Bring on the rain
Rice student leads project to evaluate green roofs

BY MIKE WILLIAMS
Rice News staff

As others duck under their umbrellas, Frances Kellerman will revel in the rain this summer.

The Rice environmental engineering student is taking the lead on a project to quantify just how well green roofs retain water that would otherwise run off into sewers or the street.

JEFF FITLOW

Frances Kellerman, who will be a senior in the fall, stands atop the green roof on Rice’s new South Plant. Kellerman will spend the summer testing a system of her own design to evaluate the effectiveness of green roofs in southern climates.

Green roofs, building tops that are completely or partially covered with vegetation, help insulate buildings and reduce storm runoff, among other benefits. Rice buildings with green roofs include Duncan and McMurtry colleges, the South Utility Plant, the BioScience Research Collaborative and the Oshman Engineering Design Kitchen.

Kellerman, who will be a senior in the fall, approached Phil Bedient, Rice’s Herman Brown Professor of Engineering, about taking part in the project that has since received support from Rice’s Center for Civic Engagement and the city of Houston’s Green Building Resource Center.

The Baltimore native is designing and building a cost-effective system to measure the amount of water a green roof will retain – or not – in a typically heavy Houston downpour. The system will be installed on two roofs at the South Plant, one green and one standard, as a control. With sufficient help from Mother Nature, Kellerman expects to have results before year’s end.

“Green roofs have been studied in other parts of the country, particularly in Portland (Ore.) and Chicago, but no substantial data has been collected for the South, especially for Houston,” she said. “Rainfall in Portland isn’t the same as it is in Houston. There, a storm drops an inch of rain over an afternoon, where we get flash floods. That’s something to consider.

“In different rainfall patterns, the growing medium of your plants absorbs different amounts of water,” she said. “There are a lot of variables with green roofs: the plants you use, the type of soil and whether it’s an engineered medium or gravel.”

Bedient, who directs the Rice-based Severe Storm Prediction, Education and Evacuation from Disaster Center and is known for his work on groundwater hydrology and flood prediction systems, said standard designs that may be fine for other parts of the country aren’t tuned to Houston’s climate. “Green roofs are all over the place,” he said. “They just put them up, but nobody’s really analyzed how well they’re doing. We may come up with some simple solutions to better address the intense rainfalls we get.”

Measuring flow through a system can be complicated. The team originally considered installing monitors at the base of drainpipes inside the South Plant. “We found out flow meters can’t be used on nonfull pipes. They need to be full of water all the time,” Kellerman said. Working inside would also have meant cutting into the plumbing to install meters, an expensive proposition. “So we went back to the drawing board.”

Kellerman settled on a combination of electronics and a PVC pipe that will slow the flow into certain drains atop the South Plant while not inundating the roof. “When rain flows into the pipes, the water will be restricted,” she said. “Water will accumulate above the hole, and the height will be calculated by a pressure transducer, which will send that data to a computer.”

Custom software designed around Kellerman’s calculations will determine how much water stays and how much goes. The system is designed for storms that produce a half-inch to an inch of rain per hour, she said.

“Green roofs really perform when you’re dealing with one to three inches of rain, and we get a lot of those in Houston, maybe 20 per year,” Bedient said.

“The beauty of the project is that we may have tripped upon something relatively inexpensive to do that the city can take advantage of,” he said. “This is exactly the kind of project the Center for Civic Engagement is looking for. I’ve done three with them now, and this one has the strongest linkage back to the city and its resources.”

For Kellerman, working with faculty and city engineers on a practical application has been a revelation. “I’ve been able to experience, as an undergraduate, how collaboration in the engineering field works, and how it’s necessary. That’s been great.”