Syracuse University Magazine

6806.jpgProfessor Cliff Davidson surveys the OnCenter green roof.

Measuring the Success of Rooftop Greenery

On a picturesque spring day, Professor Cliff Davidson and Joey DiStefano ’14, an environmental engineering major, stand atop the OnCenter roof in downtown Syracuse amid 1½ acres of green vegetation splashed with rusty gold and crimson, and discuss where to place research equipment. As one of the largest green roofs in the Northeast and the centerpiece of Onondaga County’s green infrastructure initiative, the roof subverts vast amounts of storm-water runoff and will provide Davidson and his research team with valuable data for long-term studies. “There’s not much data on green roofs around the world,” says Davidson, the Thomas and Colleen Wilmot Chair in Engineering at the L.C. Smith College of Engineering and Computer Science. “This is right at the forefront of the work. There isn’t any other place like this that is fully instrumented. We have a unique facility here in Syracuse.”

For Davidson, the project truly represents a watershed moment in sustainable engineering research. In collaboration with Onondaga County, Davidson and his research team, which includes three doctoral candidates and several undergraduates, will study the effectiveness of the green roof, using real-time sensors and specially designed equipment to examine storm-water capture and runoff, energy movement through the roof, and evapotranspiration—the natural combination of evaporation and plant transpiration. “We want to study this green roof from different perspectives to understand as much as we can about its performance,” he says. “The county has been terrific in terms of its willingness to collaborate.”

During this trip to the convention center, Davidson and Di­Stefano visit their data room, where a series of cables will feed information from sensors into a data logger. They scout the roof for a location to place an antenna that will transmit collected data to an antenna on the Whitman School of Management building and on to a campus server. They also survey the colorful sea of vegetation, which features five species of sedum and one of phedimus. “They have really shallow roots and act like sponges,” Di­Stefano says. “They absorb and retain a lot of water—the goal of the green roof.” The sturdy, drought-resistant plants sit in three inches of an artificial growth medium—a soil substitute specifically designed for green roofs—that covers a waterproof membrane atop insulation and roofing structure. Davidson and Di­Stefano consider where to place a lysimeter, a sensor-equipped plastic container filled with the growth medium and vegetation that essentially replicates a section of the rooftop greenery. It will allow them to measure the growth medium’s moisture content and determine how much water vanishes through evapotranspiration. They plan to locate four lysimeters, which DiStefano designed for the project, on different areas of the roof, resting them on load cells that will continuously weigh the containers. “I couldn’t ask for a better project to be involved with,” says Di­Stefano, who has worked on Davidson’s team since last summer. “It’s so at the forefront of everything with green infrastructure and even with wireless data sensing.” Davidson also notes they’ll be able to see if there are any substantial differences from one part of the roof to another. “At some places on the roof, we expect to have more air turbulence, and that is likely to carry more of the water away and assist in evaporation,” he says.

They’ve also equipped the roof with a weather station and two types of precipitation gauges—a tipping bucket that tips every time the water in it reaches a certain level, and a weighing bucket, which provides a continuous weight of the rain or snow. It’s surrounded by an alter-shield, two layers of sturdy metal fashioned to fend off the wind. “I’m sure it will be a constant battle against the elements to keep all this stuff going,” Davidson says. Amid all this, Davidson also has a tipping bucket and other measuring equipment on the roof of the nearby Onondaga County Justice Center that will act as a control roof, allowing them to compare measurements between the two locations.

For Davidson, the project’s every detail—and there are a lot of them—requires careful analysis. Davidson and his research team have been developing experiments and equipment for more than two years, and he expects to begin gathering data in 2014. When the OnCenter green roof construction began in spring 2011, they placed temperature sensors in the internal layers of the roof, from the top of the exhibit hall ceiling through to the growth medium. “We can look at energy transfer from the inside of the building to the outside,” he says. “We’re interested in learning about energy loss through a huge roof like this.”

The green roof is part of the county’s nationally recognized Save the Rain program, launched in 2009 as part of a legal agreement to reduce combined sewer overflows (runoff and raw sewage) into Onondaga Lake and its tributaries—the result of a longstanding problem of local sewage treatment plants being overwhelmed with storm runoff. According to the county, the $1 million initiative removes an estimated 1.03 million gallons of storm-water runoff annually from the sewer system—a figure that Davidson’s research will fine-tune.

Mallory Squier, a doctoral candidate in environmental engineering who’s helping Davidson manage the project, can’t wait for the data to start rolling in. “I love green roofs,” says Squier, who was involved in a green roof project as an undergraduate at Penn State. “It will be really exciting when we get to quantify how much water the roof is retaining.” In Davidson’s Link Hall lab, Squier is working on calibrating an electromagnetic flowmeter, or magmeter, that will measure runoff through the green roof’s drain pipes. They’re testing the magmeter on a 10-inch pipe with a 4-inch pipe diversion (for measuring lower flows). The pipes have a section of Plexiglas, so the water flow can be seen. According to Davidson, there are 25 roof drains that connect into 10-inch pipes, and inside the OnCenter exhibit hall three of them will be fashioned similar to their lab model. One is already installed and will be complemented in the exhibit hall by an educational display explaining the process.

Educating people about the green roof is part of the collaborative effort with the county. As they complete their equipment installation and begin to gather data, Davidson’s team plans to work with the School of Education to create a web site and post the real-time data, making it available for science teachers to use in their classrooms. “We’ll have web cams up there, too, so kids can look at the web site and see it raining on the green roof and see the changing colors during the year,” Squier says. For Davidson, the project is also an opportunity to demonstrate how nature’s processes can be incorporated beneficially into our built environment. “There are countless services that ecosystems provide for us,” he says. “So there are tremendous opportunities for understanding the link between human civilization and natural processes that this green roof can reveal to us.” —Jay Cox

6687_2.jpgDavidson and his team of students explore the plant life of the OnCenter green roof. In the foreground is a weighing bucket surrounded by an alter-shield, which is designed to fend off the wind.