×
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

Water level rise from pre to post

Water level rise from pre to post

Water level rise from pre to post

(OP)
Hello,

I have a project proposing an addition to an existing dwelling. The addition is so big it is causing a minor increase in peak flow in the post condition compared to the pre. An increase here in New Castle County, DE means the home owner needs to build a SWM system. I expanded my drainage are as much as I can.

The reviewer said he will accept it as "no increase" If the water surface elevation in the pre does not increase in the post by more the 0.05-ft. Problem is I have no swale in the pre condition to compare my swale in the post! Lol. Is there a calculation that would allow me to check sheet flow depth at the analysis point?

Thank you!

RE: Water level rise from pre to post

One could approximate sheet flow with a single or group of cross sections then change the single or group of cross sections to reflect the additional foot print. If you cannot approximate then you would be looking at a 2-d analysis which seems way too much for this scenario; but, I am not familiar with your local criteria or regulations.

RE: Water level rise from pre to post

JSiteDznr,

Curious:
Does this not qualify for an Standard Plan? (Residential construction less than 1.0 acre disturbed) or an exemption? (less than 5,000 feet of disturbance)

Otherwise:
If there is no channel on site - one would probably need to establish a downstream analysis point where there is channelized flow (swale, ditch, closed drainage system). This should be a point of agreement with the reviewer at a pre-application meeting.

RE: Water level rise from pre to post

This is one of those cases where you need a parameter in the existing condition to compare with Post condition. In this case, you have post Water surface elevations (WSE) but you don't have pre WSE's to compare. You would need to identify the downstream point of analysis, preferably where there is concentrated flow leaving your site. This point of analysis should be same for both pre and post. Then assume a "pseudo" channel/swale at the analysis point for the pre and perform your flow calculations. The existing "pseudo" channel or swale may not have much of a defined channel geometry, but you need to approximate.
Explain this approach in the report and let the reviewer comment otherwise.

RE: Water level rise from pre to post

Can you pop a raingarden in the front or backyard and call it a day, or use some amended soils and take credit for the higher void ratio thereby meeting the pre to post rate requirements.

RE: Water level rise from pre to post

The other way is to determine the Pre and Post runoff volume. Then determine the difference. If the difference in runoff volume is less than 2%, you can include in the report that no significant downstream impact is anticipated. If the reviewer wants you to still provide SWM system, just use Rain Barrel at the downspout to capture as much roof runoff to offset the increase.

RE: Water level rise from pre to post

Also look at your point of interest in your analysis. Where is this site located within the watershed? If you look at the larger basin, you may be able to demonstrate that the increase in runoff will be negligible compared to the overall basin. Does this development drain to an existing stormwater management facility downstream?

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