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Green-Roof Study Results Offer Positive Surprises
By Nadine M. Post
Magnusson Klemencic Associates
+ click to enlarge
Magnusson Klemencic Associates
Engineer ran study to prove value of green roofs to communities.

A green roof has the potential to lower stormwater runoff from 65 to 94%. In cities with combined sewer systems, that translates to a significant reduction in the demand placed on wastewater treatment plants during storms, according to a two-year study of green roofs in Seattle. (1.68MB)

In addition, green roofs allow building owners to reduce the size of stormwater detention tanks needed for peak rainfall. That can offset the cost of a green roof by 30 to 60%, according to Magnusson Klemencic Associates, the local engineer that led the $125,000 green-roof evaluation project. The study quantified the effectiveness of green-roof stormwater management, based on rainfall and weather conditions, from July 2005 through this January.

At best, “only 6% of the rain became runoff,” says Drew A. Gangnes, MKA’s director of civil engineering, who led the study. “That’s a huge finding.” The engineer had projected 50% mitigation based on earlier modeling.

Magnusson Klemencic Associates
Civil engineer Drew A. Gangnes, who led the green roof evaluation project, maintains that cities should give incentives to developers to build green roofs because they can significantly reduce stormwater runoff and demand at sewage treatment plants.

MKA also found the “magic number” for soil thickness to be 4 in., not 6 in., as suspected. The thinner soil bed can dry out more quickly, readying itself to absorb the next rainfall, says Gangnes.

Green roofs often get value-engineered out of a project because even the simplest installations, off limits to the public, costs $7 to $10 per sq ft. Gangnes wants more cities to offer developers’ incentives.

“Seattle fully expects to provide drainage rate credits at some level for green roofs and other low-impact development stormwater technologies beginning in the 2009 rate year,” says Dick Lilly, Seattle Public Utilities’ sustainable strategies’ policy advisor.

Calling the MKA results “really encouraging” for a small-scale study, he says the utility has a three-year project to monitor several full-size green roofs. It will look at MKA data and others.

The seeds of the MKA study were planted about five years ago by clients asking whether a green roof could replace a detention tank. To answer the question, MKA developed a tool that models water evaporation, percolation and storage in the soil matrix. Research and a tour of green roofs in Sweden convinced Gangnes that green roofs were a promising stormwater management tool. But  there were no measurements to prove it. There were green roofs, but they were not being monitored, says Gangnes.

For help in underwriting the costs of the monitoring gear and the five, 8 x 12-ft building plots at four test sites, MKA enlisted local developers Urban Visions, Unico Properties and Vulcan Inc., and one local contractor, Sellen Construction Co. Additional local contributors include Cedar Grove, for soil; landscape architect Gustafson Guthrie Nichol for consulting on planting and reports on plant materials; mechanical engineer-constructor McKinstry for weather stations and monitoring solutions; Sunbelt for scaffolding needed to elevate a test site on a parking lot; and Ness Cranes, for a hydraulic crane used to lift the dirt onto the roofs.

The $125,000 cost includes in-kind services. About two-thirds of that was for MKA’s soft costs.

Green-roof cost can be offset 30 to 60% by a decrease in the size of the stormwater detention tank.
Photo Engineer had predicted that 50% of stormwater would be mitigated, not 65 to 94%, as monitoring showed.

Each plot contained a different green-roof thickness and/or planting medium. That allowed a range of performance characteristic data. The four different downtown locations helped determine whether prevailing winds, building shadows and other variables affected performance. The weather station and monitoring equipment, wirelessly linked to MKA’s office, allowed the collection of data, including amount of rain,  air temperature, humidity, wind speed and soil moisture. MKA collected more than 1.5 million measurements.

At each site, rain was first taken up by the plants. Then, it dissipated through evapotranspiration. Finally, the water slowly made its way through the green-roof materials. Water that made its way to a drain at the corner of the installation was measured for flow and temperature before it was discharged to the existing roof. The data quantified the amount of runoff to both the building and to city infrastructure downstream.

Sellen contributed a site for an installation, and built the five plots. The contractor is now applying lessons learned on its first green-roof project to its first large-scale, permanent green-roof project, a 65,000-sq-ft installation that will top a 1,000-car garage for the 500 Fifth Avenue North Development Campus. It currently is a hole in the ground.

“The MKA project helped us work with the design team and with the city to explain the benefits of reducing or deferring detention,” says Jack Avery, Sellen’s director of preconstruction services.

Gangnes says that there is very little structural penalty for a green roof on a new commercial building. For a 12-story steel frame, the penalty is less than 0.5% of the steel tonnage. On a concrete frame, where the roof is a concrete slab, there is “virtually” no change to the structural design.

The weight of gravel ballasted to the roof to hold the membrane down is about the same as the weight of the 4 in. of soil, says the engineer.

Gangnes, who just started building a 150-sq-ft pitched green roof on an addition to his house, hopes to do more studies on the topic. “Every gallon of rainfall eliminated at the building site is a gallon that does not require treatment with chemicals, processing, etc.,” he says. “Research also suggests that this local repairing of the hydrologic cycle can reduce urban temperatures, which saves energy as building cooling demands are released.”

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