Permeable surfaces can be the future in cities

VandCenter Syd wants to find alternative solutions to surface water management in connection with climate change adaptation. Instead of diverting the rainwater in the traditional sewer system, focus is now on local management of the water according to the motto: Think big : Start small.

With this approach, VandCenter Syd has examined the traditional surfacing in Odense, in which there are large areas of asphalt and concrete which do not allow the rainwater to soak naturally into the ground. During extreme rain events and more precipitation in general, the consequences are serious with possible flooding and expensive remediation. In 2012, VandCenter Syd therefore laid a permeable car park at their own premises which is to be used to gather experience about the technology.

There is a waterworks on the site on which the permeable surfacing has been established, and there is a clean-water reservoir only 6 metres from the waterworks. Possible impacts were analysed in the planning phase through a risk assessment. The risk of impacting the clean-water reservoir was assessed to be small, and the project was therefore given the go-ahead.

Another challenge in connection with the construction of a car park with permeable surfacing was that the contractor lacked experience in permeable surfacing with the material chosen. However, this was not a problem, as the material for the bearing course was relatively easy to work with compared to ordinary surfacing material. Unlike base gravel, the bearing course material is not influenced by water to the same degree. Therefore, the construction work can even be carried out during wet weather conditions.

The car park at VandCenter Syd is a 2nd generation installation and it is the first of its kind. With an area of 430 m2, the car park has been constructed as shown in the figure. The car park has been laid with paving stones, so the water can penetrate the gravel filler. Under the surfacing, is an approx. 30 mm levelling course through which water can be transported without the levelling course losing its bearing capacity. The water is then transported to an approx. 450 mm bearing course, which can absorb large amounts of water, and then sent further down to the subsoil.

The dry well has been placed in the middle of the car park and covered by a wadi, i.e. a stormwater infiltration bed. Surface run-off-water, which cannot permeate the surfacing due to, for example, frost or water-saturated soil, is collected in the wadi. From the wadi, the water runs through a layer of topsoil and further down to the dry well. When the dry well is full, the water will be led to the sewer system through the overflow.
Moreover, a membrane has been placed under the bearing course at the dry well to separate the bearing course from the dry well. The goal is to create damming, so the water can be led to the wadi from the surface, and through this to the dry well.

If the top layer is blocked, a high-pressure washer is used to hose up the gravel filler. When the gravel has been hosed up from the cracks, it is sucked away. Then a new layer of gravel filler can be sprinkled between the tiles. This is expected to be necessary every 10-15 years in connection with regular maintenance.
As usually only the top few centimetres of the filler are blocked, just this layer requires maintenance.

Only small amounts of weed grow on the car park. Weed growing on the car park does not get a proper hold of the gravel filler, so it is easy to remove. All things considered, the surface is relatively easy to maintain, and does not entail high operating costs.
Construction costs for surfacing the car park are about DKK 640/m2 = about DKK 275,000. Of the DKK 275,000, DKK 40,000-60,000 have been spent on removing subsoil. Construction costs for traditional surfacing are about DKK 500/m2.