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Methods to determine pre- and post-runoff volumesHelpful Member! 

ewbeng (Civil/Environmental) (OP)
6 Feb 11 13:26
I am trying to determine the pre- and post-development runoff volume for the 25-year, 6-hour storm (per City requirements). Note, this is a very small project - they are proposing to add 2,067 sf of concrete and 2,624 sf decomposed granite to a 13,685 sf area (existing lawn).  

The City requirements actually state that:
"Runoff quantities shall be determined by the Rational Method using basic data supplied by the Flood Control District for the frequency of occurrence stipulated here.  The HGL for proposed drainage facilities shall be determined for the 25-year 6-hour event."  
In talking to the review engineer, he said that they assume a 6-hour duration for the peak runoff(?)  Knowing that you really can't use the Rational Method to determine volumes I've started looking into other methods.  (Note, I've had to brush up on this ... hence my handful of questions.)  

I started down the path of the Graphical Peak Discharge Method.  Where I got confused (and now understand that I'd selected the wrong method) is the TR-55 specifies the 24-hour storm.  I was hopeful that there was a way to convert this to the 6-hour storm.  

I've since been directed to the full SCS hydrograph generation procedure, but am having a hard time finding guidance on how to use it.  Note, I don't mind putting in the time to relearn the procedure, but am having a hard time getting started.  

Note, our County flood control does provide the RAINFALL DISTRIBUTION COEFFICIENTS FOR VARIOUS DESIGN STORMS as well as an S-CURVE for HMS.  

Thanks for any insight or direction on getting started.   
francesca (Civil/Environmental)
6 Feb 11 15:58
SCS method is now called NRCS method.

If you're in CA, in some places they use the Modified Rational method to determine volumes.  It's not very accurate, but for your tiny area I can't see that it matters.
Helpful Member!  cvg (Civil/Environmental)
7 Feb 11 17:09
no fancy calculations required. for a small area such as yours, volume equals rainfall depth x area x runoff coefficient. to monkey around with hydrographs, HMS etc. is a waste of time to calculate runoff volume (or peak discharge) for this small site.
gbam (Civil/Environmental)
8 Feb 11 9:42
cvg nailed it for volume calcs.  However, you will need some type of hydrograph to perform a routing analysis.   
ewbeng (Civil/Environmental) (OP)
8 Feb 11 11:36
Thanks all for the great input!  I really appreciate it ~  

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