My project features raised green roof beds (6" deep soil) located directly downstream from a tin roof. There are no gutters and for the most part runoff from the roof runs directly into the roof beds, the excess runoff from the green roof then runs into an order flat roof where it is directed to standard storm drain. My project also has a roof terrace surrounded on the perimeter by green roof bed as well. The roof terrace rain scuppers are located in the green beds, thus the runoff from the flat terrace must pass through as well. For the purpose of calculating my BMP area for capture, storage, and treatment, can I consider the sloped roof as an addition to my 1st green roof bed, and the roof terrace as addition to my 2nd bed? The EPA National stormwater Calculator doesn't have a capture ratio for green roofs, although it does have a capture ratio for rain planters. But the rain planter model requires a permeable surface beneath something I don't have.
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Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
June 25, 2014 - 11:18 am
I'm not sure I follow your question completely ....
But area is simply = (L)(W)
Volume = (L)(W)(6/12)(some void ratio dependent on the media)
Your rooftop will runoff into the green roof, which I would model as a storage basin when doing your calcs.
You really should not be using the EPA calculator for actual design purposes.
Jamison Hill
Energy Engineer/LEED ConsultantCommunity Environmental Center
3 thumbs up
June 25, 2014 - 11:25 am
I wasn't using it for design purposes, but simply to evaluate the performance of the existing design in regards to its ability to capture the 2-year, 24-hour storm. Incidentally, the EPA stormwater caculator doesn't give results for the 2-year, 24 hour storm (silly I know), so I ended using the web-based CMT calculator (the national version, not the Chicago version). But I will follow your suggestions, and treat my green roof beds as something else, see what happens. Thanks.
Steve Loppnow
Sustainability Account ManagerStok
LEEDuser Expert
294 thumbs up
October 20, 2014 - 4:54 pm
Can the EPA Stormwater Calculator be used for documentation purposes? What are it's limitations? Has anyone used it as a primary means of documenting SSc6.1?
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
October 20, 2014 - 5:06 pm
No this is not sufficient or actual design services. It is a planning tool only. It's good for determining your percentile storms but not really much else that you can hang your hat on.
The v4 reference guide mentions this as a planing tool only as well.