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Building a resilient future: the role of blue-green roofs

by Jennie Ward

With new environmental legislation set to become law next year, Nicholas Wright of Polypipe Civils & Green Urbanisation looks at new roof-level innovations that will help contractors create a more sustainable future.

As part of the UK’s aim to be net-zero carbon by 2050, there is an increasing focus on what developers and contractors can do further to make our regions more resilient to the long-term impact of climate change. This responsibility has been made even more pertinent as the Environment Bill comes to power next year bringing unprecedented reform to our foremost environmental priorities.

Above: Nicholas Wright.

In recognition of the growing pressure on combined sewers as a result of population growth, increased urban density and the challenges we face as a direct result of climate change such as increased rainfall and severe flooding, one aspect of the Bill is the need for water companies to publish data on storm overflow operations on an annual basis.

For contractors and developers, this means that there is likely to be increased pressure from water companies and planning authorities to further reduce stormwater discharge when designing surface water management systems – a challenge which, if considered in a creative and holistic way could in fact be a catalyst to create more sustainable, healthier and higher value developments and address other aspects of the upcoming Environmental Bill, namely Biodiversity Net Gain (BNG).

Under the BNG policy, developers will need to achieve a minimum of 10 per cent improvement in biodiversity on or near a new build site to get planning consent. These changes mean developers should now re-evaluate how to introduce green and resilient infrastructure to ensure that projects comply with sustainable drainage best practice and deliver an uplift in biodiversity post-completion.

New alternatives
While conventional sustainable drainage systems (SuDS), such as retention ponds, basins and swales provide a route to manage excess run-off rainwater brought about by extreme weather, they often take up significant amounts of developable space and limit the aesthetic and landscaping options.

Blue-green roofs, however, offer a hybrid solution bringing together the best of both worlds, providing breakthrough stormwater attenuation and re-use technology that transforms unused spaces into havens for wildlife to flourish, creating more resilient urban landscapes.

Blue-green roofs are designed to provide space to store and re-use surface water at source. Managing water in this way spreads storage across a development, introducing urban greening in a balanced way to maximise the biodiversity benefit.

Any water surplus flows, at a restricted greenfield run-off rate, to a stormwater sewer. Alternatively, surplus water can discharge into an above or below ground rainwater recovery tank to recharge the blue-green roof reservoir once depleted, maximising the amount of collected rainwater that can be used. And by combining the reuse tank with smart systems, further reuse efficiencies can be achieved.

The green element of a blue-green roof is planting sustained by the captured and stored water. A zero-energy passive irrigation system within the attenuation void supplies water to the vegetation above. Having access to these stored volumes builds-in resilience, maintaining the plants through extended dry spells and preserving the roof’s ecosystem.

The benefits of this approach are many: communities are surrounded by flourishing, diverse wildlife – a recognised contributor to better health and wellbeing – and developments are naturally cooled, preventing the heat-island effect and in turn, reduces air-conditioning energy consumption.

Tools in action
Bringing this approach to life, recently, in Manchester, contractors working on the redevelopment of Bruntwood Works’ latest Pioneer building, Bloc (see right), have installed a blue-green roof which will provide a greater understanding of how new construction and data technologies can help cities and urban developments to mitigate the impact of climate change and population growth, while enhancing biodiversity.

Over the next two years, the ‘smart’ blue-green roof, will be used to assess how storing and re-using rainwater at roof level can reduce the volume of surface run-off entering its sewer network. As a result, it will help to lower the flood risk associated with the prolonged high-intensity storm events that are becoming increasingly frequent as the climate changes.

Unlike conventional green roofs which use a drainage layer to simply remove rainwater, the 525 sq m blue-green structure retrofitted to Bloc’s flat roof stores rainwater beneath the planted surface where it lands. Advanced passive irrigation components within the attenuation layer draw water up through the structure to the underside of the green roof substrate to support surface planting.

The breakthrough technology protects green areas during periods of drought, reduces potable water demand during hot weather and enhances biodiversity by maintaining flora in optimum growing conditions. In the case of Bloc, planting has been specially selected from local varieties to help attract pollinators and a particularly rare butterfly, the Manchester Argus.

This net-zero re-use of water to sustain surface planting is a new approach to water management known as green urbanisation and represents the next generation of SuDS – those which are scalable and adaptable to any urban setting – allowing high natural capital ecosystems to flourish even through extreme weather conditions.

By utilising available space across the built environment to both store rainwater and reuse it to create and sustain urban green assets, developers can deliver a wide range of multifunctional benefits, enriching the biodiversity, health and wellbeing of the communities in which they are established for generations to come. And by choosing solutions which divert stormwater away from sewers, such as blue-green roofs, contractors and their knowledge of green urbanisation will also become integral to the ways in which water authorities and town planners design our cities of the future.


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