A case in subtropical climate city: Assessing the bioretention hydraulic performance on storm in response to poor permeability soil.

Bioretention has been widely used in China for the purpose of sponge city construction. In subtropical climate areas, the performance of bioretention cell under condition of low infiltration underlying soil and heavy storms is still poorly understood. This study aimed to assess the effects of low infiltration underlying soil and precipitation characteristics on the hydraulic performance of a bioretention cell using the Storm Water Management Model (SWMM). The hydraulic performance of a bioretention cell were investigated under a Typical year rainfall event (P (total precipitation) = 1299.2 mm) and seven heavy storms (i.e., P range from 53.1 mm to 287.3 mm), at different S (seepage rates of the underlying soil) (i.e., range from 2.5 mm/h to 15 mm/h). Then, sensitivity of the optimal design to the different design parameters, including the hydraulic conductivity of soil medium layer and the berm height of surface layer, was examined. The results show that the increase in S was effective in increasing the AR (annual runoff volume reduction) and R (runoff volume reduction), while little effective in increasing P (peak flow reduction). Moreover, the AR could meet the designed goal of 70% when the S was more than 7.5 mm/h. For R, the key variable of precipitation characteristic changes from P to P (maximum precipitation in 4 h) as S increases, while P remains as the key variable for P all the time. The sensitivity studies demonstrate that the hydraulic conductivity is more effective in increasing P than the berm height. For the bioretention cell under condition of low infiltration underlying soil and heavy storms, in order to simultaneously achieve expected reduction goal of both peak flow and runoff volume, and make the best comprehensive performance of bioretention cell, it requires not only a maintenance action to increase the hydraulic conductivity of soil medium layer, but also a drain pipe to be added in the storage layer, and meanwhile other LID practices should be combined.