Flood Control Effect And Grey-green Facility Optimization In Sponge City

In order to deal with various water problems such as frequent urban waterlogging,deterioration of water environment and loss of water resources caused by the increase of urban impervious underlying surface,sponge city construction has become a main means to solve the problem.The construction of sponge city can not only lay green basic sponge facilities such as green roof,permeable pavement and concave green space in communities,squares and parks,but also realize the rapid discharge and storage of rain and flood through gray infrastructure such as pipe network,storage tank and underground corridor,so as to reduce the runoff coefficient,reduce the peak flow and reduce the load of non-point source pollution.Taking Xiaozhai area in Xi’an as an example,based on the original drainage pipe network,this paper constructs a sponge city calculation model with three kinds of green infrastructure combined with regulation and storage tank by using S WMM.And the layout mode of grey green facilities is studied under the conditions of different rainfall return periods and different rainfall duration.The following conclusions are drawn:(1)For total runoff control,When the layout area of grey-green sponge facilities is different,the layout area is positively proportional to the stormwater control effect.When the space layout is different,the influence of the increase of rainfall return period on runoff control rate is mainly reflected in the rainfall with a return period of 5 years or less.The other return periods were almost not affected by the dispersity of LID and storage pools.(2)For runoff peak regulation,no matter the area or dispersion of grey-green sponge facilities were increased,at different rainfall return periods,the peak reduction rate of each layout decreased significantly.In addition to the positive effect of layout area,for the spatial layout,the reduction effect is best when LID is completely dispersed and storage pool is completely centralized.The effect is the worst when LID is completely centralized and storage pool is completely dispersed.But when rainfall is less than once every 10 years,both facilities work better when they are fully dispersed.(3)For overflow nodes and overloaded pipelines,the total duration of overflow and overload increased with the increase of return period or rainfall duration under different distribution areas and spatial layouts.For the total amount of overflow,as the spatial layout changes from fully centralized to fully dispersed,the ability to control overflow increases.(4)For waterlogging points,with one rainfall in 50 years,when the layout area is changed,the water depth in three working conditions exceeds 50 cm.Other conditions can effectively improve the water in the study area.When the spatial layout is different,only gray-green sponge facilities are centralized when the water depth is less than 50 cm.Therefore,the centralized arrangement of sponge facilities is more conducive to the control of urban waterlogging.(5)For pollutants,at different return periods,area change has the greatest influence on TN,which can increase by 24.30%at most.And the influence on COD is minimum,and the maximum increase is 7.63%.Complete dispersion of grey-green sponge facilities has the best regulation effect on rainfall that occurs less than once in 5 years.When the return period is longer than this,the optimal way is that the green facilities are completely dispersed and the storage pool is completely centralized.Under different rainfall duration,the reduction rate of SS,COD,TN and TP can be increased by 7.06%～15.78%at most by increasing the area of distribution.(6)By constructing the index evaluation system,the entropy method is used to comprehensively evaluate the adjustment effect of various layout methods.When the area of grey-green sponge facilities is different,LID with a total area of 1.947 km2 combined with the total volume of 89938.20 m3 storage pool has the best regulation effect under T=2 h,P=1.3A or P=50A rainfall conditions.When the spatial layout is different,the comprehensive effect is the best when the green facilities are distributed in 100 sub-catchment areas and the number of storage pools is 20 under the small return period.Under the large return period,the spatial layout of green sponge facilities and storage pools was best when they were distributed.