Detecting the mechanisms of longitudinal salt transport during spring tides in Qiantang Estuary

ABSTRACT Saltwater intrusion has immediate influences on the water resources utilization, aquatic environment and ecological system. Hence, it is essential to detect the mechanisms of salt transport. This study adopted observations including instantaneous flow velocities, tide levels and salinity during 10 tide cycles at 2 stations in upstream and downstream of Qiantang Estuary in China. The variables of instantaneous flow velocity and salinity were decomposed into the time-average, time-varying and vertical-varying components using the flux decomposition model. Then, the salt flux components attributed to various physical processes were quantified. During spring tides, the longitudinal salt transport is controlled by advection transport and tidal pumping transport in the upper river reaches, while being mainly controlled by advection transport in the lower river reaches. The net water flow fluxes and salt fluxes are landward in the lower river reaches. However, the transport direction of the salt fluxes is inconsistent with that of the net water flow fluxes in the upper river reaches, where the net water flow fluxes are mainly seaward while the net salt fluxes are mainly landward. In general, there are significant positive correlations between the net landward salt flux and the tidal range. Thus, the tidal range can be used as an important indicator for determining the required amount of freshwater to be discharged from the reservoir in order to resist saltwater intrusion. The vertical distribution of salt in Qiantang Estuary is relatively uniform. The vertical shear force has limited influence on the salt transport. The research results are expected to supply a reference to future research on other macro-tide estuaries.