I am trying to verify that the cisterns on site release stormwater quickly enough to capture 2-year 24-hour design storm events by calculating the minimum drawdown rate. However, I cannot figure out what the rainfall event interval is. Is this information that I should be able to find or something that needs to be calculated?

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Michael DeVuono

Regional Stormwater Leader

Arcadis North America

LEEDuser Expert

144 thumbs up

Fri, 11/23/2012 - 21:11

I assume you are talking about back to back storm events here. This topic comes up a lot in stormwater management, especially with capture and reuse. Calculating back to back events is a complex study, and requires the EPA's SWMM software. However, I don't believe you are being asked to get this involved for LEED. I have never received clear guidance on how quickly we need to free up volumes for back to back storm events. However I have successfully made the case to the DEP, just last week, that a theoretical 2 yr storm event only occurs every 2 years, and therefore providing volume for back to back events is unrealistic. I've also come across agencies wanting the water gone in 72 hours. As far as LEED is concerned, I would follow your local water quality requirements for drawdown, provide the necessary calculations that show all your reductions, and go from there. Show that you have an adequate bypass in place, in the event the cistern is full as well

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