Executive Summary

As cities continue to expand and become denser, the amount of green and permeable surfaces decreases, leading to reduced water absorption and flooding during heavy rainfall and increased heat retention during warm periods. These impacts are intensified by climate change, year by year. Traditional grey infrastructure is often ill-equipped to handle these tremendous urbanisation and climate change pressures. Amsterdam faces these very challenges and that is why it decided to test how these can be mitigated in some of its most vulnerable areas. Taking advantage of the co-funding opportunity offered by the Urban Innovative Actions (the innovation arm of the European Regional Development Fund), through the RESILIO project it delivered over 10,000 square meters of smart blue green roofs on a mix of private and social housing-estate buildings.

The blue-green roofs go well beyond what traditional green roofs offer. They are equipped with smart technologies that allow them to retain or release water based on weather predictions and real-time conditions. Sensors and remote-controlled valves manage the water storage and release, making them more effective and adaptable. By storing water, these roofs reduce peak loads on the sewer system and public space during heavy rainfall. The stored water serves to maintain the natural environment of the roofs. Particularly, during drought or hot weather, the technology has proved to offer healthier conditions for the roof vegetation. Water availability is the key for a thriving green infrastructure. In this way, concepts of circularity are effectively introduced in the urban built environment.  

While the technology was at the core of Amsterdam’s approach, the success would not have been possible without the strong partnership that was brought together, and coordination efforts put by the city. The City of Amsterdam, in collaboration with Waternet (public water management organisation), took the leading role in the project, ensuring clear communication, cooperation among partners, and adherence to project objectives and timelines. Metropolder offered the technology, while scientific partners were able to build the evidence base through experimental and modelling work to ensure the RESILIO approach is robust and fit for upscaling (e.g. biological sample analysis, hydrological modelling, heat measurements, tests of hybrid models with photovoltaic installations, economic valuations and business models for upscaling). At the same time, NGO Rooftop Revolution led a successful and innovative engagement campaign.

The RESILIO project was not a stand-alone project that suddenly came to life. It built on previous experimentations to develop a mission driven environment (aka quadruple helix partnership of public, private, academia and citizens) that invested capital, knowledge, trust, and time in the vision for a squeezable sponge city. The RESILIO project is among the largest scale cases in building this ecosystem of combined experimentation, relationship-building and concept development that leads us to the enriched multifunctional roof landscape we see today.

A year and a half after its completion, today RESILIO is seen as a pioneering project serving as a model for other cities facing similar climate-related challenges. By demonstrating the feasibility and benefits of smart Blue-Green (BG) roofing solutions, it inspires similar projects worldwide and promotes a more resilient urban future. To the testament, requests for dissemination of the “RESILIO” approach continue to arrive.

The project since its end date: a long-term sustainability plan

It has been about a year and a half since the project was completed. The roofs are in perfect working condition meeting the targets that were set in the envisioning phase of the project back in 2018. The roof vegetation is healthy. The city continues to appraise the impact of the intervention and is looking at upscaling opportunities. This is done through the mainstreaming of RESILIO within the Amsterdam Rainproof program of the city and through the policy line of the Amsterdam Rainwater Ordenance. Essentially, the aim is to integrate the RESILIO principles and technologies in the planning policies. Amsterdam requires every new building or large scale transformation to create a storage capacity of 60mm with a certain discharge rate (able to empty in up to 60 hours). This is where the RESILIO principles fit in, as this capacity can be created on a roof. Alternately, it can be created in a cistern-like system. Considering that there is also an obligation for nature-inclusive design (there is a specific parallel policy so buildings obtain “green points”), often a BG roof is the preferred solution. Of course, it is not a solution that fits all buildings and owners’ preferences. Rather, it has become part of the portfolio of options offered to developers, private owners, and any other roof-owner in the city to contribute to climate adaptation for and with the city.

Generated Knowledge: the 5 RESILIO propositions

If we had to condense the lessons learnt from RESILIO over a 4-year period, then the following 5 key propositions should be brought forward.

1. The technology can be trusted.

The project installed the technology in several roofs – after ensuring their structural integrity – and it has been a success in all cases. The technology, storage and drainage systems work uninterrupted with limited need for maintenance, i.e. not going beyond the recommended, while the vegetation is healthy supported by the stored water.

This provides the evidence that the system can be transferred safely to other buildings or cities across the world, subject of course to climatic and greening suitability. In fact, it has already been transferred to New York, and cities in Belgium, Italy and Germany. More recently, the technology has been tested in France and Guatemala as well (source: Wavin). Also, the testing answered to concerns regarding humidity and potential leakage. These have not been observed in any of the buildings (in fact, it is less likely that water will seep through than in a conventional building).  

