Study On The Influence Of Rainwater Infiltration And Purification Under Different Low Elevation Greenbelt Structures

Urban rainwater brings waterlogging and Non-point source pollution.But it's also an effective way to solve the shortage of water resources.As an effective and ecological urban rainwater treatment facility,Low Elevation Greenbelt has become the most important measure in the renovation plan of residential district.However,the efficiency of green infrastructure is uneven because of the lack of specific design standards for each facility and Due to the differences of climate and soil quality,it is difficult to design facilities locally.What's more,As a result of lacking proper operation and maintenance scheme for completed facilities,a large number of facilities block up,such as swales,raingarden and tree pits,which lead to function decline and even gradual loss.This research makes a systematic study on the design,construction and operation of the low elevation greenbelt which is used in the sponge city,and develops a full cycle solution of low elevation greenbelt for engineering applications.First,the green LID scheme and the scale of the facilities are preliminarily determined by analyzing the CAD hypsography and the current situation of the area.Selecting the low elevation greenbelt for laboratory simulation after comparing the differences of green infrastructures and experiment with rainwater.Analyzing the efficiency of rainwater infiltration and pollutant reduction in low elevation greenbelt and optimize the composition and structure of the facilities.After that,the main factors affecting the rate of seepage are determined through SPSS analysis,and further optimization was made to improve the infiltration capacity of rainwater and provide more accurate parameters for engineering.Finally,the problems,causes and corresponding solutions of the facilities in different rainfall intensity and longterm operation are studied by experiments,and suggestions for the maintenance of low elevation greenbelt are proposed.The following results are obtained:The study shows that the reactor composed of three layers of gravel gradation,red soil and Zoysia matrella Merr has better efficiency of rainwater infiltration and pollutant reduction.The effluent time,water accumulation depth,COD removal rate,NH4+-N removal rate,PO43--P removal rate and SS removal rate of the reactor are 7.67 min,18mm,70.89%,64.26%,59.82% and 80.25%,respectively.Learned through the single factor analysis of variance by SPSS software,it indicates that the main influencing factor of the infiltration is the medium layer under the 5% significant level.The medium layer is further optimized by layering and mixing.Because of the blockage of the interface between the layered media,the infiltration is poor,the average seepage rate is 12.15ml/min,14.05ml/min and 17.96ml/min respectively.And the water accumulation is serious.The maximum depth of water accumulation is 75.2mm?69.6mm and 54.4mm.so the original single medium layer has not been optimized by layering media.There is little water accumulation in the 3/7 mixed soil during long-term operation,and the infiltration is stable.What's more,the removal rate of COD,NH4+-N,PO43--P and SS in 3/7 mixed soil has no significant difference with that of the single medium.The optimized medium is tested under different rainfall intensity.It is found that When the rainfall intensity is 10mm/h,20mm/h,30mm/h,40mm/h,50mm/h and 60mm/h,the water accumulation depth of the facilities is respectively: 0mm,0mm,5mm,12.9mm,28.2mm and 47.6mm,and there is no significant difference on the removal of COD,NH4+-N,PO43--P between the soil and the red soil under different rainfall intensity.The maximum removal rate is 94.5%,obviously higher than two single mediums.Long-term operation of the facilities leads to a decrease in water infiltration and a deeper degree of water accumulation.The anti clogging performance in mixed media is better than that in a single medium.The best maintenance time for the red soil,black soil and 3/7 mixed soil is when the rainfall is 764 mm,968mm and 1146 mm after operation,respectively.For the medium which is slightly blocked,the performance of infiltration can be recovered to 95.4% by cleaning up the surface.The effect of ploughing soil is obvious when the blocking is not serious enough.But for serious blocking media,soil permeability and water accumulation can be close to the initial state by change the medium layer.Facilities should be operated by active maintenance,passive maintenance and rectification so as to achieve long-term and effective operation.