In the ‘from façade to façade’ redevelopment of Amsterdam's Nieuwezijds Voorburgwal, climate-adaptive measures were implemented as part of the Amsterdam Rainproof initiative. These included the construction of a water storage and infiltration system, which collects water from a heavy downpour and then infiltrates it in controlled doses into the ground.

"Amsterdam Rainproof is a standard we have set as a city," says Jora Slinger, geohydrologist at Waternet. "It means that the city must be able to withstand a 70 mm/hour downpour, which falls about once every 100 years. In such a downpour, the water should not rise too high or come up to against the façades. Using computer models, we looked at where in the city bottlenecks occur during extreme downpours, and the Nieuwezijds Voorburgwal turned out to be one such spot. Using the computer model, we also calculated how much buffer capacity is needed and where it should be located. The problem on the Nieuwezijds Voorburgwal was fitting it into the narrow street in the busy city centre, where the ground is full of cables, pipes and other utilities." Despite the challenges, an infiltration system with a capacity of nearly 700m3 had to be installed there, in order to meet the Rainproof standard.

Where will the infiltration facility be installed?

Timo Zijl, works planner for the municipality of Amsterdam: "The city wanted to redesign the Nieuwezijds Voorburgwal, replace the tramways and the greenery, modify all the roads, from façade to façade. Part of this plan also includes capturing rainwater during heavy downpours. Ultimately, it was clear that this would only be possible under the tramway, because that is the only space available."

But can heavy trams safely travel over a water buffer system that, by definition, consists almost entirely of a hollow space? That was the question facing the project team of the municipality of Amsterdam, Waternet and KWS. The municipality has used Rockflow infiltration systems on other projects in the past, and wanted to apply the principle here as well. "In previous projects, we chose Rockflow because of the flexibility of the system when you have to work around cables, pipes or other underground obstacles. The issue here was not cables and pipes under the tramway, but the load capacity and absorption volume. We don’t know much about water storage in general, let alone product-specific details. That's why we took advice from Rockflow when creating the design and describing it in the scope."

“The most important part is that you don't notice it’s there"

Janneke Tax, Tram Asset Manager of the GVB, the public transport operator for Amsterdam: "We were asked to agree to the construction of a water storage area under the tramway. This is a deviation from the standard requirements, whereby an enquiry is reviewed based on various criteria, such as maintenance costs, expected impact on passengers and in this case, of course, the load-bearing capacity of the system with respect to rail buckling. The rail buckling was calculated in two scenarios and in both cases little to no difference is expected compared to the normal situation. And, the most important issue for us, is that we don't notice it’s there."

Daan Los, Advisor at ROCKWOOL Rainwater Systems: "When it turned out that the only space available was under the tramway, we were asked if that was possible with our product. I contacted a colleague in a specialised business unit within the Rockwool organisation who was able to carry out the necessary calculations. We compared two scenarios. One scenario assumes the existing situation with the existing foundation under the tramway, and the other calculates what happens if you apply half a meter of rock wool as a buffer. The difference in rail buckling is minimal. On that basis, the municipality and GVB agreed to proceed with the system. Waternet then modelled the system into the overall sewer system. This then formed the basis of the calculations to verify that water can indeed flow into the system fast enough, and how the system works in terms of discharging and infiltration into the ground. Rockflow stone wool elements consist of 95% hollow space in which water is absorbed. The permeability of the stone wool is particularly high so that water can flow into it very quickly. The water can then slowly infiltrate back into the ground. When the modelling also showed this, we were able to design and work out the system in more detail, with all the connections, ducts and vents."

The special thing about this project is that concrete was poured directly on top of the Rockflow stone wool elements, which then has to support the weight of heavy trams going over them. This is not something that we do every day.

Daan Los

ROCKWOOL Rainwater Systems
Advisor Urban Climate Adaptation

Not a standard solution

"The special thing about this project," says Daan Los, "is that concrete was poured directly on top of the stone wool elements, which then has to support the weight of heavy trams going over them. This, of course, is not a standard solution. Because there is a concrete slab on top of the stone wool, the weight of passing trams is evenly distributed, preventing point loads. We also implemented the stronger variant of Rockflow elements. This variant is often chosen when there are higher traffic loads combined with high groundwater levels, with not enough space to apply adequate cover."

Efficient use of space under the tramway

The redesign of the northern part of the Nieuwezijds Voorburgwal started in September 2022 and will be completed sometime during 2023. The redesign is being carried out by KWS, which had already worked with Rockflow in an earlier project phase on the southern part of the Nieuwezijds Voorburgwal. Ruben Swier, chief contractor at KWS: "We installed a Rainproof infiltration field with 50cm Rockflow elements. These were placed on a bed of sand, thus also acting as a foundation for the new tramway. In our experience, Rockflow is quite simple to apply, and the instructions and guidance from Rockflow are clear and straightforward. The installation speed is also high. We did preparatory work and made sure the ground was compact before we laid this field in 2-3 days."

On schedule

There are always challenges. Ruben Swier: "We are here in the centre of Amsterdam, where extra rules apply regarding transport in terms of weight and direction of travel. These aspects were agreed with Rockflow in advance, so we could get the necessary exemptions and permits ready in time. The preparation went well in all respects. We made clear agreements regarding the planning and targets, to ensure materials arrived on time and work could start on schedule."

Rockflow as the basis of the foundation

"The special thing about the use of stone wool at this location is that the Rockflow elements are installed under the tramway and actually form the basis of the tramway's foundation. The entire structure consists of a foundation layer and a top layer. The foundation layer consists of a finished sand bed followed by the 50cm Rockflow elements and covered with a 22cm-thick layer of concrete. The top layer consists of the rails, which are installed on top of the foundation layer and then filled in with a 16 cm layer of concrete. The final finish is a 6cm layer of asphalt. This makes the total package just under a meter high. We believe Rockflow is a suitable material for use under a tramway because it is as strong as a normal construction with sand, and rail buckling is minimal. In this project, we are applying Rockflow at two locations, realising a combined storage capacity of around 700,000 litres of water."

Working together on innovations

Ruben: "We enjoyed working together with the different partners. Rockflow also contributed to the planning and work procedures. We also enjoyed being part of this type of innovation and so contributing to a more climate-adaptive city."

Even heavier

At one point in the project, additional calculations were needed. The initial plan was to apply the top layer of concrete with a concrete paver, but due to the width of the tramway, the caterpillar tracks of the sprayer would have to drive over the stone wool package. The model showed that the stone wool elements could also handle that load, especially if road plates were to be used to spread the pressure. In the end, due to planning reasons, it was decided to use a concrete pump instead.

Amsterdam Rainproof: integral climate adaptation

Rockflow is now part of Amsterdam’s toolbox of climate-adaptive measures. Torben Tijms – who in addition to being Climate Adaptation Research & Innovation Project Leader, is also Urban Water and Rainproof Advisor at Waternet - explains: "Among other things, Waternet is looking at what measures we can adopt to make the city climate-proof. We’re not only focusing on flooding, but we’re also looking at drought and heat. The topic of climate adaptation is all the rage. Amsterdam sees climate adaptation as one of its top priorities. Part of its maintenance budget goes into research and innovation in climate adaptation. You want to create a kind of toolbox of measures and systems that are ready when you want to do something in the city. So you know in advance what measures you can use when you’re going to run a project. This way we are aiming to create a set of integrated measures in which, for example, sewer management and road management are also coordinated at a neighbourhood level."

Rockflow project Amsterdam Nieuwe Zijde noord

Other question? Click here for general contact details and frequently asked questions: