×
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

"freeboard" on spillway?

"freeboard" on spillway?

"freeboard" on spillway?

(OP)
I have a flood control dam with an ungated principle outlet works discharging to a 100-year flood conveyance channel with levees.  During the 100-year storm, the dam fills to the crest of the emergency spillway and just begins spilling over. The outlet channel is flowing full.  The water flowing over the spillway inundates an area downstream of the dam and may end up flooding a highly urbanized area.

Questions:

1) Is the area downstream of the spillway considered by FEMA to be a floodplain?

2) Assuming it is a floodplain - if I raise the spillway crest, could the spillway inundation area be removed from the floodplain?

3) Would I have to raise the spillway three feet as required for the freeboard on riverine levees?

RE: "freeboard" on spillway?

I'd look real hard at what 3 more feet of water will do to the dam.
Call Fema about your dam for the rest of it.

RE: "freeboard" on spillway?

(OP)
I'm looking at doing a complete rehabilitation of the dam anyway, so the dam will be raised and the spillway widened.  However, the question of raising the crest of the spillway has also come up.

RE: "freeboard" on spillway?

Seems like you have several problems  which cannot be easily answered by anyone in this forum. First, who will review your dam redesign ?  Contact them to find out what they require.  What flow must you design for ?  The maximum probable flow or something less ?

  Second, consult FEMA maps to determine the existing regulatory floodplain, if any exists.  Then talk to them about what you plan to do. If a Letter of Map Amendment is required you're in for a challenging and time consuming task.

  Third, do a dambreak analysis.  What would happen if your dam fails?  In our State ( Oregon ) such an analysis is required by the State Water Resources Department for any dam impounding more than 9 ac-ft of water.  They are also the agency who would review your plans.

Good luck, hope you've done this before or know someone who has.

RE: "freeboard" on spillway?

(OP)
there are a number of problems with the dam that we are looking at.  However, we are just attempting to determine if we want or need to raise the spillway crest.

RE: "freeboard" on spillway?

If the pool level is at the emergency spillway level after a 100-year storm, I'll bet that you will have to undertake some significant modifications, including raising the spillway elevation - or something equivalent.  dicksewerrat is right - look real hard at what another 3 feet of water will do to your dam.  And at the storm frequency that would raise the pool level 3 feet.

To (generally) answer your questions:
1.Probably - but look at the maps.
2. Unlikely - this would permit development to occur in these areas, and FEMA might have to buy those properties back in another 20 years...
3. Depends on a lot of things we don't know -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.  See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

RE: "freeboard" on spillway?

(OP)
Focht

I appreciate your response and believe me, we are looking at this dam in detail.  

The FIRM maps do not show this as a floodplain, and in general, do not show this for any of the flood control dams in the area (except where those dams are not rated for 100-year flows).  We are already considering raising the spillway elevation somewhat, but the real question was on freeboard requirement.  Most opinions I have gotten so far indicate that the dam is not considered a riverine levee and freeboard is not required.  The dam safety division here does not require the freeboard, so none of the dams in the state are required to provide it.  Development has occurred in these areas and downstream of other dams in the state, and apparently the issue has not been challenged by FEMA yet.  I have done exhaustive search of the FEMA documentation and find nothing explicitly dealing with dams or spillways.

We are looking at the total design and in fact the dam and spillway modifications will be designed for the probable maximum flood and analyzed using a two dimensional routing model through the impoundment.  In addition, we will likely provide even more freeboard to account for sediment storage as well.  

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