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Blue-green roofs to prevent flooding and provide cooling

29 April 2022
Blue-green roofs can help reduce street flooding while also helping to keep buildings cool, researchers working on the RESILIO project at the Institute for Environmental Studies (IVM) have found. The institute is a collaboration between Vrije Universiteit Amsterdam and eight other partners, including the City of Amsterdam.

Over the past three years, a network of fourteen blue-green smart roofs has been installed in Amsterdam. Together, the roofs measure over 12,000 square metres, allowing for the collection of more than 650,000 litres of water. In parallel with the RESILIO project, VU Amsterdam has also constructed its own blue-green smart roof. The roofs have the ability to store water (blue) underneath a layer of plants (green), retaining or discharging water based on the weather forecast. Among other things, the IVM investigated how these roofs contribute to reducing flooding and drought and how weather forecasts can be used for optimal control. The results were published in the Journal of Environmental Management.

Collecting rainwater
The study shows that blue-green roofs that are controlled based on weather forecasts can capture between 58 and 85% more rain than standard green roofs. When heavy rain is expected, the smart roofs discharge previously stored rainwater before the downpour. Using these roofs, the amount of rainwater collected can be optimised to relieve the sewer system. This would help to reduce flooding in cities, where rainwater is less easily absorbed by the ground because of the high density of buildings and infrastructure.

When it’s hot outside, temperatures in cities can be up to six degrees higher compared to rural areas, because stone retains heat. Blue-green roofs are better able than normal green roofs to keep buildings cool in summer. This is because blue-green roofs have the capacity to retain more water, which has a cooling effect when it evaporates. In addition, the water in the blue layer provides insulation, bringing down both the inside and outside temperature.

RESILIO project
The RESILIO project (Resilience nEtwork of Smart Innovative cLImate-adapative rOoftops), which was largely funded by the European Union’s Urban Innovative Actions (UIA) programme, is now complete. Nevertheless, the research on the effects of blue-green smart roofs continues. The results are being used by the City of Amsterdam to investigate how best to protect the city from more extreme weather due to climate change. There are also other large-scale projects that are experimenting with stored rainwater to help cool cities during heat and drought.

For more information, read the project’s final report.

Photo: a student doing research on the blue-green roof on the VU Main Building. This roof was not installed as part of the RESILLIO project, but it was used for the research. Photographer: Peter Valckx.