2. Citizens demand more nature in the city.

RESILIO was deployed during the Covid-19 pandemic. While unfortunate particularly in terms of citizen engagement, the timing made it even more evident that cities need to include nature wherever possible. Feedback from the engagement campaigns was consistent: citizens request more green from the City of Amsterdam – despite the city already being among the greenest in Europe. Engagement showed that among the priorities for citizens is the protection and enhancement of biodiversity. This was not originally considered in depth by the project (i.e. in the proposal phase) but the City showed adaptability and included experts to ensure that (i) roofs are as biodiverse as possible, (ii) biodiversity is monitored.

Roofs comprise a large portion of a city’s surface, even if not on ground level, which is underutilised by people and can be a space that offers societal functions. This offers a unique opportunity for nature with multiple co-benefits for recreational space, heat and flood mitigation, biodiversity enhancements, or even food growth: all of these are ecosystem services that can potentially be offered by nature of roofs.

3. Interventions can have impact at district scale only if they are scaled further.

RESILIO roofs are decentralised pieces of infrastructure. This means that they all do their “bit” (i.e. in reducing runoff arising from the buildings where they are constructed) but cannot have a critical system level impact on their own. Their true value is realised when more blue-green solutions (BG roofs, but also other types too) are combined. This is why RESILIO emphasizes on the need to upscale. An impact assessment study carried out during the project showed that an upscaling program can have a significant impact on the reduction of district-scale flooding. Similarly, heat mitigation can be achieved at district scale through upscale – a single BG roof has negligible impact of district level heat mitigation. The cumulative effect is something Amsterdam wants to highlight through its Rainproof program campaign, as it states that “Every drop counts”.

4. Integrated solutions maximize co-benefits and maximize total value

RESILIO’s hypothesis is that smart Blue Green roof systems offer a combination of co-benefits which makes it a “value-for-money” investment decision. Co-benefits include biodiversity net-gain, water retention (representing reduction in flood damage), heat stress reduction and recreation benefits; they should then be compared to the actual costs incurred by the construction (CAPEX) and maintenance and operational costs (OPEX) over time to assess whether they are worth the investment.

The project performed an extensive economic valuation using a mix of techniques which helped inform the Cost Benefit Analysis. One critical finding is that the benefits will depend on future climate, i.e. usefulness of the measure and intensity of use will be higher under a high-emissions scenario; at the same time, the type of use of the roof, i.e. access to the roof, ensures that the recreation benefit is realised and does not remain a “potential” benefit. These different scenarios appear to influence the outcome of the CBA of the roof.

5. Grants, new business models and integrative policies can make this work.

Even if the CBA validates that the investment is reasonable (either as the only option or in comparison to an alternative grey option of sewer systems), this does not necessarily mean that there is business model to support this. This is primarily because the costs and benefits do not necessarily accrue to the same stakeholders. For instance, should a private roof owner or a social housing corporation pay for the reduction in flood risk further downstream? There is therefore a need for a business model that will ensure the investment takes place. As part of its arsenal of tools, the city developed a holistic waterproofing program, “Amsterdam Rainproof”. The program builds awareness, promotes dialogue and ownership, and ultimately action towards a climate resilient city.

Conclusions and Recommendations to urban authorities

In closing this series of four journal articles that followed the project over the last 4 years, it is safe to say that RESILIO exceeded the expectations of its co-funder, Urban Innovative Actions, and those of the City of Amsterdam itself. A project that started with many questions and uncertainties, concluded with clear answers as to how cities can innovate on a technological, procurement, governance and policy level for the benefit of the built and natural environment.

While the technology already had a prototype, it had not been tested at this mass scale before. Developing intercommunicating Blue-Green roofs on over 10,000 square meters comprises today the largest scale demonstrator of its kind. Having tested over time in multiple locations, it is safe to say that the technology is robust.

In addition, when dealing with innovation one can expect that procurement brings in some uncertainty regarding cost and specifications. Yet, very soon, the city was able to learn and adapt accordingly: now Amsterdam can procure the BG roofs and reduce costs while at the same time ensuring high-quality from their suppliers.

Last but not least, while urban authorities are not typically thought as drivers of radical innovation – besides, considering they are public-interest focused organisations, this is not their primary mission  - the city of Amsterdam made exceptional efforts to innovate on an organisational level. Through the flexible partnership formation mechanism that UIA encouraged, it managed to bring the ecosystem of stakeholders that could best deploy the project: the technologists, the water company, the city, engagement experts, and academic experts on multiple research domains that were studied to provide the evidence base behind the RESILIO approach.

These are lessons not useful just for Amsterdam, but for all European cities looking at similar Blue-Green innovation. We hope that RESILIO has provided inspiration as well as a clear pathway for other cities to lead climate resilience locally, for more liveable and biodiverse urban environments. Amsterdam showed how it is worth being mission driven, daring to create a thriving ecosystem where experiment, relations, trust, partners, failures, and involvement can grow to a new ambition for a climate adaptive urban environment.