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NC-2009 SSc6.1:Stormwater Design—Quantity Control

LEED Runoff Coefficients for BMPs

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Timothy Brown
Eriksson Engineering Associates
LEEDuser Basic Member
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I am finding it difficult to locate standard CN values for BMPs. There are not many studies out there and the ones I have found have calculated large ranges for permeable pavers, green roofs etc. Has LEED generated its own runoff coefficient and Curve Number table for BMPs? I am specifically looking for values for permeable pavers and green roofs. I have found research that indicates the CN value for permeable pavers can be taken as 79 and green roof 61. If these numbers are accurate, I can meet the SSc6.1, but if they are low, than the project falls short. Any assistance is appreciated. The project is located in the City of Chicago.

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Michael DeVuono
Regional Stormwater Leader
Arcadis North America
LEEDuser Expert
144 thumbs up

Sat, 06/17/2017 - 21:11

The LEED credits are not a Stormwater ordinance, and do not get into this level of technical detail. In these cases I've typically used the CN for the lawn condition of the underlying soil HSG
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Timothy Brown
Eriksson Engineering Associates
LEEDuser Basic Member
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Mon, 06/19/2017 - 13:36

Thanks Michael, Is that for both the permeable pavers and the green roof? It makes sense to me for the pavers, but the green roof on this project ranges from 4 inch trays to 36 inch planters. All of them have engineered soil though to retain water to nurture the plantings. I would expect that, especially for the 2 year storm, the runoff from such engineered systems would be lower than the typical soil found in the Chicagoland area. Again, I have unfortunately not been able to find any studies by reputable sources with consistent values for green roof. If there is no standard CN, what information would a LEED reviewer need to see to accept the CN values chosen in the calculations?

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