Taking Stormwater by Storm

Rainwater harvesting and green roofs not only capture stormwater for reuse, but help relieve the burden on municipal sewer systems
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Sponsored by American Hydrotech
Barbara Horwitz-Bennett

Quantifying Green Roof Performance

While green roofs can be an effective stormwater management strategy, one of the challenges associated with their application has been reliably predicting their performance capabilities.

For example, the civil engineering industry can accurately calculate storage capacities underneath permeable pavement systems, but growing medias in vegetated roofs are so varied - with different combinations of plants, aggregates, sand and composts-that it is difficult to quantify performance.

Toward the end, an ASTM testing standard was created to establish:

  • The media’s permeability-how fast water moves through it
  • Total saturated density-how much the media weighs when totally saturated with water
  • Total pore space-the air volume within the media

Taking the test results from its growing media, and combining it with civil engineering calculations based upon climatological data and stormwater modeling concepts, Hydrotech has created a special software tool-available at: http://resources.hydrotechusa.com/download.php?resource=27-that displays green roof performance data in a hydrographic format that is familiar to civil engineering professionals.


IMAGE COURTESY OF HYDROTECH

Pulling data from Hydrotech’s Hydrology Tool, a more than five-hour difference between the start of stormwater runoff on a ballasted roof, as compared to a garden roof, can be seen.

IMAGE COURTESY OF HYDROTECH

The same hydrograph also displays the difference in roof runoff flow, which is less than half for the garden roof versus the ballasted roof.

In particular, this proprietary tool incorporates storm intensity, storm duration and storm water flow data-which is available from U.S. government agencies-in addition to and selected area of watershed and release rates of that watershed.

“The Hydrotech tool is very effective, and there are some other independent calculators that can be used as well,” observes Jones. “Ultimately, one has to find numbers that the municipality you are working with will accept, because part of the goal is to reduce the amount of water one is required to detain and/or associated stormwater fees, and this is constantly evolving as municipalities begin to recognize green roofs as an acceptable stormwater runoff mitigation tool.”

In particular, Hydrotech’s Hydrology Tool calculates how vegetated roofs impact stormwater runoff volumes and the rate at which run off is slowed, demonstrates potential LEED compliance and performs long- and short-term simulations.

“The HHT software allows for the easy creation of ‘what-if scenarios’ where various combinations of media depths, vegetated roof areas and drainage/retention components can be modeled to optimize the building owner’s needs for stormwater storage,” Hayden explains.

To demonstrate the differences in total flow and runoff, HHT can be used to overlay a hydrograph curve for a standard ballasted roof, a vegetated roof and a combined roof. Although the curves will vary depending upon specific vegetated and non-vegetated roof areas, the media depth and any drainage and/or retention components used in the assembly, general estimates show that a garden roof will delay stormwater runoff by more than five hours, according to Hayden.

In addition, the garden roof reduces the runoff by more than a half, as compared to the ballasted roof.

“This combination of delayed and lower stormwater flows are very valuable to reducing the hydrological impact of a building on a storm sewer system,” notes Yanez. “The delays mean that the storm sewers can have a chance to empty enough to accept the new water flows and the flow reduction means that the storm sewers have less water to accept.”

The Outlook

Green roofs aside, experts predict that rainwater systems will continue to gain acceptance, albeit slowly, thanks to its recent addition to the national plumbing codes and support from LEED. Granted, it will take some time until the national codes are adopted by local municipalities, but Jones anticipates that in the long run, many cities, particularly major urban areas which are already leading the charge, will adopt Green Infrastructure Master Plans that incorporate rainwater harvesting, as well as green roofs and bioswales.

In addition, Rattenbury predicts that in the long term, the rising costs of water coupled with more frequent water restrictions and bans will make rainwater harvesting a much more attractive option. “As an ever increasing focus is directed to the dispelling of myths surrounding rainwater collection and the celebration of its virtues, rainwater harvesting as an industry will flourish and the built environment will be better for it,” concludes Van Giesen



MBCI

 

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Originally published in Environmental Design + Construction

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