Stream bed mods to change flood zone designation
Stream bed mods to change flood zone designation
(OP)
I am dealing with a ravine that will experience runoff during a 100 year flood.
Can installing a culvert and adding fill over the top be a justification for changing the flood zone designation?
I am interested in hearing from people who have experience with FEMA in cases like this
Thanks
Can installing a culvert and adding fill over the top be a justification for changing the flood zone designation?
I am interested in hearing from people who have experience with FEMA in cases like this
Thanks





RE: Stream bed mods to change flood zone designation
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RE: Stream bed mods to change flood zone designation
We already requested an LOMR using a log-Pearson Type III analysis which was based on 52 years worth of daily rainfall data. A spreadsheet of over 300 pages. It was rejected because it was daily rainfall not every 10 or 15 minutes.
So the question is has anyone had any experience getting an LOMR based on proposed stream bed modifications? Or do you have to get the LOMR before you can get permission to do the streambed modifications?
RE: Stream bed mods to change flood zone designation
I've done some LOMR stuff before, where streams were mismapped and we adjusted the maps based on better survey data. It's not extremely difficult, but pleasing your FEMA reviewer can be challenging.
Typically if you're planning on filling the floodplain, you get a CLOMR (conditional letter of map revision) based on your design before you do the construction, then do the construction, then the map gets changed. That varies some depending on whether you're in floodplain or floodway, which zone, and what your local flood plain manager wants you to do.
Curious - If FEMA isn't accepting your historical data, is there any reason you couldn't do a more traditional stochastic hydrology analysis?
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Stream bed mods to change flood zone designation
RE: Stream bed mods to change flood zone designation
Our case was unique in the sense that portions of the stream were going to remain active and convey local drainage, but the majority of the stream was being put underground.
Because the floodplain and floodway for much of the stream was going to "disappear," in effect, we had to get a CLOMR approved first. As beej67 implied, it took FOREVER to get the reviewer to buy off on the changes.
What is the current zoning of the stream you are working on?
What programs do you plan on using to model the changes?
RE: Stream bed mods to change flood zone designation
I think you are right on the money about the CLOMR-F being the best approach.
My project is similar to froude's except most of the stream will stay above ground and only a small portion will go underground.
You are right about FEMA being hard to convince. We are approching year two of FEMA submittals. Every time we get a rejection we get a whole new set of questions or requirements.
The project area is about 8 acres. The tributary area is over 600. On the previous rejection (two rejections ago) they required us to include the area below the project even though the flow volumes will not change and the lower areas already have modifications that are way more than adequate to handle any flow predicted by the 100 year storm.
It is a floodway that so far has never been known to flood.
I am using HEC-HMS to calculate the runoff and some basic equations for pipe flow to size the culvert.
Thanks
dpajr
RE: Stream bed mods to change flood zone designation
I ended up with 5 reaches (A,B,C,D,E downstream to upstream, where B and D were open channel sections). To avoid messing with EPA-SWMM, I ended up using a combination of HEC-RAS and StormCAD to create a "continuous" model. I used rating curves to represent the transitions from open channel to box system. Of course, there are so many ways to do this.
The box culvert basically contained the 100-year floodplain. The 0.2% chance floodplain ended up being a sticking point with FEMA. Ultimately, the overland portion of the 500-year storm was mapped using the 500-year flows, less the box capacity. We ended up removing the floodway delineation through our study reach.
Don't know if this helps, but I hope it does!
RE: Stream bed mods to change flood zone designation
Lessons learned:
1) Never ever do a LOMR on a fixed fee.
2) Educate your clients on what you're getting yourself into.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Stream bed mods to change flood zone designation
All the world's philosphers could not have come up with better advice than that.
Thanks goodness I didn't do it for a fixed fee.
Thanks,
dpajr
froude,
500 year flood yikes
What set of codes governs cleanouts and other structural details, bedding etc of putting in a culvert? I am guessing AASHTO but that is just a guess. Also was there a specific advantage of a box culvert as opposed to round? Was it just flow size?
Thanks,
dpajr
RE: Stream bed mods to change flood zone designation
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Stream bed mods to change flood zone designation
Use of box culverts was dictated by very high flowrates. The lower end of our system was basically a double barrel box that you could probably drive a car through in each box.
RE: Stream bed mods to change flood zone designation
dpajr