PMP design for small basins

[Rye1]

I need to develop a PMP for a small watershed – approx. 0.1 mi^2. I researched the topic and searched PMP on this forum, while I have found several good PMP discussions; I have not seen anything addressing small basins. HMR51 and HMR52 are for watersheds with basin sizes between 10 to 20,000 mi^2. The data and curves could be extrapolated back to very small basins, but…this maybe outside of the data population used to create the reports.

So, small mountain ponds and lakes, what do you use to calculate the PMP?

 

[RWF7437]

If we assume PMP means Probable Maximum Precipitation, then we might reason along these lines:

The basin is 0.1 sq mi or about 64 acres.

If there are nearby precipitation recording stations or streamflow records they would be the best available records upon which to base any calculations. Even if such data is available it is probably no more precise than +/- 30%.

If the PMP is the result of rainfall alone, not including snowmelt, the problem is "easier".

If your project is in the U.S. and in an area where the records have been updated and analyzed, then you can get good data from NOAA's Hydrometeorlogical Design Studies Center (Google it ).

A basin this small could be affected by a very small storm cell. A cell as small as 942 feet in diameter would just cover the basin BUT, would such a storm result in the PMP ? It seems very unlikely. What seem much more likely is that the PMP results from a large, area wide storm measuring miles in diameter.

For this reason alone it seems that "extrapolating back" would be reasonable and probably "conservative".

Other useful questions you might ask are:

Can you live with an error of estimate of +/- 50% ?
What would be the consequences of such an error ?
If this is to be used to design a dam spillway, how much difference would such an error make in my design and in the cost of the project ?
And, sadly we must ask, can I explain this in court to a hostile attorney ?

good luck

 

[Rye1]

RWF7437 - thanks for the reply. You bring up several good points. After looking at this problem and researching for several days, I've came to the conclusion that there is not a definitive PMP method for small basins beyond engineering judgment.

So what I'm going to do is extrapolate the curves from HMR51. I'm going to review all other extreme precipitation data for the area, check with my peers, and then check to make sure the estimated PMP is significantly larger than the know 500yr amount.

In the end, it boils down to engineering judgment and hoping that you never have to try to explain "voodoo" hydrology to a hostile attorney.

 

[Rye1]

I wanted to ask this question again and see if there were new ideas. I have another project with a small mountain dam. State law requires the passing of the 1/3 PMP event. The problem being the methodology to create a PMP storm event for a small watershed.

So, does anyone have a reference for developing a PMP event for a small watershed?

Robert Billings

 

[cvg]

I have yet to see a state regulated dam where the state dam safety department would not be more than happy to help you generate the PMP. Have you contacted the state and asked what method they will approve?