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# pre and post developmed modified rational method

## pre and post developmed modified rational method

(OP)
I have to design a detention pond for a 10-acre development with the modified rational method. The detention pond catches 8 of the 10 acres and the remaining 2 acres bypass the pond. There is not any room within the 2 acres to put a detention pond.

To determine the post developed 2 acre bypass peak release rate what duration/intensity do you use?

You can't get the Tc for the bypass and add it to your pond release rate because then you're adding two watersheds together that are peaking at different times.

I would think that you could use the duration/intensity for the routed pond outflow rate to obtain the bypass peak runoff rate?

For example:
If the pond maximum storage requirement was determined to happen at a duration of 2 hrs you would get the intensity
rate for 2 hrs and use that to obtain the bypass peak rate?
Then you can combine the pond and the bypass rate to obtain the post developed flow????

Anybody have any thoughts on this?

### RE: pre and post developmed modified rational method

Per VT/PSUHM, v6.2, by Kibler, Hodges, Smith and Thye:
"Several questions have been raised regarding combination of hydrographs.  A unique hydrograph results from a given time of concentration.  In the Rational Method the duration of the storm is set equal to the time of concentration (Tc).  By combining different time of concentration hydrographs, one is actually combining storms of different durations, which is incorrect.  The correct way to analyze such situations is to...analyze the longer storm routed through the basin, and then combine the latter hydrographs."

Dr. Seybert can answer your questions regarding VT/PSUHM methodology, or at least he has for me before:

Tom Seybert
Associate Professor of Civil Engineering in Surveying
Penn State Wilkes-Barre
123 Technology Center
P.O. Box PSU
Lehman, PA 18627-0217
Voice: 570-675-9139
Fax: 570-675-7713
http://surveying.wb.psu.edu/psu-surv/tombio.htm
mailto:tseybert@psu.edu

Engineering is the practice of the art of science - Steve

### RE: pre and post developmed modified rational method

(OP)
Thanks for the information. From the response you sent me it appears that I am thinking correctly. I will contact
Dr. Seybert

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