PMP design for small basins
PMP design for small basins
(OP)
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?
So, small mountain ponds and lakes, what do you use to calculate the PMP?





RE: PMP design for small basins
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
RE: PMP design for small basins
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.
RE: PMP design for small basins
So, does anyone have a reference for developing a PMP event for a small watershed?
Robert Billings
RE: PMP design for small basins