With increased computer performance and data-processing functionalities, there has been a tendency in the last few years to apply detailed hydrodynamic sewer modelling for long-term simulations, with long time series of rainfall. Although this is now fairly realistic for small networks, there is still a clear limit as to what can be done in the case of running bigger models for a long time, which need a lot more computational effort. Therefore, the idea has grown to investigate the possibilities of hybrid sewer modelling, a combination of conceptual and mechanistic modelling approaches to combine the advantages of both models, the speed of conceptual models and the accuracy of mechanistic models. Suggestions for hybrid model simplifications are presented in this paper within their application for two case studies.

The application of hydrodynamic sewer modelling allows for detailed description of complex hydraulic situations. However, for large systems long-term calculations with hydrodynamic models still require high computation times. This paper shows a possibility to overcome this problem by using a hybrid sewer model, which is a conjunction of conceptual and mechanistic modelling approaches to combine the calculating speed of conceptual models and the accuracy of mechanistic models in one model. The implementation of a hybrid sewer model was performed and tested in two case studies, in Berlin (Germany) for 6 representative catchments and in Herent (Flanders, Belgium) for one sewer system, using the hydrodynamic modelling software InfoWorks CS. Besides the motivation of the case studies on the sewer systems in Berlin and Herent this paper presents the methodologies developed for a hybrid simplification of the sewer network model, considering the calibration of the simplified network as well as the evaluation of the simplification performance. The use of a hybrid model for both case studies is then evaluated and the transferability of the methodologies is discussed.