Filling in FEMA flood plains 5

[mickcwa]

I've had a developer approach me about extending a subdivision into a FEMA flood plain. Their thoughts and mine would be to raise the elevation of the ground to above the base flood elevation. We have done this is the past with relatively small sites. However, the thing that concerns me is the length of fill along the creek itself. The fill would basically raise the ground level for about 1000'.

I am also wondering about coordination/approval from FEMA and how to go about getting it. This is a studied creek that has base flood elevations associated with it. If anyone has dealt with this situation, I would greatly appreciate some help.

Thanks,
Mick

 

[redneckengr]

Mick,

you will have to complete a hydraulic flood study of the section of stream and some distance up and downstream. The process you are about to undertake is what FEMA will call a CLOMR (Conditional Letter of Map Revision)or you might get away with a LOMR. I suggest you contact the local flood plain administrator/manager and discuss with him/her what the developer is propsing. They may allow you to just submit for a LOMR (they have a lot of power as to what you have to submit to FEMA). The fee for a CLOMR or LOMR is $4000.00 so if you only have to the LOMR then you save the developer. If they require a CLOMR then you will have to do both and pay for both.

You need to get the model used by FEMA for the stream. After that you will have to complete a duplicate effective, corrected effective, existing conditions, and then a proposed conditions model for the CLOMR or LOMR. Explainations are available on FEMA's websites along with submission forms and fee schedules.

If you need more in-depth explaination just reply. I'll keep an eye on your thread.

 

[dicksewerrat]

you will have to get this past a local watershed group too. If they say no way in He... in my backyard. Tell the developer to scale back.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[francesca]

The Cumberland River passes through the town I worked in and the local street department had authority to approve no-rise certifications, despite the Corps of Engineers having a flowage easement on that section of the Cumberland and the other agencies mentioned above having their thumb in the pie.

The engineer at the street department made it very clear that he was not stamping the no-rise certifications, and that the design engineer would take all responsibility.

If you're narrowing the flood plain, you increase the velocity in the area of flood plain constriction. This has scour consequences associated with it downstream, and ponding consequences upstream as the cross-section narrows. Beware what you stamp! Don't take on a project that could lead to you losing your license if the land upstream floods.

I'd also be wary of old FEMA flood maps. The maps in the region in which I worked are over 20 years old and the area has developed significantly in the intervening years. Of course, flood studies are littered with uncertainty (What is a 100-year storm? How accurate are the regression equations? How well do they apply to my drainage basin?) and it's a debatable point whether development increases or decreases the severity of a flood in your immediate vicinity (urban run-off flows faster and may escape before the flood peak arrives... but there's more of it).

While I did my best with the flood studies I produced, I sleep easier at night knowing that I didn't stamp them, and also knowing that the 100-year flood elevation I set was used to set property boundaries, not finished floor elevations, which were a few feet higher!

Finally, under NPDES Phase II rivers in that town now need a 50' buffer from either top-of-bank. Would your development encroach into this area? Even if buffers aren't required in your area (yet), the environment will thank you if you preserve the natural vegetation on your creek overbanks!

 

[danryan]

To add to what has been previously stated, two main issues with changing a floodplain are 1) The loss of storage when routing the flood through that section. You cannot raise water surface elevations upstream or downstream by much at all if you want to avoid liability, so you have to provide storage somewhere for the amount you've lost. 2) FEMA floodplains are based on so-called existing conditions; e.g. undeveloped in a lot of suburban or rural areas.

Therefore it is wise to assume developed conditions for your true floodplain and prudent community planners have come to require this. That is, assume no detention upstream (because detention can harm the overall watershed if improperly designed) and set finished floors based on that. It is a standard in Austin, Texas where I practice and I think it should be standard practice for any drainage engineer.