Water sensitive urban design retrofits in Copenhagen – 40% to the sewer, 60% to the city

Water Sensitive Urban Design (WSUD) is emerging in Denmark. This interdisciplinary desk study investigated the options for WSUD retrofitting in a 15 km² combined sewer catchment area in Copenhagen. The study was developed in collaboration with the City of Copenhagen and its water utility, and involved researchers representing hydrogeology, sewer hydraulics, environmental chemistry/economics/engineering, landscape architecture and urban planning. The resulting catchment strategy suggests the implementation of five sub-strategies. First, disconnection is focused within sites that are relatively easy to disconnect, due to stormwater quality, soil conditions, stakeholder issues, and the provision of unbuilt sites. Second, stormwater runoff is infiltrated in areas with relatively deep groundwater levels at a ratio that doesn't create a critical rise in the groundwater table to the surface. Third, neighbourhoods located near low-lying streams and public parks are disconnected from the sewer system and the sloping terrain is utilised to convey runoff. Fourth, the promotion of coherent blue and green wedges in the city is linked with WSUD retrofits and urban climate-proofing. Fifth, WSUD is implemented with delayed and regulated overflows to the sewer system. The results are partially adopted by the City of Copenhagen and currently under pilot testing.