LOMA Application - Floodplain for Piped/Underground Stream?

[aHandfulofDust]

Let me preface with: thanks for any help anyone can give me - really appreciate any time you spend.

I've been asked to prepare a LOMA application for a site in Brockton, Massachusetts. It's a large commercial site (~8 acres), and is partially located within an 'A' zone, so no determined BFE. I'm looking for a little help/direction in terms of technique in determining the BFE for the application.

The stream 'causing' the flood zone on the site is actually piped underneath the site. So the upstream portion of the stream is open to the air, and has an associated 'X' zone. It gets entirely piped underneath the site - no visible waterway from the bird's eye - and daylights downstream. As soon as the stream hits the piped portion, it switches to an 'A' zone, though - so I've got floodplains along the entire site in the FIRM map, but no visible water/stream.

The flood insurance study for the county only mapped the stream in question up to the downstream municipality, so no real help there. I've put in calls to the local DPW, Planning Board, and the state floodway program looking for potential non-FEMA/unofficial flood studies, so I'll see what I hear back from them.

Ultimately, I'm just not sure how to go about determining the BFE for the site to 'prove' it's not going to be inundated - it's all underground. My initial thought was to use a simplified method like contour interpolation at the up and downstream points, and (if valid) use those elevations to try and show flooding from the two daylighted points wouldn't reach the site in question. Not sure how valid that is, or if there's a better way to more experienced eyes.

I haven't prepared one before, and it looks like my firm doesn't really do them - asked around and searched the file directory, and came back with one project. So any help/direction you can give me would be great.

Thanks!

 

[Pinwards]

It seems like if you can show that the "pipe" in the piped portion was sized to convey the 100-year flood, and that inlet/outlet conditions are also sufficient to convey that flow, then the 100-year floodplain through your project area would be limited to the confines of the pipe.

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[nkrigPE]

Pinwards is correct. You'll need to either find an existing study that determined the flow rate at the pipe inlet (or nearby), or develope the hydrology and flow rate yourself, and demonstrate that the pipe can convey the 100-yr storm event.

You will most likely need to submit a LOMR to FEMA. In order to remove your site form the Zone A you'll need to establish BFEs along the stream and that = map revision.

 

[eea]

I agree, that is how I've seen bridge crossings detailed on FIRMs. I'd also be prepared to speak on the upstream inlet and the backwater it creates, demonstrating it does not encroach on adjoining properties, or if a drainage easement has been obtained, ...