Green infrastructure is gaining popularity, but the effectiveness of it is still being questioned. Researchers at the University of Illinois at Urbana-Champaign have found that using a mathematical technique can determine how well green infrastructure works. This mathematical technique was previously used in earthquake engineering. Researchers hope that their findings will help urban planners, policy makers and developers.

Reshmina William and Ashlynn Stillwell are studying the effectiveness of Green roofs (University of Illinois) Reshmina William and Ashlynn Stillwell are studying the effectiveness of Green roofs (University of Illinois)

Green roofs are flat, vegetated surfaces on the top of buildings. They are designed to capture and retain rainwater and filter any that is released back into the environment.

"The retention helps ease the strain that large amounts of rain put on municipal sewer systems, and filtration helps remove any possible contaminants found in the stormwater," said Reshmina William, a civil and environmental engineering graduate student who conducted the study with civil and environmental engineering professor Ashlynn Stillwell.

An eco-friendly solution to mitigating stormwater runoff may seem like a good idea, but there is a common concern with green roofs: variability on their performance. One challenge is trying to figure out how well the buildings that hold them up will respond to the increased and highly variable weight between wet and dry conditions. It is also hard to determine how well they will retain and process water given storms of different intensity, duration and frequency.

William came up with the idea to use a seemingly unrelated mathematical concept called fragility curves to confront the problem.

"Earthquake engineering has a similar problem because it is tough to predict what an earthquake is going to do to a building," William said. "Green infrastructure has a lot more variability, but that is what makes fragility curves ideal for capturing and defining the sort of dynamics involved."

William and Stillwell studied green roofs over other forms of green infrastructure because there was one on their campus that had the instrumentation needed to measure moisture, rainfall amount, temperature, humidity and other variables that are plugged into the fragility curve model.

The primary goal of this research is to start communication between scientists, policymakers, developers and the general public about financial risk and environmental benefit of taking on the expense.

"One of the biggest barriers to the acceptance of green infrastructures is the perception of financial risk," William said. "People want to know if the benefit of a green roof is going to justify the cost, but that risk is mitigated by knowing when an installation will be most effective, and that is where our model comes in."

The result of their research provides a snapshot of green infrastructure performance for their particular green roof. The results from this model don’t yield a one-size-fits-all approach to green infrastructure evaluation, which is one of the strengths of the technique. Adaptability across different technologies and environments is essential to any green infrastructure analysis.

This research was published in the Journal of Sustainable Water in the Built Environment