Observation and Analysis of Water and Salt Transports in the North Branch of the Changjiang Estuary

摘要

The North Branch (NB) is the first-order bifurcation of the Changjiang Estuary and is an extremely shallow water channel. The greatest characteristic of saltwater intrusion is the saltwater spillover from the NB into the South Branch (SB). A field observation was conducted in the NB from 21 to 27 February 2017 to understand the water current and salinity variation. The residual unit width water and salt fluxes, as well as their decomposed terms, were calculated to discuss the mechanisms of water and salt transport. The tidal and vertical variations in ebb and flood duration, current velocity, and salinity during the neap-middle-spring tide were presented and dynamically analyzed in detail. The direction of the Eulerian water and salt transports and tidal pumping salt transport varied by site, vertical layer, and tidal type, whereas the Stokes drift water and salt transports were landward. During neap tide, the total residual water and salt fluxes in the upper layer of the whole NB were seaward transported because of the runoff effect; the total water and salt fluxes in the lower layer were landward in the lower-middle and lower reaches of the NB, reflecting the existence of a two-layer estuarine circulation. During the moderate tide, the total residual water and salt fluxes were seaward in the whole NB except in the lower layer in the low reaches of the NB, which were landward due to the strong baroclinic effect. During the spring tide, all the total residual water and salt fluxes were landward because some flood water flowed into the SB, resulting in an imbalance between the flood and ebb water volumes in the NB. The dynamic processes of water and salt transport from the NB into the SB during spring tide were revealed by the observation and flux decomposition method.