Green Roof Study Plants Seeds for Stormwater Control

Civil engineer Drew A. Gangnes's project replaces hunches with numbers

His educated guess that green roofs can reduce stormwater runoff and therefore reduce peak demand on wastewater treatment plants in cities with combined sewer systems wasn’t enough for Drew A. Gangnes, director of civil engineering for Magnusson Klemencic Associates, Seattle. He knew he would need to arm himself with hard numbers if he wanted to try to convince politicians that giving developers incentives to build green roofs might pay off by minimizing the need for capital investment in expanding plant capacity.

Magnusson Klemencic Associates Gangnes’s green roof instruments measured the potential of an installation.

Green roofs already allow building owners to reduce the size of stormwater detention tanks needed for a peak rainfall. But that only helps reduce the cost premium for a green roof by 30 to 60%, says Gangnes. Developers need incentives to further defray the cost, he adds.

Gangnes got his numbers by organizing and running a pro-bono green roof evaluation project, supported by a group of Seattle developers and contractors. The 18-month study began in mid-2005 and provided data for the first time in the U.S. that a planted roof has the potential to reduce stormwater runoff from 65% to 94% .

Gangnes’s pet project did not go unnoticed by the city. It plans to offer drainage rate credits for green roofs and other low-impact-development stormwater technologies beginning in 2009.

While the study was under way, the civil engineer applied his green thumb to an even bigger initiative. He was influential in the 2007 adoption of the so-called Seattle Green Factor. It is the first green area ratio initiative in the U.S.

SGF is a greening requirement that lets developers achieve a certain metric by selecting from a menu of greening elements, including green roofs, permeable pavements, trees and vegetation.

Multimedia  View Award of Excellence Luncheon and Acceptance Speech of Drew A. Gangnes   April 3, 2008

Using the green area factor as a vehicle, “we at MKA started to realize how green roofs could play a role beyond reducing stormwater runoff by helping to green the cityscape,” says Gangnes.

Melissa Keeley, a fellow at Columbia University’s Earth Institute, New York City, worked with Gangnes on SGF. “What impresses me about Drew is that, for an engineer working in a firm, he’s amazingly and surprisingly big picture,” she says.