Study On Numerical Methods For Water Pollution Accidents In The Middle Route Open Channel Of The South-to-North Water Transfer

Once a water pollution accident breaks out in the middle route open channel of the South-to-North Water Transfer(SNWT), the characteristics and developing mechanism of the flow fields and pollutant concentration distributions caused by the accident must be quickly determined and effective measures are eventually taken against the accident. In this dissertation a 1-D and a 2-D numerical models are developed. Assumed accidents in a typical segment of the middle route main canal of SNWT are studied by the models.A 1-D numerical model with high accuracy and stability is developed in which the hydrodynamic and water quality equations are solved based on Preissman and Pressman-Holly schemes respectively. Based on the maximum entropy principle, a formula is originally derived to describe the longitudinal velocity distribution in the cross-section of a trapezoidal open channel. From the formula an equation to predict the longitudinal dispersion coefficient is proposed based on Fischer's triple integral relation for dispersion coefficient.A depth-averaged 2-D model is also developed. Employing a high-resolution scheme of flux difference splitting scheme (FDS) to deal with the advective terms of hydrodynamic and water quality equations, the model can handle various complicated flow fields and pollutant concentration distributions accruately. A novel upwind method by means of reconstructing vaiables in cells is designed to avoid handling the bed slope term directly. The boundary conditions can easily be specified. To model wetting and drying of 2-D shallow-water flows two novel methods called the submerged node method(SNM) and the conservational artificial porosity method are proposed. Hierarchy meshes and local time-step scheme are combined in the 2-D model to improve the model's acurracy and efficiency. Several typical problems are employed to verify the 2-D model and excellent results indicate that the model can accurately and efficiently simulate discontinuous flows such as dambreak flood, transcritical flow, or discontinuous pollutant concentration distributions. Assumed water pollution accidents in steady flows in a typical segment of the middle route main canal of SNWT are simulated using the two models. It is found that either 1-D or 2-D model can predict mean velocity of polltant clouds. The 1-D model is pretty suitable for rapid simulation. However, because mixing process in near field is not taken in account, the skewness of section-averaged pollutant concentration distribution can not be reproduced, and the velocity of the point of maxium section-averaged concentration is misvalued by the 1-D model. The 1-D model will also overvalue the length of pollutant clouds. The 2-D model can not only discribe the longitudinal pollutant distributions more exactly but account for the difference of the pollutant distribution resulting from the differennce among tranverse positions where pollutants enter flows. The faster pollutants enter waters and smaller is the transverse mixing coefficient, the more superior is the 2-D model. From the above results the characteristics of flow fields and pollutant transport in unsteady flows with fixed water level before downstream gate during pollution accident treating processes are simulated by the 2-D model. The simulations demonstrate that the upstream controlling gate should be shut during a period longer than twice travelling the channel length at shallow-water wave speed, and not only the mean velocity but the distribution of pollutant clouds are changed during shutting controlling gates. The study carried out in this dissertation has set a concrete basis for analyzing characteristics of water pollution accidents in the middle route open channel of SNWT and pursuing treatment to the accidents.