For the purpose of re-design and improvement of the combined sewer system at the Kumodraz catchment in Belgrade, measurements of rainfall and runoff at the catchment were established in late 1997. Observed data are used for calibration and validation of two rainfall-runoff models: the detailed model BEMUS (Belgrade Model) and a conceptual hydrologic model Visual OTTHYMO. The major facilities of the recommended solution for re-design of the existing system are three retention ponds and outlet into a trunk. The paper briefly presents assessment of design flows for these four locations of the catchment based on results from the calibrated models. Data on intense storms that caused severe flooding within the catchment (and also all over Belgrade and other parts of Serbia) in June and July 1999 are used for comparison of design flows with a real event. During these storms the flow gauging equipment at the catchment was destroyed and only the rainfall data was available. In order to analyze the system under extreme conditions it is therefore necessary to perform hydrograph simulations. The basic idea was to check return periods of rainfall and runoff for these storms.

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