Green roofs limit storm water run-off. If DG is to make a difference in how much stormwater we have that runs off, we will need to address things like this. From The Changing World of Stormwater Technology, by Dan Rafter, this report of a comparative testing of green roofs to determine the optimum design.
Green Roofs in Seattle
In Seattle, Magnusson Klemencic Associates are no strangers to stormwater technology. The company’s recent experience with Onset Computer Corporation’s rainfall data loggers only reinforced its commitment to new stormwater technology.
Working with local contractors, Magnusson Klemencic engineers designed five green roof test plots, each measuring 8 feet by 12 feet, in downtown Seattle. Each plot featured a different thickness of planting media and different species of plants. The goal was to see which roof swallowed the most rainfall before it became stormwater runoff.
The problem was, engineers faced a real challenge when it came time to measure the amount of runoff leaving the test plots.
“Measuring temperature and wind speed—all that is connected to a data logger. Those things are all pretty straightforward. We’ve been measuring those parameters for some time,” Gangnes says. “What we scratched our heads about was how we were going to measure the runoff leaving the different test plots.”
The solution that engineers came up with was elegant in its simplicity. They angled each plot in two directions so that the rainwater would ultimately filter into a corner drain. That drain was then outfitted with two devices: a simple water meter and an orifice restrictor device.
Engineers then used a pressure transducer to measure the depth of water over time. They used that information to determine how much water flowed from the green roofs. Finally, they relied on Onset’s logging software to reduce the data and determine runoff trends from it.
“Using Onset’s programs to do the logging worked very well for us,” Gangnes says. “We built additional routines into their program to help us take the data and use it once it was in the Onset format.”
The pilot program determined that green roofs did a rather remarkable job of mitigating stormwater runoff. The top-performing plot mitigated 94% of the rain that landed on it. Even the least effective of the five test plots prevented 65% of the rain that fell on it from entering Seattle’s stormwater sewer system.
There were some initially surprising results, though. The top-performing green roof was 6 inches deep with plants and planting materials. It was little surprise that this roof performed better than did a 2-inch-deep green roof and a pair of 4-inch-deep roofs.
But the 6-inch-deep roof also mitigated more runoff than did the deepest roof in the test program, one that featured 8 inches of plants and planting material.
Fortunately, the project called for soil moisture probes, too, which allowed engineers to chart the ebb and flow of water in the soil over time. The difference, they saw, was that the 6-inch-deep plot was the only one with a granular drainage course. The rest of the roofs featured cellular drainage boards.
Once rainfall hit the other roofs’ cellular drainage boards, it immediately began running off. The 6-inch-deep roof’s pumice-like drainage board, on the other hand, held the rainwater in place longer.
Engineers also discovered during the test program that the thinner roofs dried out more quickly following heavy rainfalls. The 6-inch-deep plot, then, didn’t hold water deep in its soil as long as did the 8-inch plot.
“We hadn’t thought through the fact that you can get soil that is too thick for optimal stormwater performance,” Gangnes explains.
The green roof experiment wasn’t done just for show. Gangnes hopes that the program’s results help convince local governments to explore adding green roofs to their municipalities’ stormwater collection systems, especially in a decentralized fashion.
“We hope that this project helps policymakers understand the value that green roofs can play in a decentralized stormwater management regime,” Gangnes says. “Rather than looking downstream as we have stormwater problems, and building bigger treatment plants and retention basins, let’s look upstream. Let’s look at low-impact design techniques. Let’s look at things like rain gardens and bioretention, in addition to green roofs. Green roofs aren’t our only tool, of course, but they can be one of them.”
As technology continues to improve, and more engineering firms conduct experiments similar to Seattle’s green roof test program, more building owners might be willing to give concepts such as green roofs a try, Gangnes says.
“If you daisy-chain green roofs with techniques that you can use at the bases of buildings, you can get to a self-mitigating building that can handle its own stormwater management needs without burdening the downstream infrastructure,” he says. “And it has to be better than just a big tank under a building. Cities should be offering incentives to folks for working with these systems. I can imagine the day when a city pays you to put in a green roof. If enough people do that, it can delay the need to build another treatment plant. Then the net burden on the system is dropping. That’s one of our goals.”
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