×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • 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!

*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.

Students Click Here

Convert CN's to Rational C's
5

Convert CN's to Rational C's

Convert CN's to Rational C's

(OP)
Is there a way to convert a SCS "curve number" to a rational "C" number.

RE: Convert CN's to Rational C's

There is no simple direct conversion but you can calculate a peak flow using TR-55 and then using that flow solve Q=CIA for an equivalent "C".

RE: Convert CN's to Rational C's

5
I would not do what RWF7437 recommends.  TR55 and Rational will almost always give vastly different Q values, as they have different applications.  Usually, I use Rational for small areas (10 acres or less) with a higher percentage of development.  Over 10 acres and Rational underestimates, I think.  I use SCS methods for mid-sized areas which are more natural and/or agricultural.  Anything over 0.5 square mile and TR55 vastly overestimates in my opinion.  Try a flood model (HEC or PSU4) above 0.5 square mile.

If you must convert CN to C, Rossmiller's Conversion can be applied.  It is a monster equation, but here it is:
C=(7.2*(10)^-7)*(CN^3)*(RI^0.05)*{[(0.01*CN)^0.6]^-s^0.2}*[(0.001*CN^1.48)^(0.15-0.1*I)]*{[(IMP+1)/2]^0.7}, where RI=return interval(yr), IMP=impervious coverage( decimal)and S=average land slope (whole number percent).  I have a spreadsheet which I have used (and has passed reviews) in the past, but it has not been QC'd by another engineer.  If you leave your email address here, I will send it to you to play with.

RE: Convert CN's to Rational C's

Iha has the correct solution. Rossmillers formula was developed by Ron Rossmiller, formerly of Univ. of Iowa. I have included it in my software for use in the rational formula method for computing runoff and call it, of course, Rossmiller Rational. CN in the main formula is computed by the formula CN=98IMP+X(1-IMP).
IMP = % Imperviousness, dimensionless decimal
X= factor based on soil type
Type A=39
Type B=61
Type C=74
Type D=80
The antecedant moisture condition is assumed as II


RE: Convert CN's to Rational C's

(OP)
Iha, I am interested in your spreadsheet.  my email address is jwledy@apexse.com

RE: Convert CN's to Rational C's

I sent it to jwledy@apexse.com this morning, it came back server unknown.  There is a nomograph available for this conversion.  I know it is Page 29 of PennDOT/FHWA's PSU-IV, and probably other places.

RE: Convert CN's to Rational C's

IHA,

I would be interested in your spreedsheet as well.  If you don't mind of course.

murrisonenvironmental@cox.net

Thanks
Scott Murrison

RE: Convert CN's to Rational C's

Just a clarification, Dr. Ronald Rossmiller taught at Iowa State University - not the University of Iowa. A small point if you live outside of Iowa, but a huge issue for those of us who live in Iowa. ;)

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

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! Already a Member? Login



News


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