Improving the performance of separate stormwater systems in the urban environment: a hydrograph-based control framework for improving stormwater outflow quality

Digitalisation and real-time control (RTC) can improve hydraulic and water-quality performance of separate stormwater systems, but adoption remains limited by the complexity and operational burden of existing approaches. This study presents a proof-of-concept hydrograph-state-based RTC framework, which uses the Storm Water Management Model (SWMM) to define water-level thresholds, estimate storage capacity and generate synthetic time series for evaluating three RTC strategies – water-level-informed, water-quality-informed and combined – against a no-control baseline. Simulations using design storms showed that all strategies reduced total suspended solids (TSS) load and event mean concentration (EMC), but with varying trade-offs between water-quality improvement and hydraulic performance. Combined control achieved the best balance, reducing TSS load by 50–78% for small and medium rainfall events and delaying first flush by up to 27 h. Under extreme events, water-quality benefits diminished as storage became limited. Practical deployment requires site-specific calibration of water-level thresholds and turbidity–TSS relationships.