CHAMPAIGN — A living laboratory atop the University of Illinois Business Instructional Facility is providing scientists with data on how green roofs can help communities better manage storm water.
A 1,200-foot-section of the roof on the fourth floor of the BIF, as it's popularly known, is covered with plants and serves as a field lab for engineering researchers.
Green roofs are said to discharge less water than a conventional roof, help filter out pollutants from the storm-water system, remain cool in warm weather rather than storing heat, and better insulate buildings in the winter.
Faculty and students from the Department of Civil and Environmental Engineering are studying how well BIF's green roof performs compared with an adjacent conventional roof.
With support from the University of Illinois Environmental Council and the College of Engineering, Professors Arthur Schmidt and Charles Werth and a team of students installed monitoring equipment on both roofs in 2008.
Two 10-foot tripods hold rain gauges and instruments that measure incoming and reflected solar radiation, air temperature and humidity. Fifteen soil-moisture and -temperature probes are scattered around the roof. Gauges inside the roof drains measure water levels.
They're all powered by a solar cell on the tripod, and the data are stored on-site and transmitted wirelessly to a computer lab in the Newmark Civil Engineering Lab Building across campus.
The instruments allow scientists to measure, among other things, how much precipitation falls on each roof, how much of that precipitation runs off into the storm system and how much water is stored in the soil of the green roof.
So far, Werth said, the roof is performing as expected overall, reducing water runoff by as much as 55 percent compared to a conventional roof.
But the data vary depending on the amount of rainfall and soil conditions. In small storms, the roof retains almost all the water, but in big storms it holds a small percentage, Werth said.
Data also showed that the green roof significantly delayed peak water runoff when the soil was dry, but had a much smaller effect when the soil was wet.
As part of her doctoral thesis, graduate student Najwa Obeid is incorporating the data into a mathematical model that will help scientists better predict how green roofs perform under different conditions.
She has applied the results from BIF to the Illinois Urban Hydrologic Model for the city of Dolton, near Chicago, which is designed to simulate how green roofs could alleviate flooding in a city. Dolton was chosen because it has extensive data on its storm-sewer system, said Schmidt, who has worked with the Metropolitan Water Reclamation District in Chicago on related projects.
Urban areas create impermeable surfaces — buildings and paved areas — that move rainwater much more quickly into storm drains. That requires huge investments in city infrastructure and can lead to flooding. It also transports more pollutants into the water system, as they're not filtered out naturally by plants and soil, said Werth, who specializes in that topic.
Ultimately, Obeid's data will help cities predict how much flooding could be reduced by different percentages of green roofs, so planners can design storm systems accordingly and decide how many green roofs make economic sense.
"It's not that every time you add a green roof you get some reduction in flooding," Werth said. "Is there some threshold for the number of green roofs where you get maximum benefit at minimum cost?"
The monitoring equipment also takes water samples from BIF's green roof to test its impact on pollution runoff, but that project proved too time-consuming for this study, Werth and Schmidt said. They hope a future student can tie that project into Obeid's data.
They also plan to use other BIF data to examine the green roof's insulation value compared to a conventional roof.