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The PMP - Tips on Modeling?

The PMP - Tips on Modeling?

The PMP - Tips on Modeling?

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 Precipiation (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?

RE: The PMP - Tips on Modeling?

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.

RE: The PMP - Tips on Modeling?

Make sure you know the area and duration of the PMP. 27" sounds like a large area and long duration event.  Here is NOAA's definition:

"The most current definition of PMP is as follows: "Theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of the year." This definition comes from Hydrometeorological Report 55A (HMR 55A). "

Good luck


RE: The PMP - Tips on Modeling?

psmart and RWF7437 are both right.  I'm just curious this pond wouldn't happen to be located in a hurricane prone area would it?  As psmart mentioned the period of the storm is going to make a big difference in the peak.  27" of rain over 3 or 4 days is lot different than 27" over 24 hours.

RE: The PMP - Tips on Modeling?

Thanks for the responses.  The PMP I was asked to design to was the 24-hour duration storm with a drainage area of 10 square miles.  Originally I used the figure for a drainage area of 200 square miles, but that was too low.

The pond is in Vermont.  Thankfully, we do not have to worry about hurricanes.

That definition sounds odd to me.  Most precip physically possible?  Isn't it physically possible to get a rain event with 100" of depth?  Interpolating the given rainfall depths for 1, 10 and 100 year storms in my area, this PMP event is a 395,000-year storm event.  Am I mixing apples and oranges in making this kind of claim?

RE: The PMP - Tips on Modeling?

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.

RE: The PMP - Tips on Modeling?

If, as you say, the 100 year (1% AEP) 24 hour (?) storm in your area is 5.2 inches it isn't possible that the 24 hour PMP is 27 inches. I agree with P. Smart and others, get the Dam Safety folks to tell you exactly what they want.  Find out the rainfall, duration, storm size, pattern, method and AEP they want you to analyze. Better yet, get them to send you an example of an approved design.

Good luck

RE: The PMP - Tips on Modeling?

Your approach needs to be modified slightly.  Reviewing NOAA data for southern VT The PMP for a 24-hour duration is approx. 23 in, and the PMP for a 48-hour duration is approx. 26 in.  In my 20-year experience, I have never heard of a reviewer asking for the detention basin being sized for this magnitude storm event, but have been asked to size the rip rap channels and overflow spillways to handle this flow from such an event.  It is a safety issue, not a mitigation issue.  Ask your reviewer if that is what he is looking for.

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