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




Why do hydrographs show no flow in or out of my pond for so many hours, then suddenly jump up? This doesn't seem logical to me. Shouldn't flow start almost right away?

RE: Hydrographs

Unlike the Rational Method (which is linear) the SCS runoff equation is non-linear. Furthermore, no runoff will occur until the initial abstraction has been satisfied.

From the web site:

Why don't I get any runoff until about 10 hours?
According to the SCS runoff equation, there will be no runoff until the rainfall exceeds the initial abstraction, which may not happen until 8-10 hours into the storm. Until that point, the runoff will be zero. This effect is most pronounced for low CN values and low rainfall depths, such as water quality events. For example, a CN value of 80 has an initial abstraction of 1/2 inch, which isn't satisfied until 11.2 hours for a 2-inch 24-hour Type II storm, and significant runoff does not occur until about 11.7 hours.

Peter Smart
HydroCAD Software

RE: Hydrographs

Regarding the pond outflow, an abrupt increase can occur if the pond is retaining the initial inflow, and then overflowing via a weir, which is characterized by a sharp increase in discharge vs. head. If you want the pond to discharge at an earlier time, then you need to have a low-flow outlet near the bottom of the pond.

Peter Smart
HydroCAD Software

RE: Hydrographs

I do have a low flow (5" orifice) outlet at the very bottom of my pond. Do you think my hydrograph seems typical?

My coworker thinks this doesn't make sense, and I have a setting "off".

The CN of my drainage area is 83, and I have a pond with a 24" outlet through a control structure with a low-flow orifice and weir.

RE: Hydrographs

I don't see anything wrong with your hydrograph. It appears completely consistent with the situation you have described.

Peter Smart
HydroCAD Software

RE: Hydrographs

Thank you Peter!

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