Event-Scale Assessment of the Effectiveness of SuDS in the Quantitative Control of CSOs

The European Water Framework Directive (2000/60/EC) promotes an integrated approach to water management, recognizing water as a shared resource and defining quality objectives. Within this framework, Sustainable Drainage Systems (SuDS) provide effective solutions to improve water quality, control runoff, mitigate hydrogeological risk, and enhance urban resilience. This study investigates the application of SuDS for quantitative stormwater management in a 290-ha industrial district within the Metropolitan City of Milan. Using a synthetic design storm as a benchmark, the study provides event-scale evidence of the performance of SuDS under observed rainfall events, a topic often underrepresented in the literature. Two hydrologic–hydraulic models were developed using SWMM ver. 5.2: a baseline model representing current conditions and a design model integrating SuDS across 24 hectares. Simulations were performed for four rainfall events representative of typical conditions and for a synthetic 10-year return period design event. Results show that, under observed events, SuDS reduce total CSO volumes by 44% and peak flows by 47%, while decreasing overflow activation by around 11%, with the highest effectiveness during ordinary rainfall conditions. Compared with the synthetic 10-year design event, SuDS exhibit similar volume reductions but lower peak-flow attenuation and overflow frequency reduction, highlighting different system responses under real and design rainfalls.