LEED SS6.2 Quality Credit - 90% Treatment Requirement 1

[kc27315]

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

 

[bimr]

What is the scope of work?

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.

 

[kc27315]

The local government requires the 10-yr frequency for storm water piping, and 100-year for detention facilities with a release rate of 0.2 cfs/acre. However, the LEED SS6.2 credit does not seem to suggest what frequency they want me to use in the 90% calculations. The previous credit, SS6.1, specifically states the 1- and 2-year frequencies with a 24-hour duration. But, SS6.2 does not seem to suggest whether to use the 1, 2, 5, 10, 25, 50, or 100 year storm. I don't imagine that they want me to use the 100-year event. In fact, the term "average ANNUAL rainfall" seems to suggest to me that they might want me to use the 1-year event, but its pretty vague based only upon that term. Any thoughts?

 

[beej67]

The truth is the guys up at USGBC who do this review aren't sophisticated enough to worry about the minutiae. If you route a SCS 24 hour storm distribution with a half an inch of precipitation, determine the peak discharge from that, and show that it's under the treatment capacity of your Baysaver, you should get through USGBC review just fine.

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

 

[kc27315]

Awesome! Thank you so much for the insight! I will probably just use the Rational Method (Q=CIA) since it's easier to me than the SCS Curve Number Method. I think I'll just use the 1-yr 24-hr storm, which has a depth of 1.03 inches. Then, there should be no way they would have any heartburn over the calcs. Thanks again!

Ryan

 

[bimr]

You might also keep in mind that the stormwater "treatment" devices are not built to any standards. The treatment claims made by the manufacturers of the devices are also not scrutinized and verified.

 

[kc27315]

That's a really good point bimr! An example of this I've found is when a manufacturer gives an efficiency of TSS removal of, say, 80% for their BMP. But when you ask them what particle size they base their 80% upon, it seems that a lot of times they can't seem to give an answer... Not that it matters much in this particular situation, since LEED doesn't specify a particle size in SS6.2 either :/

 

[Twinkie]

Plans are in the works to publish addenda to require structural BMPs to have undergone some generally accepted third party verification (NJCAT, TARP II, etc)

 

[beej67]

By the time they get all that mess standardized, the policy makers will have finally realized that the TSS isn't coming from the impervious surfaces at all, like they should have in the first place. (the old NURP data makes this obvious) And then the entire stormwater BMP marketplace is going to crash anyway, and the regs will shift wholesale to runoff volume reduction standards.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com