stormwater pipe sizing

8.5 inches per hour is a rainfall rate that is unaffected by the size of the site. It sounds like a 100-year storm intensity. Around here a 7-min duration/100-year storm would suggest an intensity of 8 in/hr whereas a 10-year storm would suggest 6.2 in/hr and a 25-year storm would suggest 6.8 in/hr. As you can tell, there's not that much in it.

Fudging your rainfall numbers probably won't help you much, certainly not as much as increasing the slope on your pipe and putting on a good headwall would. Of course, detaining your peak flow would decrease your pipe size quite nicely.

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francesca wrote: "8.5 inches per hour is a rainfall rate that is unaffected by the size of the site"

That may be the case for small watersheds, but - as pointed in various publications - pecipitation intensity decreases as watershed size increases.

See, for example; "Rainfall Input for Hydrologic Models" by David M Hershfield (
Check out the Depth-Area curves and discussion.

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

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Here's a graph of such...



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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

"The site" is 5 acres, and francesca is correct that the rainfall intensity is independent of a site that small.

Any idea what design storm your 8.5 inches/hour is? You can't just change the rainfall intensity without either increasing your time of concentration or selecting a shorter-recurrence interval storm (say 10-yr instead of 100-yr).

You wrote:

"I am probably going to use the 8.5 inch per hour intensity because it is standard design procedure."

First, there is no universal "standard procedure". You may be required to design for some reviewing agency's "standard" but that doesn't mean that that is some universal standard. It doesn't even mean that it is good practice.

Second, for this small site, the Rational Method may be OK but that also doesn't mean it is good practice or the best method to use. There may be better data available. Have you looked for it ?

Third, get a copy of the reviewing agency's standards and then ask yourself if you feel you need to be more conservative in your design. What are the consequences of failure of your design ? Can there be downstream damage, lawsuits or other problems ? What are the laws in your area ?
What are the physical constraints of the site ? What does the storm water Master Plan ( if there is one ) call for ?

Does on-site detention make sense in this case or might it be required ?

good luck

I would start with checking your time of concentration. 7 minutes seems low, especially for a 5 acre site. And many agencies limit the minimum Tc to 10 minutes.