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

Paris Region, France Nature-based solutions, such as bioretention systems, are increasingly implemented for urban runoff management. This study combined continuous monitoring with field investigations of three bioretention prototypes in Paris Region to assess their hydrological performance under constrained subsoil conditions (limited or prohibited exfiltration), identify dominant hydrologic processes and explore 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, designed to evaluate whether combining a low HLR of 4, fine-textured substrate, and IWS can enhance evapotranspiration (ET) as the main volume reduction mechanism. The three systems achieved notable runoff reduction throughout the monitoring period, despite limited or null exfiltration. For the unlined system, volume reduction was largely driven by the thickness of the IWS, with sufficiently thick IWS enabling substantial exfiltration. For lined systems, it was controlled by ET, which appeared twice as high during dry periods. A capillary barrier between transition and drainage layers enhanced 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 cracking issues which were mitigated by IWS. Additionally, potential water intrusion from perched groundwater lenses was identified. • Monitoring-based evaluation of the hydrological performance of 3 bioretention cells. • Bottom storage enhances exfiltration even for low permeability underlying soil. • Internal water storage (IWS) under silty substrate prevents dry cracks in the soil. • Evapotranspiration significantly increased in the presence of an IWS. • Water intrusion founded at the bottom of cell due to underlying perched lenses.