Research On The Design And Efficiency Of The Transformation Of Typical Public Green Space Into A Bioretention

Many cities and countries around the world are beginning to see the need for and necessity of the idea to conserve water, for many reasons. Rapid urban construction, prolonged drought and water-logging conditions have affected the city's waterways, rivers, and lakes at a large scale. Sponge city is one of the effective ways to solve the urban waterlogging and the poor quality of surface water environment. Based on the field experiments, current work discusses the feasibility of transformation of urban public green space into bioretention ponds, and provides technical support for the development of the low impact construction of the sponge city, especially the built-up area.Three different experimental designs have been used in the current work for transformation of a roadside green into three different structures of bioretention ponds (Pool A: filler height- 700mm with no submerged area; Pool B:filler height-400mm with submerged areas of 300mm; and Pool C:filler height of 400mm with no submerged area). The Chicago hyetograph method was used to simulate the rainfall data. Experimental results of the removal of runoff pollutants from 3 bioretention cells in 5 different rainfall intensity and in 10 years of rainfall return period under the 4 different rainfall duration were analyzed. Based on the experimental results, we have proposed three different types of transformation schemes for urban green space based on our experimental results.The removal efficiency of runoff pollutants has been compared between 2,5,10,15 and 30 years of rainfall return period. The results show that inlet water quality of the bioretention pond was higher than the class V standard of surface water quality. The removal efficiency of CODMn, TP, NH4+-N, NO3--N, TN and petroleum pollutants in different structures of bioretention pools is greatly influenced by the rainfall intensity, while the removal efficiency of SS is less affected by the rainfall intensity. By comparison, the removal efficiency of CODMn, TP, NO3-N and TN was higher in the 10 and 15 years of rainfall return period, whereas the removal efficiency of SS, NH4+-N and petroleum pollutants are better in the 2 and 5 year of rainfall return period. Overall, the effluent water quality of pool A, and B can stable up to the class V standard of surface water environment quality, whereas the average of effluent water quality of the pool C can reached to the level of class IV standard of surface water environment quality. For the 30 year of rain return period, the removal efficiency of 3 bioretention ponds to runoff pollutants decreased sharply and the effluent water quality deteriorated.Removal efficiency of runoff pollutants of bioretention ponds in the 10 years of rainfall intensity has been compared with the rainfall duration of 15,30,60 and 120 min, respectively. The results show that the removal efficiency of 3 different types of bioretention ponds for runoff pollutants is greatly influenced by rainfall duration. Removal efficiency of runoff pollutants was higher under the rainfall duration of 60min in pool A and pool B. Whereas, the removal efficiency of runoff pollutants in 3 bioretention pools was decreased in different degrees under the rainfall duration of 30min and15min, and the effluent water quality was worse than the class V standard of surface water environment quality.Compared with the removal efficiency of 3 different structures of bioretention pools of runoff pollutant, the results indicate that the effect of rainfall intensity and duration of rainfall on the removal efficiency is alleviated by increasing the filler height and setting anaerobic submerged area. In the 2 and 5 years of rainfall intensity, the effect of filler height is more significant, whereas the effect of submerged area is more significant in 10,15 and 30 years of rainfall intensity. Increase in the filler height can be more effective in alleviating the effects of rainfall duration because submerged area is beneficial in the removal of NO3--N and TN, but has little effect on the removal of TP, and is not conducive to the removal of SS and NH4+-N.According to the experimental results, combined with the characteristics of different types of urban public green space, a scheme for the transformation of green space into a bioretention pool was proposed in the current work. When the park green space is transformed, the filler height should be higher but does not set the submerged area. The filler height should be higher and the submerged area should be set when the transformation of the residential green space and road green space, because of the existence of the underground municipal pipelines and the safety of the road structure.The construction and demonstration area of the sponge city is in an orderly propulsion way in China. The results of the field experiments and the green space transformation norms proposed in the current work can be referred to the municipal garden department to implement the green space transformation of built-up area.