Numerical Simulation Of Hydrodynamic Field In Gonghu Bay Under The Background Of Yangtze River-Taihu Lake Water Transfer Project

In recent years, water environmental problems have become increasingly prominent. With the rapid development of economy and society in Taihu Lake basin, a large amount of sewage pulled into the lake, leading to poor water quality and eutrophication. In 2007, the northern part of Lake Taihu outbroke large aera of algae blooms, resulting in Wuxi drinking water crisis which shocked home and abroad. Currently, the government is trying to improve water environment by transferring water from Yangtze River to Taihu Lake, combating eutrophication and algae blooms. But there was a huge controversy that role of this project to alleviate the ecological problems was limited. Therefore, the present study chose Gonghu Bay as study aera which directly connected to channel of Yangtze River- Taihu Lake water transfer project. Based on data surveyed from March 2013 to February 2014, we determined the characteristic regions of Gonghu Bay and assessed temporal and spatial characteristics of water environment. Supported by lots of field survey data, FVCOM model was selected to simulate flow and wave field in Gonghu Bay. Then we analyzed the effect of flow and wave field on water quality in Gonghu Bay and discussed correlation between hydrodynamic factors and environmental factors. Lastly, we set different primers drainage scheme as model input and evaluated the influence of different primers drainage scheme on flow field in Gonghu Bay.(1) Based on physical indicators(WT, p H, DO, SD, SS), chemical indicators(nutrients, IMn) and biological indicators(Chla) surveyed in Gonghu Bay, a total of 13 environmental factors were analyzed by hierarchical clustering. Gonghu Bay can be divided into four characteristic regions. Spatial characteristics of nutritional status in each characteristic regions was very obvious, with Ⅰ aera(severe eutrophication), Ⅱ area(moderate eutrophication), Ⅲ area(mild-moderate eutrophication), Ⅳ area(moderate-severe eutrophication). Nutritional status of Ⅰ aera and Ⅱ area did not change significantly with seasons, while Ⅲ aera and Ⅳ aera changed with seasons obviously.(2) FVCOM model has unstructured triangular grids and finite volume method. Unstructured triangular mesh can better fit the boundary of calculation region. Finite volume method includes geometric properties of finite element method and simple discrete structures and efficient computing features of finite difference method. The relative error of simulated and measured flow speed ranged from 3.2 to 150%, with mean vaule of 23.7%; while the relative error of simulated and measured significant wave height was from 0.03 to 28.7%, with mean vaule of 10.7%. Therefore, FVCOM model could simulate flow and wave field well in Gonghu Bay separately.(3) Under the easterly, southerly and southeast winds, difference of hydrodynamic field(flow and wave field) in different characteristic regions of Gonghu Bay was very obvious. Hydrodynamic field was an important factor causing differences of nutritional status in characteristic regions. Significant correlation were found between hydrodynamic factors(flow rate, significant wave height) and environmental factors(total nitrogen, permanganate index, total phosphorus, dissolved oxygen, ammonia, transparency).(4) Under the influence of inflow/outflow, scope and intensity of water exchange in Gonghu Bay was growing with the increasing flow, particularly in the area of entrance of inflow. Under the southeast wind and south wind, the north shore of Gonghu Bay was the windward shore and water convergence zone, where could help accumulation of algae. Thereby, this aera was sensitive area of alage blooms. Whether Wangyu diversion or not had an important impact on velocity of the entrance of inflow, northern shore of Gonghu Bay and junction at Meiliang Bay and Gonghu Bay. The flow velocity of each layer in Gonghu Bay increased with wind speed, and flow velocity of each characteristic regions was Ⅱ area >Ⅰ aera >Ⅳ area >Ⅲ area. Under the same wind speed, the flow of different layers were the same, and the flow velocity decreased with water depth.