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₎ (total precipitation) = 1299.2 mm) and seven heavy storms (i.e., Pₜₒₜₐₗ range from 53.1 mm to 287.3 mm), at different SF₍ᵢ₎ (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 SF₍ᵢ₎ was effective in increasing the ARVR₍ᵢ₎ (annual runoff volume reduction) and RVR₍ᵢ₎ (runoff volume reduction), while little effective in increasing PFR₍ᵢ₎ (peak flow reduction). Moreover, the ARVR₍ᵢ₎ could meet the designed goal of 70% when the SF₍ᵢ₎ was more than 7.5 mm/h. For RVR₍ᵢ₎, the key variable of precipitation characteristic changes from Pₜₒₜₐₗ to P₄ₕ₍ₘₐₓ₎ (maximum precipitation in 4 h) as SF₍ᵢ₎ increases, while P₄ₕ₍ₘₐₓ₎ remains as the key variable for PFR₍ᵢ₎ all the time. The sensitivity studies demonstrate that the hydraulic conductivity is more effective in increasing PFR₍ᵢ₎ 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.