• 16 December 2012

Restoring Ecosystem Services

Restoring Ecosytem Services….One Roof at a Time

The Issue
Although not by any means a new concept, green roofs are becoming an increasingly popular trend in sustainable building design. From their origins in Scandinavia, green roofs have spread throughout Europe and the rest of the world. As well as being popular with ‘green’ building designers, green roofs are gaining increasing kudos with town planners, which can be seen by their increasing prevalence in planning policy… but what environmental benefits do they actually provide?

Historically, urban areas have been stripped of ecosystems that previously existed. Ecosystems provide environmental services such as drainage, cooling, and pollution sequestration. Some of these services are partly replaced through the use of ‘grey’ infrastructure; however, grey infrastructure tends to be single purpose with a short lifespan and historically fails to replace ecosystem services lost during urbanisation. Green roofs present an opportunity to reintroduce some of these ecosystem services across urban areas on what is otherwise unused roof space. Should we define grey infrastructure?

Climate Change Adaptation Research
Waterman has recently received funding from the Technology Strategy Board (TSB) to develop a climate adaptation strategy for an office / civic building in the midlands. As part of this study, Waterman are modelling the effects of various green roofing strategies on the microclimate surrounding the building using the software ENVI-met. Through cooling the microclimate surrounding the development as well as the inside of the building, the installation of green roofs will help the building adapt to increased temperatures associated with climate change. The model will help advice on the green roof design to ensure that environmental benefits to the project are maximised.

Green Roofs
There are two types of green roofs, extensive (shallow substrates, light weight, often planted with sedum) and intensive (deeper substrates, diverse/elaborate planting). Both types of green roofs have been shown to provide a variety of ecosystem services, and thus environmental benefits. The environmental benefits tend to be greater for intensive than extensive green roofs.

green-roof_int

Storm Water
With the increasing severity of rainfall events associated with climate change, there is going to be increased strain put on already stretched drainage systems in urban areas. In nature, rainfall is absorbed into the ground and travels through the soil, preventing build-up of surface water. However, throughout urban areas, due to the prevalence of impermeable surfaces water travels over the ground at an increased rate; collecting pollutants as it goes and often causing localised flooding as a result of blocked drains or simply because the drainage system just can’t cope with the volume of water. With climate change, the frequency and intensity of severe rainfall events are predicted to increase in the future, meaning that flood events are also likely to increase in the future.

Green roofs have been shown to decrease localised flooding events. They act in a similar way to natural ecosystems; storing storm water in the soil beneath the vegetation. This water is then released slowly over time through drainage and evaporation.

The Urban Heat Island Effect
The Urban Heat Island Effect (UHIE) refers to the relative increase in temperature of urban areas compared to the surrounding rural areas. UHIE can be linked in many ways to the removal of ecosystem services from urban areas; increased heat storage in building materials; decreased albedo; and lack of evaporative cooling. Green roofs and walls increase the albedo of the building envelope, meaning that heat is reflected from buildings rather than being stored in their fabric. Through evapotranspiration green roofs also provide cooling to both the inside of the building as well as localised microclimate cooling.

Green Roofs and PV…?
A common misconception facing green roofs is that they compete for roof space with photovoltaic panels (PV); this is in fact not true. When installed on green roofs, PV are found to be more efficient due to the localised cooling provided by the green roof. In addition, PV shadow creates a localised microclimate which can improve the biodiversity of the green roof.

Other Benefits
As well as the benefits listed above, green roofs have also been shown to absorb pollutants, increase biodiversity and increase the building occupier’s sense of wellbeing.

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