Rockflow water infiltration as a practical, sustainable measure against local flooding

Climate adaptation and challenging play

“Two things triggered us. Two years ago the student council asked for extra, more varied play options in the playground because they could really only play football or hockey. Added to this, it was often too warm in the summer because of the sun baking the paving all day. Another aspect was that large puddles of water stayed on the playground whenever it rained hard.” This is according to Margo van Holthe, head of the Paschalis primary school in the Hague, talking about their motivation for redesigning the school playground, previously covered mostly in concrete paving. When the playground was likely to be dug up due to sewage problems, the school saw this as the perfect time to put the ideas into practice. After contacting the Delfland Water Authority and the City of The Hague, a plan was conceived to make the playground green-blue, i.e. incorporating as many climate adaptation measures as possible during the redesign.

We engaged Bureau RIS, all of whose projects are based on climate adaptation, biodiversity and a healthy living environment. As a first step, the agency interviewed the end users: the pupils. “Of course, the children’s first ideas were wild, ranging from a log flume to a cable car from one side of the playground to the other”, says Margo van Holthe. “After discussing it in more depth, they realised themselves these ideas are impractical. This is how we eventually reached a design with play elements that encourage movement, a sand and water play area and panna street football courts.” These ideas formed the basis for the design that was implemented in the spring of 2020. Donny Thieme, Bureau RIS explains that “Sustainability is an important aspect here. Even when we selected a landscape gardener, we incorporated their vision on sustainability. In this project we wanted to use the expertise of the collaborating parties to move towards a more sustainable school.” Landscape gardeners Van der Heijden from Den Hoorn were therefore commissioned to implement the plan, resulting in a school playground with plenty of greenspace, challenging all age groups to take part in sport and play activities.

Greening measures

When the school was established in 1923, the playground was a green oasis, but with the advent of new buildings in 2004, the exterior space paid the price. Like many other schools, the playground was relaid with traditional concrete slabs. In the new design, removing hard surfacing and greening is the dominant theme. By removing the slabs (and reusing them, e.g. in seating walls), the ground can more easily absorb rainwater. The vegetation also provides much-needed cooling in the summer. As well as the practical aspects, the greening also has a quite different added value.

Blue measures

The least visible part of the project is a buried Rockflow stone wool water buffer. Underground storage of rainwater after heavy showers is essential because surface level measures such as a swale were not an option due to space restrictions. Underground water retention using Rockflow was easy to implement. Surface water drainage channels and drains divert water from the playground’s hard surfacing to the Rockflow infiltration system. The waterpermeable concrete slabs and wide joints allow rainwater from the panna courts to infiltrate directly into the buffer below. The 75 m3 of underground water storage is more than enough to rapidly store the water from extreme downpours and then allow it to infiltrate. This contributes to improved water management in the immediate area. In the future, there are plans to connect the overflow to surface water.

Looking at the wider sustainability spectrum

Delfland Water Authority, one of the project’s funding bodies, participated in particular because of the smart measures for capturing heavy rainfall and infiltration into the ground. “People live, work and enjoy life in the Delfland region. If we want to keep it that way in the future, then it is essential to take smart climate adaptation measures right now, together”, according to their site (climate power). The project also fitted seamlessly into their aim to achieve a climate-proof city. Arthur Hagen, Water Adviser at the City Management Service: “Of course, the initiative for the project comes primarily from the school itself. But the City and the Water Authority were both keen to actively contribute ideas, collaborate and even contribute financially. This is another good example of fruitful cooperation between various partners and official bodies towards a more robust climate situation.”

“Rockflow is practical and sustainable”

Why was the Rockflow system chosen? “At first we considered a plastic crate system, but when we contacted ROCKWOOL about Rockflow, they managed to convince us”, explains Aarnoud van ‘t Hof. “For us, the overriding argument was that the material is reusable, completely recyclable. We try to avoid putting anything into the ground that doesn’t belong there. In principle we make use of materials that can either be fully absorbed by the soil or that can be fully recyled. I am not completely convinced whether that is the case for crate systems. The fact that implementation is extremely easy was also a factor. You don’t have to completely wrap this system in geotextiles, so the entire underside and walls can be used for infiltration. The shallow depth required also means you can work with a normal slope. It connects to the existing drains so that water can also flow in the usual way across the playground. The system is practical and sustainable. We always consider the wider sustainability spectrum for our projects. Not only water retention, but also flora and fauna and how it deals with heat and heat stress. Part of that integral thinking is that you take into account what the future effects of the products you use may be. So you start with a sustainable product and end up with a sustainable product.”

Tailor-made design

The Paschalis school playground was the first project in which landscape gardener Van der Heijden used Rockflow, so in that sense it was an experiment. The fact that we immediately trusted it was because Rockflow is a ROCKWOOL product from a global company whose products you come across wherever you go. If they develop products further for specific applications, then there’s a good reason for it. It doesn’t just fall out of the sky randomly. They also have good tools such as the model that we filled in with all the partners concerned. These details enabled Rockflow to provide us with a tailor-made design.

In principle, all the water infiltrates into the underlying sand from the buffer. An overflow also allows excess water to discharge to the sewer if a number of extreme downpours occur in rapid succession, but we don’t expect that to happen very often.

Aarnoud van ’t Hof

Van der Heijden landscape gardening

The winning properties of Rockflow

Asked about their motivation for using Rockflow in the green-blue redesign of the school payground, Landscape Designer Donny Thieme from Bureau RIS sums up the main arguments:

  • Sustainable product, 100% recyclable
  • No geotextiles or casing necessary, so all sides are used for infiltration
  • Easy to incorporate, straightforward installation, simple system, modular and adaptable
  • Rockflow can absorb 95% of its own volume in water and has rapid absorption properties
  • High permeability (K value)
  • Infiltration or delayed drainage can make the system available again within 24 hours
  • Stone wool can absorb pollution
  • Only shallow excavation necessary

For the school, there was another very strong argument explains Margo van Holthe: “The system is practically maintenance-free. You bury it and don’t need to do anything more afterwards.”

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