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The calculations for this credit are always a headache! While our projects are usually awarded the credit, the equations in the LEED Reference Guide are helpful mostly for sizing a reservoir or cistern, but don't help you get to the final results. Does US

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

How does LEED SSc6.1 calculate "Imperviousness"?

How does LEED SSc6.1 calculate "Imperviousness"? I have been told that "imperviousness" means the % of the site that is impervious (paved vs green). However, that would mean that a 1 acre site with 0.5 acre of building and asphalt and 0.5 acre of mowed lawn would be considered a "largely undeveloped site" by LEED. I find that very hard to believe. It seems to me that they are actually looking at it like a composite C value from rational method (i.e. C for lawn is 0.2 and C for pavement is 0.9, so a composite C=0.55 (or 55%). Or maybe as a runoff value from SCS method which I think could yield an even higher number if the soils are bad.

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Profile picture for user Mike Dee
Michael DeVuono
Regional Stormwater Leader
Arcadis North America
LEEDuser Expert
144 thumbs up

Mon, 07/20/2015 - 14:56

This credit compares pre vs post runoff rate and volume. Using SCS methodology a CN of 98 is used for impervious areas and the appropriate CN for all other cover types based on the underlying soil condition. This is a fairly straight forward calculation and analysis. The rational method should not be used for determining runoff volume, because it doesn't determine runoff volume.

Mon, 07/20/2015 - 15:33

Thanks, but that was not my question. In order to determine whether a site falls into Case 1 or Case 2, we have to calculate the existing "IMPERVIOUSNESS". I cannot find where we are told exactly how to calculate "imperviousness". I have been told that "imperviousness" simply means the % of the site that is impervious: If that is true, then a 1 acre site with 0.5 acre of roof and pavement and 0.5 acre of mowed lawn would be a "Case 1" site and considered a "largely undeveloped site" by LEED. Really? In Case 1 you just have to show there is no increase in runoff between existing and proposed (keep it as good as you found it). Is that correct? It seems too "easy" compared to what LEED usually demands of us. (On the other hand, if the existing site is actually 0.51 acres of pavement, it would then switch to Case 2 and we now have to make the proposed runoff 25% less than the existing runoff. That is SO much harder).
Profile picture for user Mike Dee
Michael DeVuono
Regional Stormwater Leader
Arcadis North America
LEEDuser Expert
144 thumbs up

Mon, 07/20/2015 - 16:37

Percent impervious is calculated: Impervious Area/Total Area Case 2 you are only managing volume, and only for the 2-year storm, not the 1-year. A design for a redevelopment project that shows a volume reduction equal to a 20-25% reduction in impervious area is a fairly common approach to stormwater. Remember, these are Site Credits, it isn't a stormwater ordinance.

Mon, 07/20/2015 - 17:56

Michael, I have to assume you meant to say that "imperviousness" is equal to "Percent impervious" which is calculated: Impervious Area/Total Area. Please let me know if there is a different definition of Imperviousness. Thanks!

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