Analysis And Prediction For The Characteristics Of Supersaturated Dissolved Oxygen Downstream Of Hydrostructures

The dissolved gas concentration downstream of hydrostructures is often dramatically enhanced up to supersaturated due to the intense air entrainment in spillway flow, which is adverse to aquatic life. In this dissertation, BP Neural Networks, calculation method based on mechanism analysis, and numerical simulation are used to predict the dissolved oxygen percent saturation in the water downstream of hydrostructures, the results of which set a scientific and technical basis for the management of water environment of the Three Gorges Project.Considering the non-linear relationship of the pollutant and its environmental effect, a Probabilistic Neural Networks water quality evaluation model is developed, and it is applied to the field monitoring data in the dam area near the Three Gorges Project. The evaluation result of the oxygen-balance parameters is favorable. However, it is observed that the dissolved oxygen enhanced in spillway flow by means of the field data, and the downstream water appears supersaturation.Based on mechanism analysis, equilibrium concentration and equilibrium parameter, taken hydrostatic pressure into account, are prensented as the judgement criterion of dissolved gas supersaturation phenomena. The direct causes of supersaturation are the sudden change of water temperature or surrouding pressure and the diffusion of dissolved gas. In practical project, it is useful for supersaturation judgement by combination of saturation concentration and equilibrium concentration.A BP Neural Networks model is developed to predict dissolved oxygen percent saturation downstream of hydroprojects. According to mechanism study and field data analysis, the upstream dissolved oxygen percent saturation, tail water depth and discharge ratio are chosen as the input parameters of the network. By training and verifying, the BP Networks predict effectively for some cases of the Three Gorges Project and the four hydroprojects along Columbia River.Furthermore, a calculation method based on mechanism analysis for the downstream dissolved oxygen percent saturation of hydroprojects is also developed. By the anlysis of the air entrainment characteristics and the assumption of the gas bubble swarm distribution form, the presented method is developed to easily predict the dissolved oxygen percent saturation downstream of the Three Gorges Project at different typical reservoir water elevations. The results shows that, the maximum value of the percent saturation appears in the area at a short distance downstream from spillways, where the powerhouse flow and the spillway flow have not mixted together. The dissolved oxygen percent saturation at the Huanglingmiao section about 12km downstream from the dam will be as much as 140% if there were a flood occured only once every 100 years, and the water at the depth less than 4m will be supersaturated. Supersaturation phenomenon will be found in deeper water in the case of the appearance of a larger flood.Numerical computation is used to simulate the flow field and the distribution of the dissolved oxygen percent saturation of the Three Gorges Project with 175m water elevation. Equilibrium concentration is introduced in the source term for the dissolved oxygen, which presents the influence of water depth. The simulation supports the results of the calculation methond based on mechanism analysis with a good agreement, and it demonstrates the longitudinal and vertical distribution characteristics of the dissolved oxygen percent saturation.