Rockflow used in shallow swales at Duiven

Smart water management

At the time, in the context of Dutch government regulations mandating compulsory integration of water retention in the planning of new housing developments (the so-called Watertoets), these two deep swales appeared to be the only option in the plans for the Westreenen in Loo housing development in 2013. Incidentally, this water retention requirement has been fulfilled not only by the swales, but also by digging a ditch. The original storage requirement of 848 m³ for this planning area is divided between the swales (581 m³) and the ditch (267 m³). The swales therefore help meet the local planning requirements without the municipality having to sacrifice extra building space.

“In fact, the option of using deep swales was never ideal”, says Rutger Traag, civil engineering advisor to the municipality of Duiven. “They were closer to large ditches than swales.” Traag discovered Rockflow as the solution. “The Rockflow infiltration buffer is an excellent practical solution that was added to the swales within a couple of weeks”, says Traag.  “The system can cope with 97% of the water storage, so there is hardly any loss of rainwater buffering capacity. The slope of the swales’ banks has been slightly modified to create a square basin containing the infiltration buffer. “The smart water management system is so robust that a shallow covering of soil is sufficicient to allow a digger or lawnmower to drive over it without damaging the stone wool”, says Traag. “The system can remain intact under the soil for years without any effect on the material. These qualities played a decisive role in opting for Rockflow. The swales retain their function of rainwater buffers, they stay neatly maintained and they are child-friendly.

Another great advantage of the rainwater buffer system according to Traag is that, as the swales are virtually next to the residential properties’ driveways, it makes it possible to plant a beech hedge around the swales to improve boundary marking. “It is easier for us to maintain the area’s swales, greatly improving the area’s appearance”, says Traag. “Maintenance costs are now lower, too. Maintenance by hand is no longer required because we can use a ride-on lawnmower for mowing. That’s 4,000 EUR less per year.”

100% recyclable stone wool

Wim Jacobs is an engineer at Vaarkamp, Ede, and a specialist in small-scale civil engineering projects. Jacobs is supervising the management of this swale project, orders the materials required for the project and maintains close contact with the customer, the municipality of Duiven. In addition it is his job to keep local residents continually up to date in person on the project’s progress. He is enthusiastic about Rockflow. “This is an excellent system for the redesign of the swale, making it safer and easier to maintain without losing its functionality”, says Jacobs. Thanks to the fully recyclable stone wool water management system, the rainwater buffer will remain intact for many years. “The stone wool has an inherent filtering function so there is always enough water discharge into the soil or sewer”, says Jacobs.

Wout Tiedink has been living in the Westreenen in Loo area since 2017. He describes how this area is an attractive and stylish new housing development. “The untidy, poorly maintained swales were an eyesore”, he tells us. Reason for him and other local residents to band together to ask Rudi Segers of the municipality of Duiven to take action about the swales. “There were increasing complaints from people in the area about the unsafe swales that were covered in weeds that couldn’t be mown.” Tiedink says that the municipality was immediately open to finding a quick solution, but there was only a limited budget available. “The water management system was the perfect solution. It’s both unobtrusive and makes the swales shallower. The messy ditch has made way for an attractive greenspace”, says Tiedink.

Rockflow’s Ronald Kruizenga is closely involved with the swale project in Duiven. He explains that the municipality of Duiven is implementing primary water retention in this planning area to prevent the Waterschap Rijn en Ijssel water authority’s water system becoming rapidly overwhelmed during heavy showers. “Once again The Netherlands is currently dealing with a period of extreme drought. Problems caused by this drought are becoming more acute and more serious by the year”, says Kruizenga.

The Rockflow system contributes to retention and infiltration of rainwater in the planning area, rather than draining it away. “This opens up new potential uses for the swale”, says Kruizenga. “The design is based on an existing swale in which only the infiltration medium is replaced by Rockflow.” After the redesign of the swale, the bank is no steeper than 1:3 and mowing the grass is therefore no longer an issue. In addition, the swale containing Rockflow has a load-bearing base (VOSB 18/30) so that the swale can be used as a functional greenspace instead of a deep ditch. “This guarantees long-term rainwater buffering capacity.”