In a number of residential properties in the lowest area of the district of Reutsberg in Heel (Maasgouw municipality), waste water rose up via the toilets and streets and gardens became flooded during heavy showers. Enlarging the sewer was not an option and the construction of a rainwater separate rainwater sewer would be too expensive. A pilot project was therefore begun, in which rainwater was diverted and the water ran above ground into swales where it could infiltrate into the soil.

This was a good functional solution, with the exception of one location - a swale on the Boslaan which served as a playing field. The original swale configuration with lava stone in the substrate appeared not to be working as anticipated. The soil into which the collected water had to be infiltrated appeared to be silted up, as a result of which water was not absorbed during wet periods. The playing field was regularly flooded and even had to be sealed off with fencing at certain times for safety reasons. This was an unacceptable situation, all the more because the playing field had been designated as an important social area by the neighbourhood steering group at the start of the project. A solution was therefore required which combined the functions of playing field and water storage in this location.

Workable solution

Daniëlla Houben, civil policy worker at the local authority: “We cannot reach the desired depth for infiltration at this location because it is a groundwater protection area in which we are only permitted to work a maximum of three metres below ground level. This restricted us somewhat, as we had to be able to dispose of 500 m3 water. Together with Ria Smits, my colleague from sewer maintance, I began to investigate the options. Ultimately, Rockflow® appeared to be a workable solution, as it provides a great deal of storage, you can stay really close to ground level and it is easy to configure and adapt. It is also maintenance-friendly and not usually susceptible to blockages”. The last point was also important as the site was adjacent to the edge of a wooded area, where there was a great deal of sand and leaf debris.

Rockflow is a water management system which can buffer large volumes of rainwater in urban areas quickly and effectively. The innovative system is used beneath constructed surfaces such as squares, roads, streets and industrial estates that have to cope with severe flooding during heavy rainfall. The system is also used for the buffering and disposal of rainwater in swales. Rockflow consists of slim (standard element 100 x 120 x 15 cm), light (less than 20 kg) stone wool elements, which can absorb 95% of their volume in rainwater. From there the water can infiltrate into the ground layer or be discharged to the sewer system or surface water.


The available budget was mostly used up during the previous phases of the Reutsberg project, so extra funds had to be released. During the research phase into a workable system therefore, there were frequent consultations with the official bodies responsible. Houben: “We are going to peel it away layer by layer. What are the consequences if we don’t do anything? What if we only adopt measures that are within the budget? What will it cost to do it really well? The simplest solution would be to fill the playing field with sand, but then you would lose five hundred cubic metres of water storage, the water would cause severe flooding again in the properties below the gradient and the aim of the entire project would have been missed. It was in fact a process of elimination, whereby the correct solution was ultimately revealed. With the new system, we have also increased capacity by 65% and it is now calculated on showers of 50mm rather than 30mm, as in the original design. By putting forward all of these arguments, we ultimately obtained the additional budget required.

rockflow, wadi, heel reutsberg, lapinus

Construction team

Once the initial choice had been made, a kind of construction team was set up comprising, alongside Houben and Smits, advisors from the engineering bureau Kragten and from supplier Rockflow as well as Hans van der Sligte as the local authority supervisor. Houben: This worked really well; you could view the problem and the solution from all perspectives and optimise the design on that basis. The motto of our municipality is DOING IT TOGETHER with COURAGE! and courage was certainly required in selecting such an innovative system with which experience had yet been gained in only a limited number of locations. But within the team we looked very critically at the risks and how they could be overcome. Furthermore, Lapinus had carried out a great deal of research, so the most critical questions we had were answered robustly and supported by research results; this generated confidence and also provided the necessary courage.”

In practice

The final system consists of Rockflow® elements installed on a 10 cm thick layer of the existing lava stone and covered with 30 cm of soil (“the same soil used for football fields”). The water is fed into the system by a subterranean feed pipe with twenty or so internal connections which feed the water into the stone wool elements. The blocks slowly empty by the force of gravity into the subterranean lava, which contains drainage pipes discharging to the sewer system. Check-valves ensure that no foul water from the sewer enters the system. One condition of the neighbourhood working group was that the buffer system should not be filled above ground level, hence the subsurface feed pipe which is fed directly from the gutters. The feed pipe runs through the centre of the facility and begins and ends with manholes from which the pipe can be flushed out if necessary.  Houben: “That will probably not be necessary. If leaf debris entering the pipe, this is usually restricted to the first few meters and mostly rots down of its own accord. Furthermore, the internal connections have been installed at half the height of the feed pipe, so even if it is half full, this has no effect on capacity.”


A major advantage of Rockflow® is the flexibility of the system. The stone wool elements can be sawn or drilled through without any loss of effectiveness, so existing pipes in the ground can be easily circumvented or integrated. Even the root systems of the imposing trees on the site could be easily worked around. Houben: “In this case, supervisor Van der Sligte was also the contractor and as he had attended all of the preparatory sessions, he was fully aware of all of the possibilities and restrictions. As a result, he was able to make decisions on the spot, such as shortening one section if a tree was encountered or lengthening another. Even if a cable or pipe had to be removed at a later date, this was possible without any effect on performance. It is actually a huge sponge that is not very error-prone. Even the fact that tractors had to be able to pass over it for the maintenance of the trees was important. Ria Smits: “In a project like this, we always look closely at management aspects, as they are measured in years.”


One of the most significant success factors in the entire project was communication, both within the project members but also with the neighbourhood working group. Smits: “When you are going to change something in the neighbourhood, there is always resistance. And you should never treat it lightly. We therefore immediately set up a working group. We can devise excellent plans but ultimately it is going to be the residents who are going to make use of it or suffer the consequences. The working group helped us by thinking through how it works in the neighbourhood and what is possible or impossible.” In the meantime, the system has successfully buffered the first heavy rain showers and a new facility is planned for Maasbracht. Houben: “Throughout the entire process we continually requested more information and supporting evidence to convince ourselves that this was not just a pipe dream. Ultimately, we achieved an attractive solution that also performs very well for this location. Pilot successful.”

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Roy Janssen

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