Abstract
In many countries traditional simple design approaches for combined sewer overflow (CSO) structures and storage tanks are used in general practice, neglecting the significant variability observed in real systems. In this paper, two hydraulic software simulation tools and two stormwater quality model approaches are compared to assess CSO behaviour and the impact of different modelling approaches on the estimation of spilled pollutant loads in long-term simulations for an urban catchment area. A good coherence of the two runoff models is observed. The more complex stormwater quality model approach leads to significantly lower TSS interception ratios for smaller specific storage tank volumes. For both the hydraulic CSO indicators and TSS interception ratios an important annual variability is observed. Therefore, using single design value approaches or short rainfall time series in modelling can lead to significant impact on the design ratio of storage tank and CSO structures.