Case Study

Putting The Squeeze In Sponge Cities: Amsterdam's Waternet And The Innovative RESILIO Blue-Green Roof Project

The RESILIO project has helped Amsterdam repurpose rooftops as smart blue-green roofs to reuse rainwater and prevent localized flooding. This project, along with other sustainable water initiatives like the Amsterdam Rainproof program, continues to position the Netherlands at the forefront of water management. We examine the details of the project, how our software is used, and hear from Waternet about all the positive things their partners are doing to think more deeply about urban water management to meet the challenges of a changing climate.

The Dutch are perhaps a leading indicator for the rest of the world when it comes to water management, and they’ll soon be facing significant water challenges. Europe is warming at twice the rate of the global average and global sea levels are rising as glaciers melt at worrisome rates. Don’t panic if you live in Amsterdam (like I do), but were you aware that large parts of this 749-year-old city are below sea level? In fact, over a quarter of all land in the Netherlands is below sea level. Thankfully, the water management skills of the Dutch are legendary.

When confronted with difficult water challenges, the Dutch have traditionally gone big, fighting floods for centuries with dykes and windmills. In the last 100 years alone, they’ve closed off an entire sea by building the Afsluitdijk, created 13 lockable Delta Works surge barriers like the Oosterscheldekering to combat storms, and even conjured up an entirely new province called Flevoland where once there was just water. 

Lately, they’re innovating with water in smaller ways, investing in ideas like Blue-Green Infrastructure (BGI) and micro-water management in densely populated cities. But these innovations are anything but small. They have the potential for significant network-effect upsides when scaled up. 

The sky-high potential of blue-green roofs

Most people have heard about green roofs, which are covered with vegetation and provide several key benefits. They can help reduce the heat island effect in urban areas, improve air quality, increase biodiversity, and provide aesthetic pleasure to the people who live in the building or neighborhood. There’s an additional side effect with regards to localized flooding in that the vegetation helps absorb rainwater, which reduces the amount of runoff that reaches the ground from a green roof – but only by a little bit. 

“A classic green roof with a thin sub-stratum doesn’t do the trick,” says Waternet Policy Developer for Climate Adaptation Kasper Spaan. “Water availability through retention and storage is key to healthy, luscious, indigenous vegetation. Recent droughts have emphasized the value of local water storage to keep the city green and cool. When it comes to water management, the small retention capability of traditional green roofs isn’t something to take into account. Ten millimeters during an extreme rainfall doesn’t reduce water stress by a significant level.”

A blue roof, on the other hand, is all about catching and storing water. It temporarily stores rainwater to reduce flooding in the surrounding area and helps prevent sewer overflows. The water is gradually released as residents use it for various purposes like watering plants or flushing toilets, or it is simply returned to the ground or sewage system in a controlled way after the storm has moved on.  

“A blue-green roof is a classic green roof system elevated with a crate system underneath that brings significant water storage capability, as well as an additional valve system so we can control the water level in the water storage underneath the green roof.” 

A smarter way to think about infrastructure

A good blue-green roof does all of this in a smart way, automatically through a combination of hardware and water management software called a Decision Support System (DSS) that uses real-time, high-resolution weather forecasts combined with local and regional water management data.

The DSS can be programmed to do things like empty out the water buffer to prepare for an approaching storm. You can make them even smarter by plugging in macro water management data about surface water and groundwater levels, and you can fine-tune them even further using microdata to follow the demands and wishes of the building owners, who can access or monitor the DSS via a software dashboard. Seasonal weather aspects can help optimize the water retention choices when severe droughts are expected to reduce the use of drinking water for keeping the vegetation healthy.

There’s something extra smart in this kind of infrastructure, too. Implementing it allows a municipality to put a value on the stored water, issue permits, and encourage engagement with subsidies. Putting an increased value on water is a good idea in a world where access to water is increasingly limited (and too often wasted). Gone are the days when water was “too cheap to meter”. Now, the challenge for municipalities may be to cope with the complexity of decentralized water infrastructure on private real estate and the effects on public centralized systems like sewage and drinking water infrastructure. With the DSS infrastructure, monitoring of rainwater capture-and-release systems is feasible for optimization of public infrastructure.

Waternet’s key role in Amsterdam

If you’re looking for a model to follow for this kind of project, look no further than RESILIO. As a partner in this project, Waternet, the public water management organization in Amsterdam responsible for nearly all facets of water in the Netherland’s most-populous city, has built strong coalitions with the city of Amsterdam and eight other stakeholder organizations to install smart roofs in four Amsterdam neighborhoods. And they’ve been carefully measuring the results. 

“We have been working on the blue-green roof concept for over 10 years now,” says Spaan, “often intertwined with our Amsterdam Rainproof program.” That climate adaptation program has seen much success and encouraged stakeholders in the city to move beyond relying on classic, public, underground drainage systems and towards designing smarter outdoor urban spaces so rainwater can be retained and reused. “We’ve been working with knowledge institutes, financial partners, garden centers, citizen initiatives – everybody who has something at stake in a city stressed under climate change.”

