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# Rational Method - Runoff Coeficient - Newbie4

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## Rational Method - Runoff Coeficient - Newbie

(OP)
Hey guys:
I would like to know where can I find a step-by-step procedure to calculate peak runoff and volume of a 8.2 Acre site.
I am using the Rational Formula Q=CiA, I know the Area "A", and the type of coverage that is gravel so I would like to know what runoff coeficient "C" should I use for this type of coverage.
Also, how do I calculate the rainfall intensity "i" for 10yr, 25yr and 100yr storms and a 24hr duration? I am located in New England and have a IDF curve.
Oh by the way I am trying to do manual calculations so I am kinda lost...got used to using computers too much.
Thanks for any help and advice.

### RE: Rational Method - Runoff Coeficient - Newbie

Pick up hydrology book and read the section on precipt and rational methods.  Descriptions of run off coeef. and such are tablized in many resources.  My pref. is Applied Hydrology by Chow/Maidment&Mays.

### RE: Rational Method - Runoff Coeficient - Newbie

If you are going to submit your plans to a city or county for approval, you will need to use their methods and standards.  Get a copy of the drainage manual for your city.

### RE: Rational Method - Runoff Coeficient - Newbie

Download and print out Hydraulic Engineering Circular No. 22 (Urban Drainage Design Manual) from the Federal Highway Administraion web site.  It has a section on the Rational Method as well as other systems.   As cvg suggested, check out local and state agencies, they may have design manuals they want you to use.

### RE: Rational Method - Runoff Coeficient - Newbie

For I:
Find the Tc, most reviewers will accept the SCS method, free software and manual from the NRCS website.  Then read across the IDF at that time and presto, you've got I.

For C:
Any civil engineering text will have them, but cvg is right.  Get the review agency's chart of them, no sense doing everything w/ C=0.28 only to find they want 0.29.  For conservative analysis (and/or if the agency doesn't spec C):

grass C=0.3
woods C=.2
cult. farm C=.4
impervious C=.95

I used to live in both eastern MA and western MA, so if you are near either, I know you will get a lot of "I" indeed ;)

### RE: Rational Method - Runoff Coeficient - Newbie

(OP)
Thank you all!
Your help is very much appreciated.

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