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

SCS method

SCS method

SCS method

I am sizing a detention pond with a 260 ac watershed. I am using the SCS method and coming up with 22 ac-ft of storage. The actual developed property within the watershed is 60 acres. This seems to be a high required volume. When I say high volume the only thing I have to compare it to is the rational method with a smaller watershed. The time of concentrations and CN values are relatively close for the pre and post. Is their another method that I can use for a comparison other than the rational? I do not trust the rational.......

RE: SCS method

You can't do volume calculations using the rational method. There are plenty of threads that explain the ins and outs of it, and also a recent one debating the Modified Rational Method which can give a volume.

RE: SCS method

Oh, and, a 260 acre watershed almost certainly has a listed stream coursing through it, which you can't put a detention basin into.  You will have to detain the flow from the development separately before it enters the stream.  This will be much cheaper as your detention basin volume will be significantly smaller.

RE: SCS method

Modified Rational will always give lower storage, due to both the pre and post Mod Rat inflows are higher.  

Francesca is correct.  Don't analyze 260 acres, analyze your 60 acres.  Just provide a swale sized for the other 200 acres to bypass your storage volume, or provide for it to flow through the volume undetained.

22 ac-ft is more storage than I would expect for 60 acres.  Assuming post CN of 86, 50 cfs in, 10 cfs out with 8 inches of rain, you should be seeing a storage estimate of about 2/3 that.

Engineering is the practice of the art of science - Steve

RE: SCS method

A quick volume method is to use apply the SCS loss based on your curve numbers to your total precipitation to determine the total storage.  This also helps give a rough estimate for changing land use (its original intent).

Q= (P-0.2S)^2/(P+0.8S)
where CN=1000/(10+S)

The source is NRCS Hydrology Manual at:

Are you required to look at the entire watershed?  I know in the west at least, some communities require that the first developer has to put in detention for the whole watershed and then gets reimbursed for this "regional" facility as the rest of the shed is developed.  A 260 acre watershed could possibly be this.

RE: SCS method

Thanks for everyone help. Now I have another question. Where can I find the rainfall distribution curves for my area. I was using the one in PondPack for Type III 24 hr rainfall distribution for this watershed.

RE: SCS method

Most hydrology programs will include the SCS Type I, IA, II, and III rainfall distributions.  If you can use one of these tables, all you need to provide is the total rainfall depth for the n-year event.

A few locatilies have developed their own rainfall distributions.  In this case, the details should be in the stormwater regulations.  You can use this information to create a custom rainfall table for most programs.

RE: SCS method

A nice feature of HMS is running the frequency based method.  You enter precipitation for durations up to your storm duration and then weight it.  A center or 2/3 is a typical conservative weighting for most locations depending on how you model losses.

RE: SCS method

Careful iha,

There are at least two methods out there calling themselves the Modified Rational Method. One is really Hokey and the other is a little Hokey but even a Hokey Method can be calibrated to produce reasonable results.

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


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

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