Numerical Modeling Of The Dispersal And Mixing Processes Within The Plume Of The Changjiang River Estuary

A three-dimensional hydrodynamical module of COHERENS (a COupled Hydrodynamical-Ecological model for REgional and Shelf Seas), is used to study temporal and spatial variation of the dispersal and mixing processes, and their controlling factors, within the Changjiang River plume. The thesis is presented in three parts:The first part of this thesis deals with a schematized tidal estuary, representative of the Changjiang River. A sensitivity study is made using the COHERENS model to assess how the dispersal and mixing processes within the plume in a schematized tidal estuary respond to (i) the horizontal grid resolution; (ii) the vertical grid resolution; (iii) each of four different advection schemes in the momentum equations (the upwind scheme, the Lax-Windroff scheme, the TVD scheme with the superbee limiter, and the TVD scheme with the monotonic limiter); (iv) those in the salinity equation; (v) the horizontal diffusion coefficients of flow (νH) and salinity (λH); and (vi) the vertical eddy viscosities of flow (νT) and salinity (λT ), respectively. (1) The dispersal and mixing processes within the plume appears to be relatively insensitive to the horizontal and vertical grid resolutions. (2) The dispersal and mixing processes within the plume are more sensitive to the advection scheme in the salinity equation than that in the momentum equations. Application of the Lax-Windroff scheme in the salinity equation may lead to unstable results. (3) The horizontal expansion of the plume is sensitive to the horizontal diffusion coefficients of flow (νH) and salinity (λH). The northward and seaward expansions of the plume and its thickness increase with both increasingνH andλH. The southward expansion of the plume and its horizontal salinity gradient decrease with both increasingνH andλH. (4) The dispersal and mixing processes within the plume are sensitive to the vertical eddy viscosities of flow (νT) and salinity (λT ). The northward expansion of the plume and its thickness increase with both increasingνT andλT . The southward expansion of the plume and its vertical salinity gradient decrease with both increasingνT andλT . The width of the plume bulge decreases with both increasingνT andλT . The width of the coastal current increases with both increasingνT andλT . Studies are also made of the responses of the dispersal and mixing processes to the river discharge, the longitudinal bottom slope, the Coriolis force, steady winds, and the M2 tidal constituent. (a) The horizontal expansions of the plume in all directions increase with increasing river discharge. (b) The northward and seaward expansions of the plume increase with increasing longitudinal bottom slope, while the southward expansion decreases with increasing longitudinal bottom slope. (c) The Coriolis force deflects the symmetrical plume southward. (d) The southeast-northwesterly directed M2 tidal constituent suppresses the horizontal expansion of the plume, but strengthens the vertical mixing of the plume water with sea water. (e) Under 6 m/s of south-southeasterly steady winds, the plume expands northeastwards. Under 4 m/s of northwester steady winds, the seaward expansion of the plume is suppressed, and strong downwelling currents occur at its seaward edge.The second part of this thesis deals with the Changjiang River estuary with its bathymetry being divided into rectangular grids (188×140). The grid cell dimensions are 1045.5 m×1038.5 m. COHERENS is used to model the dispersal processes within the Changjiang River plume in the flood season under the influence of the M2 tidal constituent. (i) The Changjiang River plume spreading seaward in the form of a fresh water tongue is mostly discharged into the South Branch of the Changjiang River estuary. Four fresh water tongues are formed, they occur outside the North Channel, between the North Channel and the North Passage, between the North Passage and the South Passage, and outside the South Passage. The planar shapes of the fresh water tongues are modulated by the M2 tidal constituent. They advance seaward during the maximum ebb tide, while they retreat landward during the maximum flood tide. (ii) Four fresh water tongues within the Changjiang River plume spread eastwards, east-southeastwards, southeastwards and south-southeastwards. On the whole; however, the Changjiang River plume spreads southeastwards. (iii) Horizontal salinity gradients are large outside the North Channel and the North Passage, and apparent plume fronts are formed there. Both the location of the plume and its frontal intensity, being modulated by the M2 tidal constituent, display tidal variability patterns. At low tide, the plume front is farthest from the mouth of the Changjiang River estuary, while one hour after high tide, it is closest to the mouth of the Changjiang River estuary. Maximum plume frontal intensity occurs around the high slack water, while minimum plume frontal intensity occurs around the low slack water. There is one hour lag between maximum plume frontal intensity and the landward movement of the plume, and between minimum plume frontal intensity and the seaward movement of the plume.The third part of this thesis deals with the plume within the Changjiang River estuary with its bathymetry being divided into orthogonal curvilinear grids (149×69). The grid cell dimensions range from 283 m to 5583 m. An improved COHERENS model in orthogonal curvilinear coordinates is used to model the dispersal and mixing processes within the Changjiang River plume under the forces driven by M2, S2, K1, and O1 tidal constituents in the flood and dry seasons, respectively. (i) Depending on the Changjiang River discharge and tidal regime, the Changjiang River plume spreads southeastwards in the form of jet flow, or a circular bulge, or a fresh water tongue. (ii) There is a two-layer structure along the longitudinal section within the Changjiang River plume: the upper buoyant plume and the lower vertically homogeneous layer. The thickness of the upper buoyant plume is smaller in the flood season than in the dry season, larger during the spring tide than the neap tide, and smaller at the maximum flood tide than that at the maximum ebb tide. (iii) The salinity stratification within the Changjiang River plume appears to be controlled by tidal mixing, and estuarine circulation resulting from interaction between the Changjiang river discharge and tides. The salinity stratification is stronger in the flood season than in the dry season, weaker during the spring tide than the neap tide, and stronger at the maximum flood tide than at the maximum ebb tide. The salinity stratification displays seasonal/fortnightly/tidal variability patterns within the Changjiang River plume. The improved COHERENS model is also used to model the dispersal and mixing processes within the Changjiang River plume under the action of combined tidal constituent and steady winds in the flood and dry seasons. (i) Under 6 m/s of south-southeasterly steady winds, a part of the surface Changjiang River plume expands eastwards in the form of cloudy patches during the spring tide in the flood season, while the Changjiang River plume expands east-northeastwards as a whole during the neap tide. Under 4 m/s of northwestly steady winds, the seaward dispersion of the Changjiang River plume is suppressed in the dry season, and it expands southeastwards in the form of a fresh water tongue. (ii) Under 6 m/s of south-southeasterly steady winds, the thickness of the upper buoyant plume within the Changjiang River plume increases in the flood season. Under 4 m/s of northwesterly steady winds, the vertical salinity gradient within the upper and lower layers of the Changjiang River plume is small, and thus the plume is nearly vertically homogeneous, except at the maximum flood tide during the neap tide in the dry season. (iii) Under 6 m/s of south-southeasterly steady winds, the stratification within the Changjiang River plume is enhanced. Vertical stratification was estimated by using Simpson's stratification parameter (φ). The stratification parameter increases about 20-80﹪more than without wind in the flood season. Under 4 m/s of northwesterly steady winds, the stratification at the upwind side of the Changjiang River plume is enhanced, while destratification occurs in other parts of the plume in the dry season. It is suggested that the stratification or destratification within the Changjiang River plume appears to be controlled by the direction of the wind. Furthermore, the upwelling-favourable wind drives the plume to disperse seawards and tends to enhance the stratification, while the downwelling-favourable wind drives the plume to move landwards and causes destratification within the Changjiang River plume.