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 Concentration

Time of Concentration

Time of Concentration

I have been told that the time of concentration can be lengthened by 5-15 min. when storm water is routed through a catch basin. I have not been able to find any technical data discussing this and was wondering if anyone has information on this subject.

RE: Time of Concentration

Sounds bogus to me, I would make you prove that.  I would guess water takes somewhere around a nanosecond or two to enter and pass through a catch basin.

Engineering is the practice of the art of science - Steve

RE: Time of Concentration

Sniff - Sniff...smells like fertilizer.  The incoming water has to reach a point where the catch basin will discharge (if there is any storage).  Depending on the rate of inlow, this could be a couple of seconds to a couple of minutes.  Once the flow is going out, your Tc through the basin is essentially zero.

RE: Time of Concentration

Do you mean catch basin or detention/retention basin?  Detention basins can increase times of concentration for hours.

RE: Time of Concentration

Crystal has a point - catch basin is usually an inlet.
The detention/retention basin or pond will hold water, and could make an impact.

Your pipe flow time may be used in the Tc, but it won't be very much time.  

RE: Time of Concentration

Thanks for the input. I was in a teleconference on detention basin design and the presentor specifically stated that 5-15 min. inlet concentration times could be added to the time of concentration. I had not ever heard that before and it seems like is it not a reasonable assumption.

RE: Time of Concentration

The travel time for a lake or reservoir can be estimated by:

Tt = L / V

where V = sqrt( g D )

Tt = Travel time (seconds)
L = Flow length
V = wave velocity
g = gravitational constant
D = mean depth

RE: Time of Concentration

I wonder if the presentor was refering to the minimum Tc of 5 minutes commonly used in Rational method calculaitons.

RE: Time of Concentration

The overland travel time (time to reach the inlet) for a one acre parking lot will be about 100 ft/2fps = 50 seconds. For this reason computer models that require a minimum Tc of 5 minutes are inherently invalid.
The Rational Method was never intended to be used with Tc --this misconception was invented in 1890.
The Rational Method uses the IDF correlations that estimate the ability of a raincloud to generate rainfall. This ability is determined by rain gage data.
The minimum duration for IDF correlations is 5-minutes. This duration can be used as long as the pipe network travel time is less than 5 minutes. For example, a 1000-ft long storm sewer flowing at 3.3 fps has a travel time of 5 minutes.
Rational Method Runoff coefficients can be reduced to allow for ponding and/or the longer travel time for heavily vegetated areas.

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