The importance of building coalitions

They’ve picked up a lot of partners and learned a lot along the way, particularly from the original Polderdak (polder roof) created in 2013 by De Dakdokters (The Roof Doctors), in conjunction with the Green Business Club Zuidas, the municipality of Amsterdam, and Waternet – among other constituencies.

It’s important to emphasize that one reason these projects have found so much success is because the Dutch are especially good at working with everybody who has something at stake, via careful consensus-based decision-making. This kind of approach when applied to economics is sometimes called “the polder model”. 

If you’re not familiar with the word “polder”, it essentially means “reclaimed land” and the idea behind polders is deeply ingrained in the Dutch psyche. Governing in the Netherlands arose from water boards, who were the original coalition builders, tasked with, among other things, combatting floods and draining polders to repurpose them as fertile farmland. That work required countless community hands and lots of windmills to pump out the water. You’ll find these polders everywhere in the Netherlands, usually surrounded by shallow canals that work as fencing and being enjoyed by countless birds and grazing cows and sheep – along with cyclists (like me) who navigate between villages along the raised edges (“dijks” in Dutch). 

The Polderdak project’s way of working was not simply to install a single roof as a proof of concept and wait for others to adopt it. Like the water boards of yore, this was a mission-driven, full-ecosystem approach. They wanted to address important details like market development through tendering, the concept of water neutral building envelopes, and eventually zoning plans.

The Polderdak success brought more partners to the table, often initiated by the Amsterdam Rainproof program and national dialogue arranged by Green Deal Green Roofs (predecessor of the National Roof Plan), which spurred on the RESILIO four-neighborhood network idea, which has now led to an Amsterdam rainwater ordinance that requires builders to include rainwater storage in all new building projects and large scale transformations. Progress, bit-by-bit, through partnership, open communication, and a commitment to sustainability.

Which roofs are suitable for blue-green treatment? 

“For new developments, the introduction of blue-green roofs is easy, relatively cheap, and can be integrated into the design process of buildings,” says Spaan. “More difficult is the existing stock of real estate.” As you can imagine, some of those buildings are quite old. But Waternet knew right away which roofs would be easiest to consider: social housing. 

“In Amsterdam, social housing corporations own over 40% of the real estate stock in the city,” says Spaan, “so we focused on encouraging social housing corporations to adopt the RESILIO project.” This approach of bringing rooftop oases to those who may never be able to afford a home as a public amenity resonated deeply with all of the partners, who hope to set an example for how innovation can go hand in hand with social and environmental justice. 

As part of the initial suitability analysis, they determined that a roof should be at least 200 m2, have a slope of no more than 8 degrees, and be constructed after 1950, when buildings were more and more made with concrete instead of wood, allowing for a larger carrying capacity on the roof. 

With basic roof suitability sorted, Waternet then set out to examine two neighborhoods with a strong stock of social housing: Slotermeer, a newer neighborhood with many non-western migrants with low socio-economic positions, and Rivierenbuurt, a gentrifying older neighbourhood with a considerable socio-economic divide between people with higher incomes and those who struggle to make ends meet. 

Waternet Hydraulic Analyst Jora Slinger and Project Intern Anne van Rijn, who wrote a masters thesis about the project, focused their hydraulic and hydrological studies on creating flood maps for the two neighborhoods to understand how effective the idea could be. 

“When looking at all the roofs which are suitable for blue-green, it results in roughly 17% of the two neighborhoods,” says Slinger. “Of course, when taking private ownership into account, this would be a much larger suitability.”

The hydraulic and hydrological impacts of a blue-green roof

The Slotermeer neighborhood is newer, with a separated sewer system. They chose to perform a simple 1D SWMM analysis with precipitation of 20 mm/hour, looking closely for flooding at manhole locations. “Just with one roof it is a reduction of about 4%,” says Slinger. “But looking at the suitability of all roofs which could be blue-green in this neighborhood, we saw a reduction of over 60% of flooding in those manhole locations.”

Determining the flood risk for Rivierenbuurt was a little trickier. It is an older, more densely populated neighborhood located beside the Amstel River with a combined sewer system. It has a much higher risk of flooding, and they went deeper with their flood mapping. Slinger turned to InfoWorks ICM to perform a more detailed 2D analysis that considers the movement of water through the underground sewers. She also wanted to fine-tune her model by incorporating a special standard. 

“We calculated that with our so-called ‘rainproof precipitation event’, which was 60 mm/hour at the time of this study,” says Slinger. Interestingly, this 100-year standard has since then been increased to 70 mm/hour, which shows how prudent the Dutch are when it comes to modeling for accuracy and planning for a climate-stressed future. 

