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How Much Storm Water Do Eco-Roofs Capture?

It’s pretty well known that eco-roofs can reduce pollution from storm water runoff by capturing it in plants, and and slowing its flow into rivers and streams.

But exactly how much storm water an eco-roof captures is less clear.

For the past three years, Portland-based Columbia Green Technologies has been selling eco-roofs.

But according to chief operating officer Greg Love, his company could improve its sales pitch to commercial businesses if there were better information about just how much storm water an eco-roof can control.

This week, the company got an $80,000 grant from Oregon BEST to do a study with Portland State University in a lab that will actually simulate different storm conditions and model an eco-roof’s performance.

“We’re trying to take it up a notch,” said Love. “How much does an eco-roof retain or slow down in what kind of rainstorms? The green roof industry is still in its infancy, with lots of claims being made but very little data to back up those claims.”

Barge builder Gunderson LLC has built 3,500 square feet of eco-roof on its 60-acre site in Portland while working to reduce storm water pollution.

Barge builder Gunderson LLC has built 3,500 square feet of eco-roof on its 60-acre site in Portland while working to reduce storm water pollution.

“With our model, you should be able to plug in where you are and find out with this much green roof this is how much much storm water we can control,” said Love. “Up to now that has been impossible to show.”

As I reported earlier this year, Gunderson Marine has built two eco-roofs on its 60-acre industrial site in Portland and is planning a third. Gunderson Environmental Director David Harvey said the eco-roofs are a small part of the company’s effort to meet new storm water regulations.

However, the company has deliberately built habitat roofs over hazardous material storage areas to further reduce the rainwater exposure in those places.

The company also has a vacuum sweeper truck that clears dirt off pavement and filters on its storm water drains to reduce the amount of pollution running off the site with rain water.

“Our area is so large even if we put in a lot of eco-roofs it doesn’t have as much of an impact as putting one on an office building,” he said.

But it would be helpful to know how water much an eco-roof could retain, he said. New rules for storm water runoff ratchet down on the level of zinc that can be released in water leaving the site. With the change, Gunderson’s storm water went from being well below the limit to being over it.

Now, the company has two years to reduce the zinc in its storm water.

“People are spending a lot of money on storm water systems, and a habitat roof or eco-roof can be one of the ways you could reduce the loading of something in particular like zinc,” said Harvey. “You’ll get zinc run-off from a galvanized fence or roof, and we have existing roofs that are galvanized. So it could pay for us to convert the roof to a habitat roof.”

Portland State University Stormwater

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