Numerical Simulation Of The Hydrodynamics And Water Quality Of Confluence Of The Two Rivers In Chongqing

Yangtze River and Jialing River are the main sources of living and industrial waterin Chongqing. With the development of economy, the living sewage and industrialwaste water into Yangtze River upstream is increasing day by day, in addition, thecompletion of Three Gorges Reservoir will weaken the self-purification of water in thetwo rivers, therefore, the water-pollution problem in Chongqing becomes more andmore severe. This paper takes the confluence of Yangtze River and Jialing River asstudy object, and simulates the hydrodynamics and water quality with two-dimensionalfinite element numerical model. Then validates and analyses the computed results, inorder to provide scientific date and qualitative consultation for the water environmentprotection and sustainable development of Chongqing city.Numerical computation in this paper bases on the theory of shallow waterhydrodynamics. Considering the width is much more than the depth in natural river, theassumption that the horizontal flow velocity in the calculated domain is depth-averageignoring the vertical velocity and acceleration, is taken in this paper. The 2D shallowwater equations, which is depth-integrated of the 3-dimensional incompressible fluidmovement equations, is applied to describe the river's flow. The diffusion-convectionequation is numerically solved to compute the transfer and diffusion of contaminant,with the hydrodynamics results as its input data. The contaminant is considered asconservative constituent with an assumption that it has a low concentration withoutdecay, and has no influence on the river hydrodynamics.Finite element model is applied to the numerical computation, using the Galerkinmethod of weighted residuals to discrete the hydrodynamics and water qualitygoverning equations. The two kinds of governing equations are solved by combination.The SMS model (Surface-water Modeling System) is introduced to the pro-process,after-process and numerical computation in this paper for the finite element model oftwo rivers in Chongqing. By comparing with measured data, the computed resultsaccord with the reality approximately, the computing accuracy is good enough.The flow result shows that due to the wriggling land boundary and complexunder-water topography, there exists an obvious vortex at Liangtuo which located nearconfluence downstream. The velocity varies acutely in this area, and it becomes abackwater. The drifters will be lingering here and difficult to move downstreamnaturally, so drifters elimination should be paid more attention in this area. In addition, afew of drogues will be lingering temporarily in the area near Jialing River Bridge andisland Shanhuaba, where have a large boundary curvature. Diffusion of conservativeconstituent from the assumed discharge point is modeled, the result shows that, in thegiving hydrodynamic condition, the transfer and diffusion rate of contaminant is high, itwill not influence the calculated domain obviously. In the early stage of sewagedischarging, it forms a pollutant band at the boundary near the discharging point. Thenit will diffuse entirely in the whole riverway down to the convergence. So it is suggestedthat the sewage should be cleaned before discharged, or else it would deteriorate thewaterquality down from discharge point.