New, permeable road surfacing in Frederiksberg instead of expensive drain pipes

In May 2014, around three months after the new climate-friendly road surface on Helenevejhad been taken into use, the area experienced another cloudburst, and the new surfacing passed the test: The water seeped down through the pavement as planned.

2010: Frederiksberg Municipality required that Helenevej be refurbished.

July 2011: The greater Copenhagen area experienced a large cloudburst which also severely affected the area around Helenevej.

Late summer 2011: A resident on the road proposed an alternative, permeable road surfacing.

November 2012: Homeowners on Helenevej held a meeting to discuss the proposal. Frederiksberg Municipality, Frederiksberg Forsyning and the residents on Helenevej subsequently developed a project together with the Faculty of Science at the University of Copenhagen to ensure a new road surface able to absorb large amounts of rainwater.

Spring 2013: Preliminary surveys, preliminary works and re-routing of drain pipes etc. commenced.

Summer 2013: The actual construction works began.
The work was completed in spring 2014.

The area around Helenevej has been repeatedly flooded during heavy rainfall. The area was hit most severely in the summer of 2011 when an extreme cloudburst caused extensive damage to both municipal and private property.

Helenevej in downtown Frederiksberg is situated in a low-lying area. The sewer system under Helenevej was additionally challenged because the road slopes slightly and because there are no green areas close by onto which the water can be diverted. Furthermore, traditional underground sewer systems are expensive to establish.

Therefore, an alternative solution to manage large amounts of rainwater locally was needed; a solution which could take the load off the city's sewer system.

The asphalt on Helenevej has now been removed and replaced by special paving slabs, joints and a drainage layer that absorbs surface water into the road surface.

The permeable surfacing consists of quadratic slabs and 15-milimeter joints with an 'easy-drain' filler material. This means that the water will quickly seep down into a 40-centimeter-thick layer of special drainage aggregate, which has been developed by NCC Roads and IBF, Ikast Betonvarefabrik.

The aggregate layer has a porosity of 30%. That is, the aggregate layer can absorb up to 30% water.

From the aggregate layer, the water percolates further down into the subsoil.

The road slopes by around 60 centimetres from one end to the other. To prevent the water from collecting in one end, transverse water barriers have been established at every 20 meters along the road. At the end of the deepest place is an emergency overflow for the sewer.

During the cloudburst that hit Copenhagen on 13 August 2014, the University of Copenhagen performed measurements in the road to document the effect of the new road surface.

The ability of the road surface to absorb water will likely diminish with time, as the joints can become clogged by small particles.

NCC has developed a machine which can loosen and suck up around 50% of the filler material without damaging the surfacing. The joints can then be refilled with new material to re-establish the road's original efficiency. It is estimated this type of maintenance will have to be carried out every three to five years for the road to maintain its original ability to absorb water.

The road surface has an expected lifetime of 25 years.

The residents on Helenevej now have a road that can absorb large volumes of rainwater. Furthermore, they are pleased to have a road that looks much more attractive.

With the new surfacing, the road can divert large volumes of rainwater - corresponding to a 100-year event.

In addition to helping efficiently remediate the rainfall problem, the new surfacing is a cheaper alternative to establishing new sewers.

The residents on Helenevej have been very active in the process and by choosing slabs rather than asphalt they have ended up with a road that has a much friendlier expression, encouraging slow driving.

The new road surface on Helenevej is an example of how the co-financing scheme can pave the way for climate change adaptation projects that are a collaboration between citizens, the municipality and utilities.

The conversion to permeable surfacing has cost DKK 920,000, whereas a traditional solution, e.g. with expansion of the sewer system, would have cost DKK 3.5 million.

The work was performed under the co-financing scheme and the costs have been divided between Frederiksberg Forsyning and the residents.

The scheme makes it possible for the wastewater utilities to co-finance municipal climate change adaptation projects on roads, watercourses and recreational areas.

In connection with the project, the road changed status from private to municipal.

Frederiksberg Municipality is in charge of operation and maintenance of the road.

Frederiksberg Forsyning pays for the part of the operation to ensure the road's permeability is maintained.

A new, permeable road surface is no guarantee against flooded basements. Not everyone on Helenevej was aware of this.

When Helenevej experienced a cloudburst, only three months after completion of the new road surface, sewage water intruded into the basements of residents without a backflow blocker or a defective backflow blocker.

Explaining this to the citizens was a communications challenge for the utility company.

"As a utility company, we cannot protect individual properties against cloudbursts. We invest in the general protection of Frederiksberg against flooding but it is the responsibility of the individual resident to protect their own property, for example through investing in a backflow blocker," said Henrik Bay, head of the water division at Frederiksberg Forsyning.