Hydrological performance of bioretention systems under constrained subsoil: A study based on three bioretention prototypes in Paris, France

Nature-based solutions, such as bioretention systems, are increasingly used for urban runoff management. This study combined continuous monitoring and field investigations on three bioretention prototypes in Paris region to explore the runoff volume reduction efficiency under constrained subsoil conditions (limited or forbidden exfiltration), the importance of different hydrologic processes and the ways to enhance them. The prototypes consisted of: 1) An unlined system with a hydraulic loading ratio (HLR) of 13 over clay subsoil, to assess how internal water storage (IWS) may enhance exfiltration; 2) Two lined systems, to assess whether combining a low HLR of 4, fine-textured substrate, and IWS can enhance evapotranspiration (ET) and serve as the main volume reduction mechanism. Overall, the three systems achieved significant runoff reduction over the monitoring period, despite the lack of exfiltration or low permeability of the subsoil. For the unlined system, the volume reduction was largely controlled by the thickness of the IWS. For lined systems, it was controlled by ET, which more than doubled in the presence of an IWS. A capillary barrier between transition and drainage layers promoted soil water retention in the unlined system, yet its ET remained lower than potential ET. Silt loam substrate in lined systems supported vegetation but led to clogging and cracking issues which were mitigated by the IWS. Water intrusion risk from perched lens was also identified.