×
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

Time-of-Conc Question

Time-of-Conc Question

Time-of-Conc Question

(OP)
Hello everyone.  I am presently crunching through some detention basin design calculations and I came across a minor snag.  I think it's a fairly common problem, but I was wondering if anyone had any feedback about how to deal with it.  Here's what I have.   
 
1) I set up pre-development hydrographs using the Rational Method, using one large watershed (about 25 acres) to model the pre-development runoff.  The max Q came out to 36.31 CFS for the 100 year storm with a 17.44 min Tc.
2) For post-development, I divided the pre-development watershed into routed and bypass sub-areas.  I set up my post-development hydrographs and modeled it without the basin routing to quantify how much the runoff would increase as a result of the added impervious surface area.
 
Here's where I run into a problem. The combined post-development hydrographs (unrouted) resulted in a REDUCTION in the peak runoff, rather than an increase.  For the 100-year storm, the max Q was 33.73 CFS with a 17 min. Tc.  It seems pretty clear what the problem is.  Simply put, the peaks of the sub-watersheds do not line up anymore.  The increased runoff from the developed area occurs at Tc=5 min, which is well before the Tc of the much larger bypass watershed, which occurs at 17 min.  The developed runoff is already off-site by the time the bypass runoff peaks.
 
Has anyone dealt with a similar problem before?  I don't think it would fly if my SWM report indicated that the development resulted in a decrease in runoff.  My instinct is to manually add the peak flows of the individual watersheds.  It's over-conservative and it does not accurately model the actual conditions. However, I cannot think of a better solution.  Suggestions anyone?  Thanks.

Damon

RE: Time-of-Conc Question

You shouldn't really be using the rational method to design a detention basin. That's problem number one. The SCS curve number method is much better. In the past, my company sized detention basins on the Rational Method and now that the City is requiring the SCS curve number method for volume calculations, any old detention basins that need to be recalculated end up much too small. The problem is that the rational method gives a peak flow, but not a volume, which the SCS CN method provides in addition to a peak flow.

However, your biggest problem is that you appear to be using a 5-minute design storm.  In that case, perhaps your peak is lower.  However, if you use a 15-minute design storm, and compare that to the pre-developed, you'll almost certainly get a different answer.  In this case, while your post-developed flow peaks within 5 minutes, because it keeps raining, the 5-minute time of concentration basins remain at peak flow, allowing you to add the peak flows together. This sum should be higher than than your pre-developed peak flow.

Remember that you have to make post-developed run-off equal to or lower than the pre-developed rate/volume/velocity for all storm durations and recurrances.

Ches

RE: Time-of-Conc Question

(OP)
Of course! It doesn't make sense to add a 5-minute design storm in my routed watershed to a 17-minute design storm in my bypass watershed.  I will modify my calculations accordingly.  Thank you so much.

With regard to the use of the Rational Method, I should clarify.  I am actually using the Modified Rational Method to size the basin.  Thanks again.

RE: Time-of-Conc Question

You actually need to determine the critical duration.  This is the rainfall duration (and the corresponding intensity from the IDF curve) which produces the highest flow at a particular design point.

For a single subcatchment, the critical duration is equal to the time-of-concentration.  But for multiple subcatchments, the answer is not so simnple.  In your case, the critical duration for the combined flow can be anywhere between 5 and 17 minutes.  Note that the critical duration may be different at different points in the watershed.

The SCS/NRCS method is much easier to apply, and is far more appropriate for detention pond studies.

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


Resources

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