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.
You rely on LEEDuser. Can we rely on you?
LEEDuser is supported by our premium members, not by advertisers.
Go premium for
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
187 thumbs up
June 17, 2017 - 5:11 pm
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
Timothy Brown
Eriksson Engineering AssociatesJune 19, 2017 - 9:36 am
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?