Influence of river inflow and its impact on the salinity variations and flushing time in a networked system, northwest coast of India

A numerical hydrodynamic modelling study has been implemented based on the seasonal salinity variations in a networked system (comprising creek and an estuary), which is the first of its kind attempted for the Indian subcontinent. Salinity variations in the estuaries and creeks exhibited unique characteristics caused by the combined effects of various external forces such as tidal flow, freshwater runoff, wind and geometric effects. Precise understanding of dynamical conditions in estuaries and creeks is necessary to address pertinent issues related to oceanography, water quality and ecosystem dynamics. In a broader perspective, it is noted that due to the influence of winds during monsoon, the salinity fields in the estuarine environment are not in a steady state. However, in creeks, tidal flow plays a major role in altering the salinity structure apart from runoff. The results from this study decipher the fact that the networked system was vertically homogenous during all seasons. However, a horizontal salinity gradient was observed in the system depending on the river runoff. The flushing time for the Ulhas estuary was about 1.5 and 2.57 days during the monsoon and non-monsoon seasons, respectively. Similarly, for the Thane creek, tide-driven flushing time was about 3.68 days. The low flushing time during the wet season provides a suitable dynamic environment for effluent discharge in the mid and upstream reaches of the estuary, wherein the freshwater influx is higher. On the contrary, during the dry season over this region, the low runoff and the highest flushing times can increase the pollution or can support the growth of phytoplankton biomass accumulation.