Could anyone give me an advice which of the following interpretation complies with the credit language? There are two possibilities how to construe this requirement:
a) 90% of the average annual rainfall is equivalent to 1/0.75/0,5 inches based on climate as states in LEED Reference Guide. As explains Gregory Hurst down here (Aug 26 2011), this is the depth of rain that equates to 90% of the daily rainfall amounts, 90% of the days will have rainfall amounts under the calculated value. From this point of view 90% determines only the rainfall depth.
For example if I have 1000 sq ft of absolutely imperviousness site in humid climate, I must capture and treat 900 cu ft of rainfall (1000 sq ft * 1 in) to meet the credit intent.
b) The numbers of rainfall depth 1/0.75/0,5 inches represent the average annual rainfall and 90% of the calculated runoff must be treated and captured. This interpretation conforms to the possibility to calculate 90% rainfall compliance based on areas (see Simon S., Nov 29 2011).
In this case if I have 1000 sq ft of absolutely imperviousness site in humid climate, I must capture and treat only 810 cu ft of rainfall (0.9 * 1000 sq ft * 1 in) to meet the credit intent.
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
187 thumbs up
November 7, 2012 - 7:26 am
Check your math.
1 inch of rainfall over 1000 sf is 83cf
(1/12 ft)(1000 ft^2) = 83.3 ft^3
Now typically, when agencies say they want "1 inch of runoff" I use a depth of 1.2" rainfall to account for evaporation, evatranspiration, etc.
So...
(1.2/12ft)((1000ft^2) = (100 ft^3)(.90) = 90 ft^3
Lyle Axelarris
Building Enclosure ConsultantBPL Enclosure
64 thumbs up
April 23, 2014 - 10:23 pm
I don't believe this question has been answered. My interpretation of the Reference Guide language coincides with "a" above.
The Reference Guide says that "90% of the average annual rainfall is equivalent to treating the runoof from the following (based on climate)" - Humid Wastersheds - 1 inch of rainfall, Semiarid..."
I believe that the Ref. Guide is doing the rainfall calculations for us in order to keep it simple. If you are in a dry climate, for example, then you must remove 80% TSS from a 0.5" storm event. Can anyone else confirm this?
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
187 thumbs up
April 24, 2014 - 3:00 pm
The percentile storm is based off of very site specific rainfall data collected over decades. If this is not very clear in previous versions of LEED, it will become more obvious with v4.
For example, Philadelphia, the 90th percentile storm is 2.04" for 24 hours. Well above the 1.0" and we are hardly a rainy climate.
I don't have a copy of the reference guide with me, but I will look at this tomorrow. I know what it is "supposed" to mean. If it was desired for you to treat 80% of a 1" design storm it would say as much. The methodology to calculate the design storm is outlined in EPA 438 (I need to check that reference tomorrow, going from memory).
**FOLLOW UP**
I totally agree with the above. 1", 0.75", and 0.5" design storms. It is completly incorrect, as this is not the "90th Percentile Storm." This is what is commonly referred to as a "Water Quality Storm" basically a jurisdiction dictates the depth.
I can't believe I never picked up on this. v4, this does not apply. You will definitely be calculating the real 90th percentile storm.