Confirming hunches and learning new things 

The modeling results were positive for properties with blue-green roofs and for the surrounding areas. “In 2D, the maximum water depth using the 60 mm/hour standard shouldn’t pose any risk of flooding at the property locations,” says Slinger. “But there’s also a standard for the streets themselves. When the maximum water depth is higher than 25 centimetres, then it wouldn’t be accessible anymore for emergency services. That’s a situation we want to prevent.” 

For Rivierenbuurt, they determined a maximum of 2.7% volume decrease at high-risk flooding areas. This more detailed model also provided an additional learning. “The impact is much larger when the roofs are located more closely or more locally to those flood risk areas,” says Slinger. The 2D InfoWorks ICM model also helped them calculate the effects of a so-called second wave. “We looked at second-wave occurrences as well to determine what would be the impact of discharge from those roofs once the precipitation event has passed, but there is no risk of flooding due to a second wave, so that’s not something we must worry about.”

Ultimately, the hydraulic and hydrological models helped determine that, depending on the suitability of the roofs, between 11% and 19% of the water volume which remains in the streets during an extreme rainfall event can be prevented. 

The RESILIO project estimates that these four locations will increase Amsterdam’s rainwater resilience by adding 5,600 m3 of water managed over a year without any damage, with the ultimate goal of building functional rainwater buffers covering 10,000 m2 of smart blue-green roofs.

“I think we can conclude that blue-green roofs are really a part of the solution,” says Slinger. “They help store water during those peak moments when we really need it, but it should always be something we use in combination with other types of climate adaptation measures.”

Putting the “squeeze” in sponge cities 

The implementation of the RESILIO blue-green roofs in the four neighborhoods ended two years ago, but the social housing corporations will continue to maintain them for at least five years. Now, it’s time to spread the word, encouraging both home-owning citizens of Amsterdam and municipalities worldwide to adopt the practice. 

“Since then, we are working hard on the dissemination of our knowledge and bringing our concept of the squeezable sponge city to the broader public,” says Spaan. At conferences like Amsterdam International Water Week, he’s been seeing keen interest in the micromanagement capabilities of storage systems. “We see that it’s a growing industry, and we are proud that we could play a part of it.” 

He has some advice for builders and planners who want to make a case for blue-green roof systems in their own cities: “Creating a single innovative project is relatively easy, but implementing it on a broader scale is complex because it has to battle a lot of the existing regulations and cultures. But dare to start with the project, involve partners, and think beyond the horizon of the project end.”

“Investing in a blue-green roof system has many benefits because it creates, as we call it, a Lego landscape on the roof where you can add very easily other functions.” He recommends proposing rooftop water storage crate systems like this as a canvas upon which to paint other functions. “The crate systems are a perfect mounting surface to add fences and safety measures next to PV or pergolas – you name it – without piercing the primary roof surface!”

“It’s essential to work within the investment cycles we see in the city from other sectors like solar or insulation, maintenance, or renovation of buildings. If you work contrary to that rhythm, it’ll be too expensive,” he says. “Bringing multifunctionality to a roof can help with multiple financial streams that, when combined, can deliver a cheaper and more robust solution.”

Expanding the roofprint

The city is helping push the idea further with a grant scheme for private building owners and new regulations. “Nowadays in Amsterdam,” Spaan says, “every new building or big transformation is obliged to store 60 mm of water on its plot with a certain release of 1 mm/hour, so it has an empty storage capability in time for other rainfall or a dynamic control system.”

Can this idea of installing rooftop BGI go even further? If you think about it in bigger and broader economic terms, it most certainly can. The sheer number of unused rooftops that provide no real value to developed cities around the world is, in essence, a brand-new avenue of infrastructure hiding in plain sight.

To that end, there is another ambition being considered in Amsterdam. “We are working on creating an office or program for a so-called ‘dakregisseur’ or roof director assisted by a group of experts on roofs and accompanying fields to facilitate the further development of the multifunctional roof landscape.”

That concept is strongly connected to the program called the Nationaal Dakenplan (National Roof Plan) which is made up of (no surprise here) a coalition of municipalities, businesses, and knowledge organizations who want to create as many multifunctional roofs as possible in the Netherlands. That coalition works with the idea of multibeneficial roof agreements and through stimulation of policy instruments and new building codes, sharing of knowledge and creating of awareness, fitting financial schemes and education lines for employees and staff of partners.

Roofs are by nature valuable spaces that have traditionally been undervalued and underused, and Amsterdam is doing a smart thing by utilizing them to capture water and decrease flooding during extreme weather events. Forward thinkers like Waternet and its many partners are doing something extra innovative by attempting to create a brand-new layer of public/private space, one whose climate-conscious benefits can be applied to all of its citizens – not just the people who live and work in the buildings. 

Go deeper into the story 

There is a lot to learn from the experience of Waternet and all of its partners in the RESILIO project – and a lot to shout about from the rooftops to other water professionals: