Study On Low Impact Development And Scale Management Of Urban Green Space Based On Multi-Objective Control

The rapid development of city brings significant benefits.However,urban diseases such as air pollution,water environment deterioration,and urban heat island effects are also increasing because of population expansion,traffic congestion,and hard surface pavement.The ecological background and hydrological characteristics have been affected.Traditional drainage mode and runoff management concept cannot meet the current requirements in urban sustainable development.The theory of low-impact development is aimed at solving the problem of "more runoff,less water,dirty water".Urban green space is a vector tor implementing the low impact development.However,the current urban green space has poor water quality,low permeability,and ignores the consumption and storage of the surrounding runoff.It has caused the lack of resolution function of“water,problem.In this study,the pollutant concentration level of runoff and soil characteristics in different urban green space were analyzed.Combined with low impact development measures,the structure parameter and spatial combination of measures were designed for different types of green space.The system of infiltration,retention,transmission,purification,and storage was formed.Furthermore,with the total runoff control rate,runoff pollution control rate and cost as the control indicators,the critical values of measures scale were determined under different return period.First of all,the premise of designing low impact development measures is to evaluate the runoff pollutant characteristic of different urban green space accurately.Field work was done on the pollutant concentration level,drainage characteristics,and correlation between pollutants of park green space,residential green space,industrial district green space,cultural district green space and road green space.It was found that pollutant concentration in runoff of different urban green space all exceeded the standard.The comprehensive pollution degree of road green space runoff and industrial district green space runoff are the highest,park green space runoff residential green space runoff and is lower,the cultural district green space runoff is the lowest,and the initial erosion effect is not strong.The occurrence state of pollutants is different.Total nitrogen and heavy metal elements have good correlation with SS,mainly in the torm of particles.COD,total phosphorus have poor correlation with SS,mainly in the dissolved state.And then,soil is the base of urban green space and its permeability has great effect on storing and the cycling rainwater.The permeability test on soil with different amendments was done.Results showed that organic waste,inorganic materials and soil structure amendments are good amendments improving soil permeability.Under the same conditions,the stable permeability rate is sorted as:crust>sand>straw>ceramsite.When the content of the PAM is 0.1 g/kg,the permeability rate is the best.For meeting the requirement of rainwater infiltration for 1or 2 years,10%of the crust,sand or PAM can be selected;For meeting the requirement of rainwater infiltration for 5 years,20%of the crust,sand or 0.1g/kg PAM can be selected;For meeting the requirement of rainwater infiltration for 7 years,25%of the straw or 20%of the crust can be selected.Aggregates play an important role in maintaining soil structure,retaining water and improving soil fertility.The soil in a good aggregate state contributes to the migration of water and nutrients.Through the aggregate test and long-term effect test,it shows that the content of water stable aggregates whose particle size is bigger than 0.25mm is more,the the soil pores,the aggregates structure and the soil permeability are more improved.The pores formed in the initial stage do not disappear with the decay of organic waste.The pore structure is more stable and the permeability is also improved.Finally,the theoretical formulas of runoff reduction are constructed,including permeable pavement,sunken green space,grass swale and bioretention facility.Combined with the characteristics of different types of urban green space,spatial combination design and the methods of calculating measures scale are determined under different return period.This study results provide a theoretical basis for better realizing the runoff control management function of urban green space,alleviating the current situation of water problems and promoting the recycling of rainwater resources and the construction of the sponge cities.