Modified rational method - rainfall intensity

Hana Raitrova

LEEDuser Basic Member

0 thumbs up

November 13, 2015

Dear All, I´m still wondering how to get a value of rainfall intesity in inches/hour or mm/hour for the 95th percentile (option 2 - modified rathional method) based on time concentration. I understand I simply use a different design storm with LEED v4 (the 95th percentile as opposed to 1- and 2-year 24-hour design storm), but to get the amount of rainfall intensity I need to know if I should count with 1- or 2- or x- year storm (to choose the right amount from the graph). Is it correct to choose the value for 2-year storm event? Or am I completely wrong? Thanks in advance

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Michael DeVuono

Regional Stormwater Leader

Arcadis North America

LEEDuser Expert

144 thumbs up

Fri, 11/13/2015 - 16:47

You aren't going to get a runoff volume from the Modified Rational Method. I know it's in the guide, but it just isn't correct. The 95th percentile storm is a depth, its not an intensity. I recommend using SCS methodology. As for the 95th percentile depth, the EPA National Stormwater calculator is pretty good. You can also get from most local DOTs or weather agencies. Or collect local data from NOAA or your local equivalent and plug into the rainfall calculator spreadsheet which should be in LEED online.

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Modified rational method - rainfall intensity

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