LEED SS6.2 Quality Credit - 90% Treatment Requirement
LEED SS6.2 Quality Credit - 90% Treatment Requirement
(OP)
I've been asked to pursue the SS6.2 Storm Water Quality Credit for one of my projects, a 3 acre parking lot. The requirements state that the BMP(s) need to be "designed to treat stormwater runoff from 90% of the average annual rainfall." I also understand that treating 90% of the average annual rainfall is equivalent to treating 0.5 inches of rainfall in Arid Watersheds, such as where my project is.
The treatment device selected (5K Baysaver) has the ability to treat 11.1 cfs. My question is:
How do I compare 0.5 inches of rain with the 11.1 cfs? Doesn't it depend on whether the 0.5 inches comes down really fast, or takes its time? For example, the 1-year 2-hour storm would drop about 0.5 inches near my project according to NOAA depth-duration-frequency chart. However, the 100-year 5-minute storm would also drop about 0.5 inches. Therefore, the 1-year storm would be dropping an average intensity of 0.5/2 = 0.25 in/hr, while the 100 year storm would drop an average intensity of 0.5/(5/60) = 6 in/hr.
Therefore, 0.5 inches for a 1-year storm would produce a rate of Q=CIA=0.9*0.25*3=0.68 cfs. And 0.5 inches for a 100 year storm would produce a rate of Q=CIA=0.9*6*3=16.2 cfs.
So, for a 1-year storm, I would meet the 90% treatment requirement. But, for a 100-year storm I would not.
How can I reconcile this? Am I going about this calculation the correct way?
RYan
The treatment device selected (5K Baysaver) has the ability to treat 11.1 cfs. My question is:
How do I compare 0.5 inches of rain with the 11.1 cfs? Doesn't it depend on whether the 0.5 inches comes down really fast, or takes its time? For example, the 1-year 2-hour storm would drop about 0.5 inches near my project according to NOAA depth-duration-frequency chart. However, the 100-year 5-minute storm would also drop about 0.5 inches. Therefore, the 1-year storm would be dropping an average intensity of 0.5/2 = 0.25 in/hr, while the 100 year storm would drop an average intensity of 0.5/(5/60) = 6 in/hr.
Therefore, 0.5 inches for a 1-year storm would produce a rate of Q=CIA=0.9*0.25*3=0.68 cfs. And 0.5 inches for a 100 year storm would produce a rate of Q=CIA=0.9*6*3=16.2 cfs.
So, for a 1-year storm, I would meet the 90% treatment requirement. But, for a 100-year storm I would not.
How can I reconcile this? Am I going about this calculation the correct way?
RYan





RE: LEED SS6.2 Quality Credit - 90% Treatment Requirement
Local governments normally specify the design criteria such as design frequency (return period) for the collection and conveyance of runoff water on different types of developments. A minimal design storm frequency for a parking lot is a 2-year event. Why are you using a 100-year event?
Refer to the pdh course.
RE: LEED SS6.2 Quality Credit - 90% Treatment Requirement
RE: LEED SS6.2 Quality Credit - 90% Treatment Requirement
Now, you do probably need to check your local or state code to find out what rain even they consider to be the "first flush." LEED's "90% of annual rainfall" criteria is supposed to emulate first flush, so if your locals are calling first flush something different, then I would go with the local criteria, and only default to your half-inch-for-arid-areas if the GBC gives you grief on your submittal.
Keep in mind, the USGBC isn't really a regulatory agency. They aren't really in the business of holding you to specific engineering standards, and their LEED accreditation procedures are built for architects and policy makers.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: LEED SS6.2 Quality Credit - 90% Treatment Requirement
Ryan
RE: LEED SS6.2 Quality Credit - 90% Treatment Requirement
RE: LEED SS6.2 Quality Credit - 90% Treatment Requirement
RE: LEED SS6.2 Quality Credit - 90% Treatment Requirement
RE: LEED SS6.2 Quality Credit - 90% Treatment Requirement
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com