The main advantages of the probabilistic modelling of urban drainage systems are the possibility of quantification and efficient reduction of the risk of ‘wrong’ management decisions or engineering designs, as well as the less biased mathematical modelling. Such probabilistic modelling achieves quantification of the uncertainty in the field of model-output. In the modelling of sewer system overflow emissions, a lot of uncertainties are involved: uncertainties about the spatial rainfall-input, model simplifications of the sewer system and runoff catchment, and biases and uncertainties about the model parameters.

A methodology has been presented to quantify the different uncertainty-sources in a decomposed way by separating uncertainties about the model structure, the model input and the model parameters. It has been applied to an existing sewer system and WWTP, for which the emission modelling has been performed both by a full hydrodynamic and a simplified conceptual model. In this application, the importance of having accurate (spatial) rainfall data and a thoroughly calibrated runoff model has been noticed. A well-considered simplification of a full hydrodynamic model, for instance by a conceptual reservoir model, has a negligible influence on modelled values of sewer system emissions. Such simplification achieves a reduction of calculation times, which is a necessity in probabilistic modelling.

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