Study On Flow-pollutant Improvement And Diversion Optimization Of Xihu-zuohai Lakes Group In Fuzhou City

The urban lakes play an important role in tourism, local microclimate regulating, rainfall and flood discharge, fishery, Industrial and agricultural production, and so on. It just likes heart and lung of a city. While for chasing maximum economic benefits, people have begun the predatory development and utilization of the lake. The quality of lakes water is becoming worse and worse. To protect environment of urban internal lakes is a fundamental requirement of the harmonious society, so it is really urgent to control the pollution of urban lakes.In this paper, the lakes of Xihu-zuohai were taken for study, it can be considered as shallow water lakes group to be composed of several lake units. Considering the flow and water quality interrelationship among lakes units, the lakes group’s water environment improvement is more complex than single lake. To solve water environmental problems urgently needs support of corresponding scientific theory and related technology. This paper uses a planar 2D hydraulic and water quality mathematic model to analyse the appropriate proposals of diverting and flowing water. Research contents and results are as follows:(1) Based on the idea of the Analytic Hierarchy Process (AHP) applied in the evaluation system and the water characteristics of the Lake Group of Xihu-zuohai in Fuzhou city, this paper adds two evaluation parameters, engineering cost and turbidity change rate, constructing a comprehensive evaluation system to evaluate the proposals of improvement of lakes’diversion water referred as "ESLI", which include water quality improvement effect indicators, ecological dynamics indicators, economic indicators and sensory indicators. The theory of this evaluation system is multi-objective linear weighting method. Furthermore, the system combining quantitative analysis with qualitative analysis, which can reflect the improvement effort of the lake diversion program more reasonably and objectively. Therefore, this system can be used for comparison and selection of lakes water quality improvement program.(2) Considering the current situation of too much stagnant water, poor water quality and water turbidity of Xihu-zuohai in Fuzhou city, this paper analyses the link among the units of the lakes group and the water source, selects a reasonable diversion path, and forms four flowing water ideas as follows. ①Set pump stations to pump lake-water centralized, and then revitalize water with adding distributed inlets at the bottom of lake after the water was pressured. ②Based on the previous idea, the zuohai upstream was chosen as source water. ③Communicate the lake units by tubes. ④Set submerged weir locally. After a comprehensive comparison of various diversion scheme with ESLI, we can find that adding inlets and outlets can reduce the area of stagnant water, and can improve the water quality and sensory, but it costs too much. Setting submerged weir can significantly improve the local hydrodynamic, water quality and turbidity, but it’s less economic. The method of setting communicating pipe uses the diversion path that most conducive to the diffusion of pollutants to achieve a significant effort to improve the water quality and sensory conditions of the lakes group, which cost the least money. The above water-revitalization idea follows the standards of minimizing stagnant water area and fastest of facilitating the pollutant diffusion at the lowest of engineering cost, which has a certain reference value to the lakes flowing water program design and implementation.(3) Setting submerged weir can obviously improve the local water environment, but it also has some drawbacks like poor water quality in water-blocking zones, front of the weir and high engineering cost. Consideing the advantages of conductive to artificial adjust lakes internal flow state and is good for operations management of the inflatable rubber weir, this paper optimizing diverting and flowing water combined with engineering measures. The results show that the reasonable planning of the rubber weir can achieve the organic unity of good ecological benefit, economical benefit and social benefit. The elements including human being, lakes, weirs and surrounding environment also can achieve a harmonious coexistence.