Estimation of flushing time in a monsoonal estuary using observational and numerical approaches

Abstract

Estimation of flushing time (T F) in an estuary is important for water quality analysis, and it is one of the major transport time scales used in estuaries to quantify the hydrodynamic processes and for water resources management strategies. Thus, the main objective of the present study was to estimate the T F in the Mandovi estuary (a monsoonal estuary) on the west coast of India. In the study, field observations of salinity (FOS) and a numerical model are used to describe the T F during the south-west monsoon (heavy river discharge), post-monsoon (moderate river discharge) and pre-monsoon (negligible river discharge). T F was calculated for 12 (months) river discharge scenarios (the calculation was done under monthly mean flow conditions). Among the 12 scenarios, four scenarios were within an extremely low flow range: 0.8–2.2 m3s−1; four were in a moderate flow range: 4.3–67 m3s−1; and the remaining four were in a high flow range: 119.0–506.6 m3s−1. The T F calculated from FOS and numerical model showed good matches during the periods of heavy river discharge and moderate river discharge. The results from FOS (numerical model) showed that the T F was 1.12 (1.11) days for a river flow of 506.6 m3s−1 (during the south-west monsoon), and the T F reached 251.28 (592.38) days under extreme low flow of 0.8 m3s−1 (during the pre-monsoon). Regression equations fitted by power and exponential functions were derived from FOS and numerical model simulations to correlate T F with monthly mean river discharges. The power regression equation derived from FOS (numerical model) showed good statistical fit with data (r = −0.997 (−1.0)) for any given river discharge compared to the exponential regression equation (r = −0.81 (−0.80)). Hence, the power regression equation can be used for the rapid evaluation of the impact of drought (during the pre-monsoon) and flood (during the SW monsoon) scenarios in the estuary.