The calculation of flushing time for the upper Pasur River Estuary, Bangladesh

Abstract Estimation of estuarine flushing time, a time required to transport of pollutants or any other properties from estuaries to the coast, is very important for its resource management. In this study, we estimated flushing time (T) of the upper Pasur River Estuary (UPRE) for understanding the water quality condition in the dry and wet seasons. High-resolution salinity data were collected from the PRE at high water in the dry and wet seasons in 2014 and 2019. Flushing time was calculated using the freshwater fraction method (FFM) as well as e-folding flushing time scales was estimated empirically using the salinity (non-reactive conservative) and monthly river discharge data. System flushing during the dry season was thirteen times weaker than flushing during the wet season owing to decreasing river discharge by nearly 94%. In addition, the daily exchange volume was decreased by eight times during the dry season than during the wet season. As a consequence, the conditions of the UPRE are more dynamic during the wet season due to receiving huge amount of river discharge. During the dry season, only the e-folding time scales showed higher values in the salinity maximum zone (salt plug area). This implied that the e-folding time scale is an empirical approach and was able to encompass the tidal dispersion process whereas the FFM was unable to include that process. As the PRE is a macrotidal estuary, the tide assists to flush dissolved substances from the UPRE to the coast during the dry season having negligible river discharge. In addition, there was no significant variation in water quality parameter between the salt plug area (SP) and downstream of salt plug area (DSP) during the dry season. In order to have more accuracy, a three-dimensional hydrodynamic model would be useful to compute estuarine time scales precisely.