The impact of urbanization on the hydrological regime is complex and depends on different urban features. Increased imperviousness is the most studied one, but the reduction of buffers (river bed, ponds), increased drainage efficiency, or underground infrastructure interacting with subsurface flows can also be relevant. Classical assessment methods use data analysis on coupled catchments: they are limited by case-studies availability and cannot discriminate between different urban features. We associate here data analysis on 6 catchments in Brussels to detailed modelling, allowing to assess separately the role of different features of urbanization, i.e. imperviousness and ponds filling.