OK it seems like, from reading the questions and responses here, that there is absolutely no clear guidance on how to come up with the "rainfall interval event" in order to perform the drawdown calculation. Am I missing something, or is there clear language from LEED somewhere that will clear this up for people?

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Michael DeVuono

Regional Stormwater Leader

Arcadis North America

LEEDuser Expert

144 thumbs up

Mon, 03/14/2016 - 19:56

Keep in mind the LEED rating system is not a stormwater management ordinance. There is no (to my knowledge) specified dewatering time. 72 hours is kind of standard industry practice, but is no way a hard and fast rule. This actually stems from detention basins, and helping to stave off mosquitoes. The dewatering equation that appears in the reference guides looks like it was taken from the MD BMP manual, which required a certain time for some BMP while LEED doesn't. I would suggest having your civil engineer make a determination as to what you believe your best option is for your project. In the absence of dewatering in 72 hours, I would calculate your actual drawdown for those 3 days, then provide the delta volume for a second 2-year storm event (still only like a 0.5% probability of happening back to back within 72 hours).

ethan adams

UUSC

LEEDuser Basic Member

4 thumbs up

Mon, 03/14/2016 - 20:29

Thanks for the quick reply Michael. I'm actually hoping to reuse the stormwater for toilet flushing, and at a required 3,000 gallon storage capacity (for the 2-year, 24-hour event) 72 hours is not going to happen for us as we don't use anywhere near 1,000 gallons per day. I'll use your suggestion for the delta volume calculation and see how it goes.

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