Quantifying the Hydrological Performance of Urban Rain Gardens under Simulated Extreme Storm Events

Urbanisation reduces permeable surfaces and increases susceptibility to surface water (pluvial) flooding. Nature-based Solutions (NbS) and Green Infrastructure (GI) have emerged as key components of sustainable flood risk management, complementing conventional grey systems through hybrid designs that enhance resilience and deliver multifunctional benefits. Sustainable Urban Drainage Systems (SuDS) are a prominent example, integrating the four design pillars of water quality, water quantity, public amenity and biodiversity by capturing and attenuating stormwater before it reaches combined sewer outflows (CSOs).

This study evaluates the hydrological performance of urban bioretention rain gardens across multiple sites in Edinburgh and Glasgow, Scotland. A combination of desk-based site characterisation, in-situ hydrological and hydraulic testing and distributed environmental sensor networks are used to establish baseline behaviour and storm response. These networks include volumetric water content sensors to quantify soil water storage, attenuation and drainage capacity, alongside local meteorological measurements to characterise inflow and evapotranspiration dynamics.

To assess system performance under high-intensity rainfall, controlled storm events are simulated using a portable rainfall simulator developed for site-based SuDS stress-testing. Sixty-minute design storm profiles of varying magnitudes (10-, 30-, and 100-year return periods) are applied to standardised 1 m² test plots isolated by custom-built separator trays. This setup enables consistent cross-site comparisons and links hydrological mass balance responses to site-specific conditions such as soil texture, infiltration rate, vegetation structure and planting density.

Preliminary findings demonstrate that vertical soil moisture dynamics during simulated storm events, reflecting the combined influence of soil hydraulic conductivity, antecedent moisture and vegetation cover on infiltration and retention. Measurements from sensors installed at 0–40 cm depths show rapid wetting of surface layers followed by delayed responses at depth, consistent with progressive infiltration through the soil profile. Under moderate (10–30-year) storms, soil columns exhibited sustained storage increases and slow drainage recovery, indicating effective attenuation of runoff generation. Under more extreme (100-year) events, near-surface layers reached saturation thresholds rapidly, producing short-term ponding and reduced percolation efficiency. Despite this, the monitored profiles retained measurable storage potential compared with non-vegetated controls, demonstrating capacity to buffer surface flow during extreme rainfall.

These findings provide empirical evidence on the hydraulic resilience of current NbS implementations to extreme pluvial conditions. These insights will inform design optimisation and future-proofing of rain gardens and related SuDS elements, supporting the development of more resilient and multifunctional urban drainage networks that safeguard both communities and infrastructure.