Infiltrating and retaining water under a 'climate bioswale' in the Netherlands

Perennials in a bioswale called for innovation

"In this project, we very much considered sustainability and the climate," says Paul Geluk. "For example, nesting boxes have been incorporated in the facades, there are green roofs on the garages, and solar panels and heat pumps have been installed. We also tried to design the outdoor space in a sustainable and climate-adaptive way, with as much greenery as possible. But the space was limited, hence the idea to plant the bioswale with perennials. The look you get with perennials is much more colourful, and you considerably increase biodiversity. Bioswales are often seen as grass strips and, if not constructed properly, they can quickly become dry trenches, or they retain a layer of smelly water. In addition, rubbish often collects in such a bioswale. On a working visit to Amsterdam, I came across bioswales with perennials, and the idea stuck. But it’s not easy. There are plenty of plants that can handle wet feet, of course, but will they survive dry spells in the summer?"

Capillary break as a solution

For the bioswale at Klaverweide there was only a narrow strip available between the road, the green parking spaces, and all the cables and pipes. In order to achieve the specified water storage capacity during peak downpours, the plan was to place a Rockflow infiltration package beneath this bioswale, which the surrounding paved surfaces could drain into (with gluttons/spillway as a back-up). A layer of soil for perennials was then to be placed on top of the Rockflow package. A thick layer, because soil lying on top of the Rockflow buffer is no longer in contact with groundwater and will therefore be vulnerable to desiccation.

The problem was that there was no room for such a thick layer of soil. In the end, the solution was to apply a capillary break in the form of a gravel layer on top of the infiltrating Rockflow package. On top of that, came an extra layer of Rockflow, about 10 cm thick, and on top of that, a relatively thin layer of soil for the plants.

Project leader Daan Los of ROCKWOOL Rainwater Systems: "Rockflow has a certain capillary force and so can hold water. You have to make sure that the water is not sucked out at the bottom by the sand, because sand has a higher capillary force. This is the principle how Rockflow infiltration buffers work. But Rockflow is also used to retain water for longer periods of time. This means you have to capillary seal the package from the ground at the bottom, for example by applying a layer of foil or, as we did in Zoeterwoude, with gravel."

Two systems working together

The remarkable thing about the Klaverweide is that these two systems have been combined here for the first time. The lower stone wool package is there to buffer and infiltrate the bulk of water during peak rainfall events and prevent flooding in the neighbourhood. Moreover, the infiltrated water replenishes the groundwater in the immediate vicinity. On top of the Rockflow infiltration package now lies a second, much thinner package of stone wool elements.

A capillary break, in the form of a gravel layer, has been installed between these two rock wool layers. Gravel is not, or is hardly capillary and, therefore, the water is retained in the upper Rockflow layer. The soil on top draws the water upwards by capillary force and it is then available for the plants in the bioswale. In the Zoeterwoude set-up, the upper package has no ducts or pipes, and consists of elements of 10 and 15 cm thick. If this layer is completely saturated with water, the plants can, in theory, draw on it for a summer dry period of about 4 weeks. This will create a better natural water balance and reduce the need for irrigation by the municipality, reducing management and maintenance costs.

The substrate and the plants were specially selected for this construction by Griffioen Wassenaar, applying their GreentoColour^® concept.