The Dutch capital Amsterdam is a trendsetter in many areas. Situated below sea level in a vast polder landscape, it's not surprising that city and region are also working hard on water management. One of the initiatives is Amsterdam Rainproof. A master plan with the aim of making Amsterdam resistant to increasingly frequent downpours. This means both preventing damage and making better use of the free rainwater that is currently drained via the sewers.

Rockflow stone wool infiltration buffers are already being used at various locations in Amsterdam to collect rainwater and delay delivery to the sewage system or infiltrate it into the soil. But that is not the only Rockflow solution being used in Amsterdam. Bluelay is being used under a growing number of sports fields in the city. Sturdy stone wool mats retain water directly beneath the field. It provides additional playing comfort and contributes to reducing the city heat island effect through evaporation.

In the Amsterdam toolkit

Torben Tijms, Project Lead Research & Innovation Climate Adaptation and advisor Urban Water and Rainproof at Waternet explains: "Waternet looks at what measures we can take to make the city climate proof. We're not just looking at flooding, but also drought and heat. As project leader, I help determine which projects we will investigate in the context of climate adaptation. This subject is very hot as all kinds of things are happening in the world. Climate adaptation is high on Amsterdam's agenda; a large part of the maintenance budget goes to innovation and research in this field. You actually want to create a sort of toolbox of measures and systems that are ready when you want to do something in the city, so that you know in advance which measures you can use when you start a project. In doing so, we aim for integrated measures in which, for example, sewer management and road management are also coordinated at the district level."

Integral solutions

Integral solutions are an important concept when implementing climate adaptive measures in the inner city. Solutions often consist of multiple components that all have to fit into the existing structure. This requires intensive consultation with various stakeholders. In the first half of 2021, an extensive redevelopment project was carried out in Tuindorp-Oostzaan (Amsterdam North) with an explicit climate adaptation assignment: showers of 70mm/hour must be absorbed within the district. The initial option of working with water-storing road foundations proved inadequate in view of the necessary habitat improvement for existing trees. Rockflow, with 95% hollow space, has a much higher water storage capacity, so less space will suffice.

Torben Tijms: "In Tuindorp-Oostzaan we are now investigating the contribution of the Rockflow package to combating flooding in the neighbourhood. It is a problem location where we will be monitoring exactly what the system does and how it contributes to the task for a period of five years. We are also investigating what the degradation of a particular system is over time. Only when you start getting all those key figures you can take them into account in the models at some point. This also involves the relationship with the surrounding measures. We are striving for integrated measures. We've been replacing sewers for a long time, but now there are extra requirements and provisions. And the subsurface is only getting fuller and busier; it has to fit in. It's a huge task to coordinate all this with the various parties. You have to coordinate with the policymakers and managers at the front end. You have to organise your toolbox so that an extra function can be added."

Rockflow in Tuindorp-Oostzaan

Between the beginning and the end of May 2021, trucks carrying Rockflow stone wool elements drove into Tuindorp-Oostzaan almost daily according to a tight time schedule. Intended for the water buffers that were planned at various locations in the context of climate resistance. To cause as little inconvenience as possible in the narrow streets, each time a limited trajectory was completed. This required precise logistics and well-coordinated cooperation between all parties. Daan Los, technical advisor at Rockflow, says about the project: "In the context of Rainproof Amsterdam, contractor KWS asked Wareco to advise them on finding a system to collect and store the prescribed capacity of water. The firm was already familiar with Rockflow through previous projects. Together with the consultancy firm, we informed the contractor extensively about its operation and functionality. It helped that Rockflow is now a proven system with sufficient documentation and answers to all questions. The circular character of the basic material stone wool contributes to sustainability, which was also an important factor in the climate-proof redesign of the neighbourhood. Also, in some places you have to deal with a high groundwater level, so there is little installation space. Rockflow can be used there because it is stronger and can handle the same traffic load with less coverage. This allows us to install more volume under the cover layer and above the groundwater level."

Convincing arguments

Daan Los: "At first glance, Rockflow elements seem quite massive. We therefore often demonstrate how quickly and how much water such an element absorbs. That is surprising to many people and changes their perspective. It's a convincing story if you then also point out how much more traffic load the system can take compared to other systems and how flexible you are in design and implementation. Also, the ease with which the elements can be installed without need to use geotextiles or to interconnect the elements. They are just placed next to each other. If you encounter an obstacle - tree roots or unforeseen cables, for example - you simply saw the elements on the spot. Volume that you remove in one place you simply add in another so that the calculated volume is maintained."

The exact performance will be closely tracked over the next five years. Torben Tijms: "We will survey the systems with extensive measuring equipment and simulate showers with full-scale tests, adapting the construction of a shower every five minutes. All measures and systems are examined and a lot of data is collected. Information that can improve the operation of a system is also shared with the cooperation partner. Ultimately it is published on the Waternet Winnovatie website. Other municipalities then also benefit from this, because yes, we have to do it together."

Sports fields also help with water buffering       

Near De Meern stadium are the field hockey fields of HC Athena. There, the municipality of Amsterdam opted for an innovative new main field with water buffering in the hollow Permavoid foundation. A BlueLay shock pad made from ROCKWOOL stone wool is installed on top of this. Stored (rain) water is automatically absorbed by the BlueLay through capillary action and released through evaporation. In this way the hockey field helps to collect rainwater during heavy showers and to reduce the urban heat island effect. Thanks to the technical know-how of both Rockflow and development partner DutchBlue, the product also has excellent sports technical properties. Moreover, in summer the field is considerably cooler than a conventional artificial turf field; a big advantage for the club and the athletes.

Torben Tijms: "The added value of such fields for Waternet increases as the system is applied on a larger scale. If all sports fields could be used to retain water, this would clearly relieve the burden on the larger system. Athena's field is located at the deepest point of Amsterdam Watergraafsmeer. That is quite a vulnerable polder, so it would be ideal if more water could be captured there. With climate change and rising sea levels, every drop that you don't immediately send to the sewer makes a difference. That saves problems at the end of the storage basin, in IJmuiden. By using sports fields to collect water, you contribute to the sponge effect of the city and thus to relieving the burden on the larger system. This sports field is a small trickle, but if it is used on a larger scale it can make a significant contribution, especially during peak loads. Results achieved here we take back into our larger model."

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