Study On Hydrodynamic Simulation And Regulation Of "Lougang Polder Field" River Network

South China,with flat terrain and horizontal river network,is not only an important food production area in China,but also one of the regions with high degree of urbanization development.Highly dense population and various industries gather,which makes the ecological carrying capacity of river network in the southern plain reach the limit.In order to meet the production and living needs of residents in the river basin,a large number of water conservancy projects and facilities are built on the river network.In addition to the special subtropical monsoon climate in South China,a large amount of rainwater is concentrated in summer throughout the year.Flood control and drainage have also become the key target of local river network reconstruction.With the increase of river network reform in recent years,the original orderly river network has been artificially separated,some rivers have been silted up and lack of hydrodynamic force,large area of river network has become dead water area.The ponding water in the river network can not be discharged during rainstorm,the water body can not be effectively replaced when the water body deteriorates,and the problems such as inability to make up water quickly to the river network through external rivers when the river network is short of water become more and more serious,thus the water environment becomes worse gradually.Aiming at the problems of insufficient hydrodynamics and deteriorating water environment in river network area of Southern Plain of China,the river network of"Lougang Polder Field"in Taihu Lake Basin of China was selected as the research object.As a typical case of river network modification by river basin residents,the river network modification of"Lougang Polder Field"is of high degree,which is an indispensable choice for studying hydrodynamic regulation of river network in southern plain.In this study,8 drainage schemes and 5 hydrodynamic control schemes were designed by grid topography of river network in the study area,collecting local hydrological data,building a two-dimensional hydrodynamic-water quality coupling model of river network,combining with the layout of local water conservancy projects.All schemes are simulated and their effects are analyzed to provide reference for river network reconstruction project and hydrodynamic regulation in polder area.The main conclusions of this study are as follows:（1）When the river network is drained by opening the gate（scheme 1-scheme 3）,the water level of the river network can be lowered to the lower limit within 24 hours,and the best drainage effect is scheme 3;When the gate can not be opened,only when water pump is used for drainage（Scheme 3-Scheme 8）,the river network needs to open at least 2 pumps to ensure that the water level can be lowered to the lower limit within 24 hours under any working condition.The more pumps are opened,the shorter the drainage time is.The best drainage scheme is scheme 8,scheme 4 can not satisfy any operating condition,and scheme 6can not satisfy the drainage requirement in case of 100-year rainstorm.（2）Whether it is a closed living water scheme or a diversion and drainage scheme,the flow rate of the river network can be effectively increased.The flow rate of the river network is mainly concentrated in the outer circle of the river.The hydrodynamic driving effect of the water body in the central pond is general,but it is still greatly improved compared with the general conditions.The outer river flow rate of the live water scheme is more uniform.The flow rate of pond water in diversion and drainage scheme is faster.The best hydrodynamic improvement is scheme II,in which the overall flow rate of the river network is the fastest and the flow rate distribution is uniform.（3）Hydrodynamic regulation scheme III-V can effectively replace water body in river network.After 7 days of diversion,most of the water bodies in the river network are replaced.The best replacement effect is the outer river,with the replacement rate of more than 90%.The replacement rate of water body in the replacement area of central pond is 48-88%.The longer the diversion time,the larger the proportion of water body in the area with replacement rate over 72%.The best replacement effect is scheme III.（4）Hydrodynamic regulation scheme III-V can effectively improve DO content and BOD₅ of river network.After 7 days of diversion,DO and BOD5 of all waters in the river network have been improved,and DO and BOD₅ of outer ring channel water body have reached Class III water standard and above.DO and BOD₅ in the central pond can reach the index of Class II water because of the increase of flow rate and large contact area with air.BOD₅ in the central pond can even reach the index of Class I water.The best way to improve water quality is Scheme III.In this paper,by combining field data acquisition with simulation model construction,different control schemes are designed based on the hydraulic engineering in the river network after modification to explore the drainage capacity of the river network in the face of rainstorm and the feasibility of hydraulic engineering to control the hydrodynamics of the river network.The results of various schemes are analyzed from flow rate,flow field distribution,water displacement rate,DO content and BOD₅.The research results can provide reference for hydrodynamic regulation of hydraulic engineering in southern plain river network.