Study On The Optimal Water Surface Ratio Of Pond-drainage Channel Wetland System In Lixiahe Polder

Lixiahe area is in Low-lying terrain and the flood disaster is very frequent,the flood usually leads to serious economic losses.Beginning in the 1950s,In the hinterland of the Lixiahe area,3.6 million acre of retting field has been reformed to grow grain,the water level that the area can be controlled gradually reduce,and the area of pond together with lake is becoming smaller and smaller,and the function of natural regulation and storage weakened gradually.Later on,With the development of industry and agriculture as well as the acceleration of urbanization,a large number of water area in Lixiahe area has been occupied.In addition,point source pollution that comes from some industrial and mining enterprises along with breeding farms is emitted disorderly.Fertilizers and pesticides are overused,agricultural non-point source pollution is increasing year by year,the water environment and water ecology in Lixiahe area has been deteriorating in the past 20-30 years.Especially during the summer rainstorm,the water level outside the river is often higher than the ground level inside the polder area,which leads to the internal waterlogging water can’t be discharged in time,the flood and waterlogging disaster is formed frequently,which seriously affects people’s production and life.Ponds and lakes are not only the important carrier of water resources,but also it should take into account the prevention of regional flood disaster and the maintenance of regional ecological environment.Therefore,it is of great practical significance for improving local people’s life and production and ecological condition to do research on the optimal water surface ratio of Lixiahe polder.The main research contents of this paper are as follows:（1）For the distributed river network wetland system composed of the pond water surface and drainage channel system in the polder of Lixiahe water network area.This paper does quantitative research on the main non-point source pollution factors that includes the degradation mechanism and influencing factors of nitrogen and phosphorus.Besides,this chapter builds the the quantitative relationship between these influencing factors and the water surface ratio that mainly refers to the water volume.This research provides a theoretical basis for establishing water quality restriction of the polder area in chapter 3.（2）Then,this paper builds a nonlinear mathematical model of optimal water surface ratio in Lixiahe polder area,which takes account of both drainage design standards and water quality improvement requirements.The objective function of this model is the minimum present value of total fee of river network water system reconstruction project in polder area,decision variables include pond water surface ratio,drainage channel system water surface ratio and drainage design flow,constraint conditions mainly include the water storage capacity of pond water surface and channel water body.Pollutant removal capability of pond and drainage channel wetland system,which mainly includes the removal rates of total nitrogen,total phosphorus and chemical oxygen demand of chrome.requirements of local relevant planning departments on the upper and lower limits of water surface ratio,etc.Genetic algorithm is used to solve the mathematical model.（3）Then,the example analysis is used to Funing Qunan irrigation district.Which is located in Lixiahe area of Jiangsu Province.Conclusions are achieved by calculation and analysis,when the programming water surface ratio of pond is 8.15 percent,the programming water surface ratio of channel system is 3.2 percent,the drainage designed modulus is 0.86 m3/（s·km2）,the optimization scheme can meet the requirements of the regional once every twenty years flood drainage standard.At the same time,when the water body quality outside the polder is Class V,the water body quality inside the polder can reach Class Ⅳ standard.（4）Finally,this paper uses VB software to develop an optimal design system for water surface ratio of agricultural polder in Lixiahe Area,this module can provide reference for land use planning and rural water conservancy planning and design personnel.rural water conservancy planning and design personnel.