You might already be wondering “I know what a green roof is and how cool it is, but what’s the blue colour about?”. Well, that’s the technology that Amsterdam will test, pilot and the upscale to manage rainfall on building roofs. Put simply, there is a water storage layer below the green areas throughout the surface of the roof. When it rains, water does not run-off to the drain pipe, instead it is stored there. Then, it slowly supplies the necessary soil moisture for the roof greenery, the plants and trees. That’s done through multiple capillary suction tubes that connect the soil to the underlying water reservoir (nothing fancy, do not get intimidated!).
Multiple Benefits of smart blue-green roofs
You might now wonder “All this effort to save some water?”. There is much more to water-saving and the multiple benefits arising from the smart Blue-Green roof interventions make it possibly a good value-for-money option.
Neighbourhood scale flood control
In fact, the primary reason for storing water at the roof is flood protection during extreme events (intensity of up to 60 mm/h). As the housing complexes are in flood prone areas, storage offered by their BG roofs results in flood risk reduction. In contrast, hard impermeable surfaces such as traditional roofs, roads and pavements contribute to run-off generation. When the sewer capacity is no longer sufficient to convey this excess water, then flooding occurs.
Now imagine several buildings with BG roofs coordinating between them through a smart ICT decision support system for micro-water-management that controls the volumes and timings of water storage and release, and there you have a neighbourhood scale flood risk reduction from building scale interventions.
Of course, one can argue that it is still costly and it might be better to upgrade our sewer systems. This is a valid point and certainly a cost-benefit analysis comparing the two options should be performed. However, this would require that the additional benefits (below) are also co-valued. Along this direction, the BeST tool developed by CIRIA (UK) has been used to perform monetary valuations of multiple benefits.
In addition, one should consider that sewer upgrades require one-off interventions to carefully decide on the upgraded pipe dimensions. However, as one can hardly predict how much climate will change the intensity and frequency of extreme rainfall events, it is difficult to get the new pipe size “right”. Too big would possibly be unnecessarily expensive, too small (minimal upgrade) would be too expensive for a marginal benefit. On the other hand, installing smart blue-green roofs as part of a long-term plan, gives flexibility to the city to adapt the rate of delivery of blue-green to the actual rate of climate change. Additional blue-green measures (e.g. swales, raingardens) can also be part of an integrated solution.
Hard engineering pipe-upgrades slowly lose their effectiveness as more and more areas become urbanised: this is because the sewer systems can no longer cope with the increased runoff contribution to be conveyed. Rather, the paradigm globally has changed to “first hold stormwater at source, then allow it to infiltrate or release it slowly to reduce the peak discharge”.
In summary, it is not about one type of infrastructure (grey or blue-green) being better than the other as many times argued; it is rather about integrating them in the optimal manner to reduce flood risk.
Aesthetic and Recreational Value
A pipe under the ground is useful when it rains, but does not really offer any other service the rest of the time. Blue-green roofs, on the other hand, have an aesthetic value which is maximised through landscaping that make these roofs conspicuous, that allow human participation to aid ecological knowing, and engagement through maintenance (Sutton, 2014). These dimensions will not necessarily be all present in all RESILIO blue-green roofs, but will be explored depending on the intended use.
In addition, the blue-green roofs practically covert empty spaces into parks. Pavilions and playground areas that are suitable for recreation, socialization and relaxation for local residents. While open spaces are slowly disappearing within cities (luckily Amsterdam still has plenty), green roofs can partially mitigate this loss of recreational space.
Although disconnected from the natural environment, blue-green roofs can surprisingly contribute to urban biodiversity. This contribution is maximised when the design and construction is based on planning strategies that take into consideration surrounding environment and wildlife, as well as the conditions of the exposed roofs (Brenneisen, 2017). While ground conditions cannot be reproduced (particularly considering the low depth of soils). A wide range of planted or seeded wildflowers can be included as well as sedums. Depending on design and type of vegetation, a wide range of insects and birds will soon find their way to the green roof (LivingRoofs). The RESILIO interventions will be monitored and evaluated to assess the actual impact of the interventions on urban fauna and flora.
Micro-climate moderation benefits
The roofs offer improved building insulation and also support micro-climate regulation by mitigating the Urban Heat Island effect. As a result of the evapotranspiration of the stored water and the improved thermal comfort, a more liveable environment is offered at the top building floors. Such bioclimatic design can help reduce energy bills and also help the City to contribute to its energy efficiency goals. Similarly, RESILIO will study the efficiency to incorporate in the business case of the project. More energy savings make the investment more attractive.
Why should you care?
You might think that this is a luxury for your city and that this can only happen in visionary Netherlands. As was shown, this is certainly not a luxury, as the solution is quite practical for many urban areas under the risk of flooding. The alternative of replacing pipes for bigger ones can be more expensive and might not even solve the problem; besides, this does not deal with the root cause of the problem but with the impact instead. In addition, Amsterdam is planning to incorporate RESILIO in its wider strategy to protect and enhance biodiversity in urban space. Similarly, many cities that suffer from a decline in biodiversity and lack sufficient community space can benefit from the RESILIO approach. Finally, most cities suffer from increased, unpleasant temperature that could be moderated through natural processes. Surely you have found some relevance with your local environment.
How to get involved
It is very simple, just get in touch. Amsterdam is looking for cities that would like to test the idea. In particular, we are looking for cities with different characteristics, e.g. different climates, types of roofs, water governance structures. We wish to test our technology and approach to all sorts of different conditions and adapt them to make them applicable to other regions. Similarly, we want to build business cases that are suitable in different contexts.
For this reason, we have recently prepared the RESILIO Amsterdam Agreement as part of the Amsterdam International Water Week to commit in knowledge transfer with interested Cities.
Get in touch!
This is the first web article of a series to follow between 2020-2022 presenting news about RESILIO. Stay tuned!