Flood Plain Mittigation

[bank]

A project I am working on involves constructing new buildings, roads, detention pond, etc., within an area designated as a 100-year flood plain. By code I am required to set finished floor elevations at least one foot above base flood elevation, with no net increase in fill within the flood plain. To achieve the required elevations I proposed to use dirt excavated from the detention pond. Local code enforcement people told me, however, that for mittigation purposes, I could only use fill from the detention pond in excess of the required storage volume. The amount removed to provide detention storage could not be counted against the amount added to raise the site. I don't get it; I just cannot grasp how using fill excavated from the detention pond to raise elevations on the site would affect the pond's performance or contribute to local flooding. The site drainage is such that all runoff is carried to the detention pond. Would there be a difference if I lowered the parking lot and used that dirt for fill? I don't see the difference. I welcome your advice.

 

[francesca]

The area below the 100-year flood elevation is considered to be flooded. If you're placing a detention pond in that area, then you can't double-count it for both detention and flood plain.

I'm somewhat flabbergasted that you're allowed to do a development in a flood plain, and that they'll allow a detention pond below the 100-year flood elevation. Water will back up the detention pond outlet and flood your detention pond regardless of how high the side walls are built up.

It certainly isn't a set of plans I'd stamp.

 

[Callin]

FEMA can look at that detention pond as a levee under new standards. I common way to avoid the levee certification is to map the revised floodplain BFE within the pond. You would be suprised francesca on how much this is occuring in the US. Developments can be placed in the floodway fringe, but the process is lengthy and heavily reviewed.

I had trouble reading the question but I agree that you can not use the area of the detention pond as conveyance in the 100-year floodplain, especially if you are not looking to certify the embankment as a wall.

I would suggest that the site plan be revised to encroach less into the floodplain if I am understanding the situation correctly.

 

[bank]

On the Gulf Coast, particularly in Texas and Louisiana, there are entire cities located within the 100-year flood plain. Despite that, the normal design activities, whether drainage, detention, or whatever, still have to be performed. My project is a major expansion of a college campus that is located entirely within the flood plain. It's true that the detention pond might be innundated during the 100-year storm event; can't help that. Nowhere on the campus is above the 100-year base flood elevation. I still have to provide detention. For the normal 2-, 10-, 25-, and 50-year events it will work fine. My question was, since all fill has to come from within the 100-year flood plain, what would be the problem with using dirt from the detention pond to raise ground elevations where needed?

 

[Schopsy]

Bank, it seems like you've misunderstood the Code Enforcement folks. I can't imagine that they would care where the fill comes from.

 

[bltseattle]

My interpretation of your query is that the agency does not want you to consider the "scooped out" detention pond as mitigation for placing the floodplain fill for your building pad (and roads, etc). The detention pond is considered full of water and therefore not available for flood plain functions. I agree with Schopsy that they probably don't care where your fill comes from. It sounds like you will need to scoop out some storage somewhere else within the floodplain to make up for the fill placed for your site improvements.

 

[Lih]

In response to "My question was, since all fill has to come from within the 100-year flood plain, what would be the problem with using dirt from the detention pond to raise ground elevations where needed?" -

I assume the detention pond will be "wet" and have a permanent water level? Then your required storage volume would be the volume between the permanent (low) water level and the maximum design water level.

I think as long as you can demonstrate that you are able to provide this required storage volume you should be ok.

If the space where the fill is removed from is subsequently replaced by water in the pond, of course this will not provide any detention. This may be an issue the authorities have in their minds, you'll have to demonstrate that the volume taken up by the permanent water is separate and additional to the detention volume that you will provide.

 

[blueoak]

I actually think you may be lucky. I am used to any filled areas must be mitigated by increased storage at the same elevation. For larger flood systems timing is important and trading upper storage (or later) for low or (early) storage is discouraged.

I think you may need to look at the whole flood plain and what impact your actions have. If you are just a tiny part of it, what really is the point of the detention except maybe stormwater quality. I think presenting this to the officials would be the best bet. If you can show your fill doesn't impact others and additional storage doesn't affect the floodplain it seems you meet FEMA requirements. The whole fill must equal storage is so that consultants and reviewers can get through work without reanalyzing things for every little project. Not really meant as a logical solution to all situations.

 

[Callin]

Agreed. We are working in floodplain area right now where the on-site detention standard is the 10-year storm event. If you can compare hydrographs it may not be necessary to detain up to the 100-year event.