Copenhagen’s stormwater detention roads are everything but.

By Kamila Grigo

As part of its climate change and urban flood mitigation strategy, Copenhagen aims to build 300 stormwater management projects over the next 20 to 30 years. Among the projects are a series of detention roads, entire streets redesigned to convey and detain rainwater locally to relieve the existing storm sewer system. It’s an ambitious target that reflects the city’s understanding that investment in these projects is a way of managing greater long-term risk to city infrastructure while providing citizens with multifunctional spaces in the short term.

The Sankt Kjelds Square and Bryggervangen by SLA is a pilot of the detention road concept. Completed in 2019, it comprises the entirety of the 2,300-foot-long Bryggervangen road and Sankt Kjelds Square, the roundabout in the middle. “It’s quite a simple project,” says Bjørn Ginman, a project director at SLA, who says that the fundamental concept is about seeing water move through the site. Rain gardens lining the pedestrian rights-of-way receive rainwater from sidewalks and the roofs of adjacent residential buildings, while road runoff is directed into larger infiltration ponds at the roundabout and at intersections, though not before an in-ground diverter (one of the municipality’s first applications in a public road context) deals with the most polluted first flush.

In winter, the custom-designed stormwater inlets that are part of the linear drain system are closed to prevent salt runoff from entering the rain gardens, and the first-flush diverter similarly diverts road salt runoff from the infiltration ponds. The project is designed to handle a 10-year storm event, though Ginman says it performed well under conditions of prolonged saturation this past winter. In terms of pedestrian safety, the municipality accepted the use of the lush, biodiverse vegetation as a subtle barrier around the infiltration ponds.

Half the length of Bryggervangen and completed the same year, the Scandiagade detention road by 1:1 Landskab is designed to handle a 100-year storm event. An existing median provided ample volume for eight roughly four-foot-deep basins evocative of swimming pools with minimal utility reconfiguration. Under typical rainfall conditions, runoff from the west part of Scandiagade flows to a catch basin with a pipe that discharges into the northernmost basin, explains Jens Ulrik Praefke Jensen, a senior consultant at Atkins, the civil engineer. During heavy rainfall, runoff is directed into the basins via curb cuts in the custom polished concrete curbs that cap the basin retaining walls. Only when stormwater drains through to underdrains is it slowly released into the storm sewer system.

Embedding a series of brightly painted “swimming pools” into a public road might seem like a whimsical concept, but it’s a logical approach to passively delaying a substantial amount of water from the storm sewer system. Jacob Kamp, a partner at 1:1 Landskab, says that yellow guardrails around the basins ensure safety while bright blue pavement markings create informal pedestrian crosswalks into the park. Each basin has its own theme and planting palette, and only the northernmost basin is inaccessible as it consistently receives street runoff. Kamp calls this the “experimental basin” and explains that the municipality is interested in seeing how the planting performs under polluted and saturated conditions.

Copenhagen’s utility company, HOFOR, pays for the stormwater management projects and their maintenance, while the city funds additional amenities. Martin Abrahamsen Vester, a planner at HOFOR, says that the utility performs a per-project cost comparison between green-blue and gray infrastructure and has found that in retrofit conditions, the life-cycle cost of green-blue projects is lower than the alternative, even when factoring in higher maintenance costs. Although the approach benefits the utility company by easing the load on gray infrastructure, Vester says it’s as much about contributing to a greener city: “I’ve been out talking to a lot of the neighbors at Sankt Kjelds, and they’re so happy about these solutions.”