Smart questions
Smart answers
Smart people
Join Eng-Tips Forums
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

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. http://onlinemanuals.txdot.gov/txdotmanuals/hyd/nrcs_runoff_curve_number_methods.htm

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. http://njscdea.ncdea.org/The%20Modified%20Rational%20Method.pdf
 
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 ~  

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close