"freeboard" on spillway? 1

[cvg]

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?

 

[dicksewerrat]

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.

 

[cvg]

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.

 

[RWF7437]

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.

 

[cvg]

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.

 

[Focht3]

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. [blue]dicksewerrat[/blue] 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?"

 

[cvg]

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.