Influence of Dam-Controlled River Discharge and Tides on Salinity Intrusion in the Godavari Estuary, East Coast of India

Abstract

The Godavari is an important tropical monsoonal river located along the east coast of India. The Gautami-Godavari is one of the two major estuarine branches of the Godavari River bifurcated by a low dam at Dowlaiswaram. The study focused primarily on the dam-controlled freshwater fluxes using extensive field data on hydrodynamics and numerical modeling of tidal currents. A two-dimensional hydrodynamic model was used to simulate the tidal circulation for the period 29–21 by forcing it with predicted tides, river discharge from the dam, and rainfall. The model produced accurate results with negligible bias in the current speed during zero-discharge conditions and a bias of approximately .8 m·s− 1 during moderate- and high-discharge conditions. The observations and numerical modeling, suggest that the estuary is dominated by tides during the dry season (December–May) and by high freshwater fluxes during the wet season (June–November) associated with the Indian Southwest Monsoon (ISM). During the ISM, the salinity and stratification were found to be highly variable and were mostly controlled by the dam’s discharge. The characteristics of the estuary were transformed from a salt-wedge type in February to a river type in November, instead of during the peak monsoon season (August), indicating extensive variability in the dynamics linked to the freshwater fluxes from the dam. From the hourly observations of salinity at Yanam station over a tidal cycle, the presence of saltwater intrusion was observed even during high river discharge in the flood (currents moving from sea to the estuary) period. The magnitude of the currents strongly varied between .2 and .8 m·s−1 during different tidal phases. It was found that the flushing times were mostly controlled by river discharge during the wet season and by the tides during the dry season.