Continuous simulation-based assessment of SuDS retrofit strategies for CSO mitigation under observed rainfall variability

Combined Sewer Overflows (CSOs) are a recurrent source of pressure on receiving water bodies, especially where their activation occurs even under ordinary rainfall conditions. Sustainable Drainage Systems (SuDS) can contribute to CSO mitigation, but their effectiveness may be affected by rainfall regime, implementation scale and retrofit strategy. This research investigates different SuDS retrofit strategies for CSO mitigation in Sesto Ulteriano, Northern Italy, through a two-year continuous simulation framework based on observed rainfall data. Three drainage configurations were analysed in SWMM5.2: the current Business-as-Usual scenario, a Distributed Retrofit Strategy (DRS) with distributed small-scale SuDS interventions, and a Large-Surface Retrofit Strategy (LSRS) focused on retrofitting extensive impervious surfaces. The system response was assessed at the rainfall-event scale in terms of CSO discharged volume, peak overflow discharge and number of activated overflow structures. Events were grouped into four clusters according to rainfall characteristics and performance indicators were also normalized with respect to the retrofitted area to compare strategies with different implementation extents. Results show that LSRS achieved the highest overall mitigation, with mean reductions up to 47% in overflow volumes and 56% in peak overflow discharges. DRS showed lower absolute mitigation (on average 32% and 42%, respectively), due to the smaller extent of the retrofit. However, when assessed on a unit-area basis, DRS outperformed LSRS, suggesting a possible influence of the type of SuDS interventions implemented. The rainfall cluster analysis showed that CSO mitigation was more consistent under low- to intermediate-rainfall conditions, whereas higher-magnitude events led to a more variable response.