4 - Changjiang Estuary

Abstract

The Changjiang Estuary has an extremely dynamic hydrological environment due to runoff, tide, wind, mixing, topography, and continental shelf current, which are the main dynamic controlling factors on hydrodynamic processes. Significant semi-diurnal, fortnightly, and seasonal tidal variations have been found in the estuary. The maximum monthly tidal range exhibits two peaks in March and September and two valleys in June and December each year. As the tidal wave propagates up-estuary, the tidal range becomes smaller due to bottom friction and runoff. Considering the combined effect of river discharge, tide, wind, and baroclinic force induced by the density gradient, the simulated winter residual currents in the South Branch (SB), South Channel (SC), North Channel (NC), and North Passage (NP) flow seaward due to river discharge, and have a larger amplitude in these channels than over tidal flats. In the sand-bar areas between the South Passage (SP) and NP, water is transported northward across the tidal flats. In the North Branch (NB), the residual current is weaker, and there is net water transport toward the SB in winter. The summer residual currents in the SB, SC, NC, and NP flow seaward, and have a larger amplitude than the winter residual currents due to high river discharge. Saltwater intrusion in the Changjiang Estuary is controlled mainly by river discharge and tide, but is also influenced by wind, topography, river basin and estuarine projects, and sea level rise. There is a net landward flow in the NB when river discharge is low during spring tide, resulting in a type of saltwater intrusion known as the saltwater-spilling-over, which is the most striking characteristic of saltwater intrusion in the estuary. Strong northerly wind enhances the net water and salt fluxes from the NB into the SB, and the wind-driven circulation flows landward in the NC and seaward in the SC, which enhances the saltwater intrusion in the NC. Wind is an important dynamic factor of saltwater intrusion in the estuary. Both the intensity of saltwater intrusion and stratification increase as sea level rises, and there are obvious inter-annual changes in different river discharges. The Stokes drift transport is the major mechanism for the up-estuary salt transport in each channel, while the seaward Eulerian transport is of the same order in amplitude as the Stokes drift transport. The impact of the deep waterway project is that the saltwater intrusion has been alleviated distinctly in the NC because the dykes of the project blocked off the southward drift of the brackish water plumes under the northerly monsoon and the Coriolis force. The saltwater intrusion in the NP has been intensified at the upper section and alleviated at the lower section. The impact of the Three Gorges Reservoir (TGR) is that saltwater intrusion during the dry season is suppressed as the TGR supplements river discharge. The operation of the TGR is basically favorable for reducing the burden of freshwater supply in the highly populated estuarine region. Hypoxia occurs off the Changjiang mouth because of the presence of excess nutrients, which can cause algal blooms, and of freshwater input, which can intensify stratification. The seasonal variation of pycnocline is consistent with hypoxia, indicating the pycnocline plays an important role in preserving hypoxic condition. The residual current speed at the bottom is small and favorable for the maintenance of hypoxia during summer.