Based on a coupled hydrodynamic–ecological model for regional and shelf seas (COHERENS), a three-dimensional baroclinic model for the Changjiang (Yangtze) River estuary and the adjacent sea area was established using the sigma-coordinate in the vertical direction and spherical coordinate in the horizontal direction. In the study, changing-grid technology and the "dry-wet" method were designed to deal with the moving boundary. The minimum water depth limit condition was introduced for numerical simulation stability and to avoid producing negative depths in the shallow water areas. Using the Eulerian transport approaches included in COHERENS for the advection and dispersion of dissolved pollutants, numerical simulation of dissolved pollutant transport and diffusion in the Changjiang River estuary were carried out. The mass centre track of dissolved pollutants released from outlets in the south branch of the Changjiang River estuary water course has the characteristic of reverse current motion in the inner water course and clockwise motion offshore. In the transition area, water transport is a combination of the two types of motion. In a sewage-discharge numerical experiment, it is found that there are mainly two kinds of pollution distribution forms: one is a single nuclear structure and the other is a double nuclear (dinuclear) structure in the turbid zone of the Changjiang River estuary. The rate of expansion of the dissolved pollutant distribution decreased gradually. The results of the numerical experiment indicate that the maximum turbid zone of the Changjiang River estuary is also the zone enriched with pollutants. Backward pollutant flow occurs in the north branch of the estuary, which is similar to the backward salt water flow, and the backward flow of pollutants released upstream is more obvious.
Based on the Environmental Fluid Dynamics Code(EFDC),a three-dimensional baroclinic model for lower reaches of the Yangtze River,Yangtze River Estuary,Hangzhou Bay and the adjacent sea areas was established with the sigma-coordinate in the vertical direction and the orthogonal curvilinear coordinates in the horizontal direction.The fine silt particle tracers were injected during different tides with the calibrated three-dimensional numerical model in the Yangtze River Estuary to simulate the migration tracks of the tracers in the following three days,and thereby study the migration behavior of the fine silt particles of dredged sediment after dumping at different locations in the navigation channel in the Yangtze River Estuary.The study shows that the particle paths of the tracer particles basically migrate to the open sea along the middle axis of river course in the form of reciprocating oscillation,the migration and diffusion directions of tracer particles are basically parallel to the navigation channel in guide levee,similar to long belt distribution,and the tracer particles migrate to the southeast out of the guide levee.The particle which is on the southern and northern sides enter groin and cross the navigation channel in the process of movement,and is easy to deposit in the navigation channel.The migration tracks of the particles released at different layers are basically similar at the same release station,but specifically different,varying at different cross sections.The result can be used for the choice and evaluation of dredging schemes in the Yangtze River Deepwater Navigation Channel.
