Combining grey and green infrastructure

Urban Life 10 May 2016 Henrik Søgård Olsen

Trench excavation is the traditional method used to enlarge sewer systems, but tunnelling makes the operation smoother for the public. In the same area of Copenhagen 10 municipalities are collaborating on a major recreational climate adaptation initiative. The two projects combined lower the risk of flooding dramatically.

8 min

Three times in just one year the citizens of Hvidovre, a suburb southwest of Copenhagen, found themselves shovelling sewage water out of their basements. On 2 July, 2011, the worst cloudburst ever registered in the area caused the Harrestrup stream to flood several parks and roads, leaving people standing knee-deep in water in their own gardens. In a nearby scout cabin the level exceeded 1.2 metres.

Hvidovre cannot totally avoid problems with sewage water. But these problems will soon be dramatically reduced.

This is the result of a major sewer and climate adaptation project that Ramboll has undertaken for the Greater Copenhagen Utility (HOFOR) along the Hvidovre-Copenhagen border. The goal is to prepare the area for a new climate reality involving more frequent and more severe cloudbursts.

“The key here is that a traditional sewer upgrading would not be enough. Nor would a so-called blue-green infrastructure approach, where you keep the water on the surface in parks, on football pitches and the like. It’s the combination that does it,” Henrik Søgård Olsen, Country Director, Ramboll Water, explains.

Tunneling leaves the surface intact

In fact, there is little traditional about the sewer upgrade, although the aim is the same: to reduce overflows along the 4.3-km sewer route. To expand sewage capacity, most municipalities dig deep sewer pipe trenches. In Hvidovre, however, boreholes are limited, because Ramboll’s specialists have used another method entirely, drilling tunnels through the fissured limestone and other layers instead.

This spring, the tunnelling machine will excavate 970 metres in a single go, one of the longest stretches ever undertaken in Denmark. The tunnelling method will enable Hvidovre residents to enjoy a park and other areas unmarred by a 970-metre-long trench. Neither will the infrastructure be affected, as most of the tunnelling is being done under existing transport corridors, including several residential streets, a four-lane motorway and a railway.

A cheaper and better combination

The new pipeline has a diameter of up to 2.5 metres. Even pipes that big cannot prevent floods in Hvidovre, but nine other municipalities along the Harrestrup stream system are joining forces with Hvidovre on an innovative climate adaption project that will further strengthen the area’s resilience to flooding. The 10 municipalities share a vision where the almost 30-km system of streams with mostly paved edges returns to a natural state with greener banks, clear water and fish – and where sewage sludge only threatens to overflow into the system or nearby basements after exceptionally violent cloudbursts.

“It is unique both in Denmark and Internationally for so many municipalities to be working together on a climate adaptation project like this that also holds big recreational benefits,” says Henrik Søgård Olsen from Ramboll.

The head of Copenhagen’s Climate Unit, Lykke Leonardsen, agrees that the projects are innovative and gives the municipalities and their citizens more for less:

“All our calculations on climate adaptation show that it is cheaper and better for citizens to combine traditional sewer solutions with surface greening initiatives like stormwater storage in parks, recreational schemes and other urban development. In popular terms, you get benefits in two areas, but only pay for one,” says Lykke Leonardsen.

Written by Michael Rothenborg.