95th Percentile Storm in terms of Design Storm

[MWC PE]

Does anyone know of a way to calculate the 95th percentile storm depth based on the NOAA Atlast 14 storm design storms? Our new MS4 permit is requiring this and the guys at the regulatory agency looked at me like I have a third eye when I asked this question.

Anyway I cobbled together this approach, does it makes sense?

95th percentile storm is a storm with a depth greater than 95% of all storms in the rainfall history of the site, however without history could it be assumed to be a storm that has a 5% probability of occurring in the design life?

Assuming that is true then set P = 0.95

and P = 1 - (probability of 24 hour design storm X not happening per year)^(design life)

Assume design life is 20 years

then

0.95 = 1 - (X)^20

X = (1-.95)^(1/20)

X = 0.86

24hr-Design storm X thus has a 14% chance of happening -> 100/14 = 7.14 24 hour-Design storm.

I haven't really looked at statistics in very long time so this may all be nonsense. Could someone that is good with statistics tell me if that seems reasonable or how to actually calculate it?

 

[John QPE]

You could always use EPA's National Rainfall Calculator. It will give you site specific 95 percentile.

There are also step by step instructions in: Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act.

We developed a calculator for the new LEED rating system which is requiring infiltraton of the 95th percentile storm: https://www.mediafire.com/?bu816m6w4hm4stw