Roadside trees drink stormwater in innovative solution for urban climate adaptation

A new method for combined retention of rainwater and greening of the city provides climate resilience in dense urban areas.

Densely populated areas face a challenge on the rise. Intensified rainfall due to climate change means that increasing water volumes need to be managed either by extending the capacity for transporting the stormwater or by building retention basins. Retention can be solved by a new innovative product that combines storage and delay of road water with watering new roadside trees. The product benefits are climate adaptation of roads as well as a greener city!

Road water is collected and directed to the innovative tree plant holes that water the trees from below according to the same principle as when potted plants are watered in the saucer. The plant drinks as needed. In winter, the water may contain salt as a result of deicing of slippery roads, but it is not an obstacle to the innovative plant hole, because the salty water is not sucked up by the tree in winter.

The soil is designed to handle large amounts of water. With 50% cavities, distributed with 30% field capacity and 20% buffer volume, the soil can store 20-50% of the soil volume of water. This provides a unique opportunity to use the soil for storage during heavy rainfall events.

Moreover, the soil is developed with reference to the German FLL2 standard, so it can be built in as a foundation under the paving and compressed into proktor 95. Unlike root-friendly base layer, the soil has a high topsoil content, which provides the good growing conditions for trees, and with oxygenation pipes the individual trees need a very small footprint on the terrain.

The coupling of innovative plant holes with larger volumes built in elsewhere in the road as plastic cartridges in membrane and the unique water brake (SUDS regulator) that can throttle down to 0.4 l/s makes it possible to delay the precipitation for 24 hours or more if necessary.

A series of cassettes are built into the ground around road trees, so they can provide storage for rainwater.


The new climate adaptation product has been developed as part of an innovation partnership between Team NIRAS and Frederiksberg Municipality. Team NIRAS consists of NIRAS as advisor, Arkil as contractor and product suppliers Mattle and Hydrosystems. To date, Team NIRAS as a total contractor has climate-adapted two roads in Copenhagen, Denmark. In 2021, additional three roads in the Danish capital will be climate-adapted with this innovative approach.

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Esben Ravn Iversen
+45 8161 4142


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