Probable Maximum Precipitation (PMP) event for stormwater pond

I have been working on a retrofit to an existing stormwater pond (increasing its detention time for a 1-year storm event). In the state review process, the project has been referred to the Dam Safety section, which is concerned with the pond’s performance during the Probable Maximum Precipitation (PMP) event. Reviewing the data for my region, the design storm they are looking for is a 27" storm event. Plugging this storm event into my model as a 27", 24 hour storm event (predictably) overloads my pond by quite a bit (The 100-year storm event in our area is 5.2")

Has anyone else here designed stormwater ponds with the PMP in mind? Is it really on the order of 5x larger than a 100-year storm event? Have I erred in my modeling at some point?


Peter Smart
HydroCAD Software

Make sure the 27" is intended to be applied using a 24-hour Type II rainfall. It might be for a longer period and/or a different rainfall distribution, either of which could lower the peak considerably. Of course, you’re still looking at an incredible volume of runoff, so I presume the concern is to handle the overflow without structural failure of your pond.
I think you need to request details on the basis of the 27" value. Exactly how was this arrived at? The number really isn’t plausible for 24-hours, so I suspect it applies to some multi-day event.

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