Indian Summer Monsoon (ISM), approximately June-September. The Mandovi, in Goa, is typical of these estuaries and is the best studied among them. In this talk I will first discuss the characteristics of freshwater runoff into the Mandovi estuary using observations and simulations of a hydrological model. Hydrological modelling is important in India because very few of its rivers are regularly gauged. The nature of salinity variation in the estuary will be described next using a year-long set of observations on salinity field. Then a numerical model of the estuary to simulate its annual cycle will be discussed. The runoff (from the hydrological model) and tides at the mouth of the estuary are inputs to the model. Salinity and tides within the estuary simulated by the model compare well with the observations. This increases confidence in analysis of model output to study dynamics of the estuary, its residual (time-averaged) circulation, for example. This analysis shows that because of the occurrence of ISM, the estuary goes through all classical estuarine classifications: well-mixed, partially-mixed and salt-wedge type. In fact, the estuary turns into a river during intense spells of rainfall over the catchment area of rivulets and streams that feed it. This special character of the Mandovi and of other estuaries along the west coast has earned them the name monsoonal estuaries. Many of our estuaries are under threat because of excessive dumping of sewage and industrial waste. They deserve to be kept healthy enough to support their unique environment which serves as nurseries for fisheries along the coast.