Floods and combined sewer overflows in Brussels: The limits of the current mitigation measures and the benefits of the infiltration-based best management practices

The Brussels-Capital Region (BCR) suffers from recurrent floods on the whole territory. Summer storms constitute the principal cause of inundations in the city. Rainwater runs on the steep slopes and loads the unitary sewer system. The constant increase of impervious surfaces significantly reinforces this topographic effect leading to even higher run-off. This situation has long been recognized and the mitigation measures advocated by the different storm-water plans (Plan Pluie, Plan de Gestion de l'Eau) mostly rely on end-of-pipe infrastructures that are dimensioned for one type of rain and placed where the most severe floods occur. However, some CSO events are caused by rains less intense and more frequent than the ones generally considered. Flooding risk is also more complex than
presented and some other minor components (fluvial risk, groundwater risk) are not considered. The fluvial and groundwater risk also involve longer and less intense rains. Moreover all these phenomena do not appear at the same place and require a management at catchment scale. Geological and hydraulic data have been processed to throw light on the complexity of the flooding risk in Brussels, to detect the actual gaps in the management of floods and CSOs, to assess the impact of infiltrationbased best management practices on the different types of floods and overflows. The research has been carried out at the scale of the region and in a district (Forest) in the South of Brussels. The main conclusions are that Low Impact Development measures are complementary to the current mitigation infrastructures and unavoidable to mitigate all the forgotten components of the flooding risk in Brussels. The infiltration-based Best Management Practices can reduce the different type of floods and overflows but their installation, in densely urban areas, depends on the local geological conditions.