Urban green roofs could ease flooding woes — environmentalresearchweb

Urban green roofs could ease flooding woes - environmentalresearchweb

For many of us it is becoming a familiar sight: water bubbling up out of the gutter and roads turning to rivers, as urban drainage systems struggle to cope with an intense downpour. As well as closing roads and flooding houses, this mucky water pollutes our streams, rivers and lakes. Climate modellers predict that the issue is only likely to become worse as global warming brings more of these extreme storms. One way of easing the problem could be to install more "green roofs" to help absorb the heavy rainfall and release it slowly into the drainage system. So how much difference could urban green roofs make? A study undertaken in New York City has some answers.

New York, like many other cities, has combined sewer systems to manage both storm water and sewerage. Even moderate rainfall events (3 mm in one hour) can lead to overflow problems, releasing polluted water into local waterways. In the US an estimated 850 billion gallons of water pollution is caused this way every year and in New York alone an average of more than 400 overflow events happen annually, releasing around 20 billion gallons of polluted water.

Green roofs growing plants on a roof can help by capturing some of the rainwater and releasing it into the drainage system more slowly. But some green roofs may be more effective than others. To assess how effective green roofs might be, Patricia Culligan from Columbia University in New York and her colleagues compared three of the most common types of green roof installed on buildings in New York.

First was a "vegetated-mat" system installed on a residential building at Columbia University. Next was a "built-in-place" system installed on the United States Postal Service (USPS) Morgan General Mail Facility. Finally the researchers studied a "modular-tray" system installed on the Con Edison Learning Center.

"For the vegetated-mat and built-in-place systems, substrate is installed over a specialized drainage course to prevent ponding and surface runoff that could cause soil erosion," explained Culligan. The difference between the two lies in the installation. "Vegetated-mat" systems come attached to their drainage mat and are rolled out like a carpet, whereas "built-in-place" systems are constructed and landscaped on-site in special bounded regions. Modular trays are different again. "The walls of the trays restrict surface runoff, while the tray bases provide corrugated air space for drainage, and therefore may be placed directly on a roof’s waterproof membrane," said Culligan.

Culligan and her colleagues collected continuous rainfall and runoff data from each of these three roofs between June 2011 and June 2012. Their data included 243 storms with rainfall ranging between 0.8 mm and 180 mm in depth.

Urban green roofs could ease flooding woes - environmentalresearchweb

So how did each roof perform? Over the study period the team found that the Columbia University roof retained 36% of the total rainfall, USPS retained 47% and Con Edison was the winner with 61% retention. "We believe that during these extreme events, green roof performance is mostly a function of the total potential storage volume, which is thought to be highest on Con-Edison, due to the regulation of runoff by outlets at the base of each tray, and lowest for the Columbia University vegetated-mat roof, based on its thinner substrate depth," Culligan told environmentalresearchweb. The findings are published in Environmental Research Letters ( ERL ) .

However, Culligan and her colleagues caution against jumping to conclusions. "It is important to note that no two green roofs are likely to perform identically, even those of the same construction type, due to changes in the location of non-vegetated spaces, local weather patterns and selected vegetation, as well as the quality and type of installation," she said. Nonetheless, these results make it clear that green roofs can play a significant role in reducing the incidences of flash flooding in our cities.

Related links

  • Hydrological performance of extensive green roofs in New York City: observations and multi-year modeling of three full-scale systems, T B Carson, D E Marasco, P J Culligan and W R McGillis 2013 Environ. Res. Lett. 8 024036
  • ERL
  • Patricia Culligan

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