RESUMO
Engineers generally use simulation tools such as SWMM, MOUSE, HYDROWORKS, etc. to simulate the operation of sewer systems. Equations used by these models to represent the hydraulics of sewer systems are well known as well as their performance. Thus assessment of the performance needs to suppose that the user is an expert who is able to describe each facility in the most accurate way according to the model. In Western Europe, important parts of sewer systems are old and combined structures. Such structures very often include very complicated special structures, especially within CSO facilities. Experience shows that the performance limits of the models do not rely on the accuracy of the equations, but on the ability of the modeller to describe these special structures properly. The objective of this research is to assess the part of errors and uncertainties which originate from modellers' inconsistencies in the description of special structures. Even if the results are obtained with specific software (CANOE), results can easily be generalised to other models, since the equations used (Barré de Saint Venant, etc.) are almost the same.